In Shiguhao gas area, Dongsheng gasfield, Ordos Basin, most gas reservoirs of Shihezi 2 and Shihezi 3 Member (He 2+3 Member) are lithological-structural water-bearing sandstone ones with extra-low to low porosity and permeability. There are some issues during their development, such as highly difficult well deployment and poor effect just because of rich water in those wells at the lower position of gas reservoirs or part connecting with the lower water layer after fracturing. So, large amount of data from core mercury-injection experiments was analyzed and the difference among production results in natural flowing wells was found out. Then, gas-water differentiation height under different physical conditions was quantitatively characterized by using capillary-pressure and relative-permeability curve. In addition, many standards to screen the structure amplitude of small structural trap during well deployment were determined. Finally, a development-well pattern for different sandbody superimposition modes was pointed out, and both geological and development conditions to construct natural capacity were clarified. Results show that (1) the gas-water differentiation height is related to physical property. For the gas reservoirs of He 2+3 Member in this area, the permeability is 2.1 mD, and the differentiation height is 22 m; (2) the gas reservoirs are developed by deploying wells in the structural high. The vertically overlapped channel bars are developed by using vertical or multi-target directional wells, while the laterally cut and overlapped channel bars by horizontal wells, respectively; and (3) the screening standards of natural-capacity construction include smoothbox-shaped gammy-ray logging curve, no barrier and interbed, interval transit time greater than 260 μs/m, porosity and permeability higher than 15% and 1 mD, initial and cumulative production more than 1.0×10⁴ m³/d and 0.21×10⁸ m³, respectively.

There are abundant unconventional oil and gas resources in China. In order to effectively reduce construction cost, improve operation efficiency, and shorten development cycle in the development process of low-permeability shale-gas resources, based on one large-scale shale-gas development mode used in North American, another factory-like mode was established for Weiyuan shale gas in southern Sichuan Basin. However, in China, dense population and complex mountainous environment lead to some difficulties in shale-gas development, such as low sharing rate during resource development, too much occupation on water resources, mulitple construction procedures, and low efficiency. So, this foreign mode shall not be copied, and it is necessary to establish the domestic factory-like shale-gas development mode. Results show that (1) well location, drilling platform, equipment, and materials shall be involved in this factory-like mode; (2) water/power resources shall be unified in one region; (3) not only personnel but also equipment in some drilling at the same block along with platform shall be reused; (4) drilling and fracturing fluid shall be recycled; and (5) some support technologies of drilling and completion shall be optimized. In conclusion, the pipelining operation flow is preliminarily established, the regional factory-like mode is explored further, the seamless connection of drilling and completion is realized, the equipment installation time is shortened by 70%, the equipment utilization rate is up to 100%, the recycling rate of both flowback fracturing and drilling fluid is individually over 90% and 80%, and the operation efficiency is increased by more than 100%. What's more, this factory-like shale-gas development mode may provide effective technical support for speeding up shale-gas exploration and development in China.

In order to sufficiently understand and support an efficient development of high-temperature deep shale gas, southern Sichuan Basin, both reservoir characteristics and drilling situation in this block were analyzed, and an engineering technological support management mode was innovatively put forward by taking the drilling management mode as an objective. A suite of support team was built by integrating directional-well drilling, drilling fluid, rate of penetration (ROP) increase and optimization, and well control comprehensively. And a kind of three-level working mode of "providing technological support, participating in Party A's technological decision making, and assisting field management" was adopted. Specifically, one team of frontline support permanently should stay at Party A's base to fully participate itself technological research and organizational management, and another team of rear expert should provide remote support through engineering information technology. Results show that this management mode has been applied 120 well times in Company A and B since 2020. The average drilling cycle in L203 well block of Company A is shortened from 110 d to 100 d and then to 90 d, and two-round ROP increase is completed. The average drilling cycle in Company B is 23 d shorter than that in 2019, the ROP is increased significantly whereas the drilling cycle is shortened continuously. In conclusion, this mode provides a new definition of cooperation between Party A and Party B, which customizes the support range according to Party A's needs. It can be popularized to other complex blocks in Sichuan Basin to increase both ROP and efficiency.

Since 2016, four technologies such as wellbore, atomization, small skid-mounted, and centralized desulfurization have been applied successively to Daniudi gasfield in Ordos Basin. And the productivity in the Lower Paleozoic sour gas wells has been released by means of various chemical test, contributing to production increase in this field. In the test process, however, desulfurization cost is higher, economic benefit is not good, and serious scaling in pipelines may affect safe release. In order to sufficiently and accurately understand the influence of H₂S concentration in these reservoirs on the technological adaptability and provide decision-making reference for scientific development, both adaptability and application effect were analyzed for different desulfurization technologies by taking the H₂S content in natural gas treated by existing desulfurization as the reference value. In addition, the adaptability of wellbore, wellhead atomization, wellhead skid-mounted, and surface centralized desulfurization technologies was evaluated comprehensively. It is ascertained that, in Danniudi gasfield, there are four main desulfurization modes, including chemical adding in the wellbore for desulfurization, atomization at the wellhead for desulfurization, wellhead small skid-mounted desulfurization, and surface centralized desulfurization. In conclusion, the adaptability of these four desulfurization modes is analyzed and evaluated comprehensively based on the distribution characteristic of sour gas wells and the difference of H₂S content, combined with the development and deployment plan of sour gas wells and the principle of "benefit priority", so as to promote economic feasibility on the development of Lower Paleozoic gas reservoirs.

In order to better figure out production behaviors of each fracturing segment in a horizontal well of tight sandstone water-bearing gas reservoirs, some horizontal wells in Dugui gas province of Dongsheng gasfield were taken as objectives to analyze the correlation of gas/water production capacity in different fracturing segments to both physical properties of reservoirs and fracturing parameters by means of horizontal-well production profile test. Furthermore, the contribution and influential factors of the fluid in each segment were pointed out. Results show that (1) in Dugui gas province, the production mode of staged fracturing is mostly adopted to horizontal wells, so related production profiles shall be tested to make clear the contribution of each segment to gas/water production and the production behaviors under different physical conditions; and (2) parameter comparison demonstrates that the fluid production rate of each segment is in a certain correlation with porosity, permeability, gas saturation, and shale content. In conclusion, production profile test can quantitatively describe the fluid contribution of each segment. Furthermore, the gas production capacity of each segment may be influenced mostly by physical properties. The better the physical properties and oil-bearing property of the reservoirs drilled by horizontal segment is, the greater the contribution of horizontal segments to gas production is. What’s more, in view that each segment is different in geological physical properties, it is recommended to perform perforation in the zone with lower Gamma ray value and shale content, and high total hydrocarbon content, porosity, interval transit time, and gas saturation, which can improve stimulation pertinence and effectiveness.

Because of high fracturing pressure, most marine reservoirs in western Sichuan Basin can be hardly stimulated effectively in the process of completion. So, some deep marine reservoirs in western basin were taken as objectives to analyze the main tool and form some processes to reduce the fracturing pressure in these tight reservoirs. Results show that (1) high confining pressure and temperature should be the key factors affecting perforation depth. In extra-deep high-temperature tight reservoirs, conventional deep penetration charge is not adequate for reducing the fracturing pressure because the charge is hard to pass through stress-concentrated regions; (2) abrasive perforation, featured by larger perforation size, greater perforation penetration, and difficult generating a certain compaction zone around wells, may effectively reduce the fracture pressure in bottomhole; (3) if a little acidizing fluid seeps into these tight reservoirs, the absorption pressure can be reduced effectively, resulting in a decrease of stimulation difficulty. In conclusion, the implementation process of abrasive perforation in ultra-deep reservoirs is developed by optimizing string structure and sandblasting parameters, which can realize wellbore-reservoir connection and effectively reduce stimulation pressure. Furthermore, the absorption pressure can be reduced effectively by developing and optimizing the liquid for reducing the fracturing pressure, and matching the testing string and process. Thus, a large-scale stimulation may be achieved successfully while string safety is ensured. These processes may provide the reference for similar ultra-deep tight reservoirs.

Productivity is an important index to evaluate the development effect. In order to sufficiently make clear the production capacity of horizontal wells in the process of gasfield development and establish a reasonable method for evaluating the productivity, 9 horizontal wells in Daniudi gasfield of Ordos Basin were taken as objectives to create a sample database containing geological and development data. Then, the main factors influencing the productivity were figured out from two aspects of both geological cognition and fracturing operation, and 17 factors were screened out to establish a productivity evaluation and characterization model, including vertical depth of gas reservoir, length of horizontal section, sandstone length, drilling rate of sandstone, show length, show drilling rate, average total hydrocarbon, maximum total hydrocarbon, porosity, permeability, gas saturation, thickness of gas reservoir, number of fracturing stages, total liquid volume, total sanding volume, single-stage liquid volume, and single-stage sanding volume. In addition, these factors were optimized by means of a grey-correlation algorithm. Finally, after establishing one mapping relational equation between QI value and absolute open flow (AOF), the productivity was evaluated quickly. Result shows that the real AOF in Daniudi gasfield is less deviated from the value from the productivity evaluation equation, less than 10% of relative error. In conclusion, the new equation can not only satisfy the field application very well, but also provide the guidance for horizontal-well drilling, and the prediction of fracturing effect, and the theoretical support for the long-term stable production of 3 billion cubic meters in this field.

In order to sufficiently understand the development laws of large, ultra-deep, ancient, karst, and fractured-vuggy gasfields, provide reference for making scientific development countermeasures, and guide an efficient development for similar complex gas reservoirs, the Upper Sinian Dengying 4 gas reservoirs, Gaoshi 1 wellblock, Anyue gasfield, Sichuan Basin, were taken as objectives. And after implementing development schemes, the main geological understandings obtained were analyzed, and some development effect was summed up. Results show that (1) the gained understandings on the structural details, stratigraphic distribution characteristics, and reservoir thickness during scheme implementation are basically accordant with those in the stage of reserve submission. Compared with the scheme design, other understandings on the sedimentary mode of platform margin, the reserve producibility of Dengying 41 reservoirs, and the producibility of Dengying 42+3 reservoirs with porosity of 2%-3% are deepened, and the actual producible reserves have been increased by more than 600×108 m3; (2) aided by an increase of producible reserves, both single-well production and reservoir development scale are much higher than those in the designed scheme, and the development effect is better than expected; and (3) quality benefit and cost control are remarkable, the main economic indexes are superior to the designed values (IRR>20%), resulting in an efficient development on these ancient gas reservoirs. It’s comprehensively concluded that reservoir development shall follow scientific procedures to carry out an evaluation firstly to determine the main technical indexes and then extensively select the best one for productivity construction. Accordingly, the large Dengying 4 gas reservoirs with low porosity and permeability and heterogeneity can be developed efficiently. Furthermore, it’s necessary to speed up not only the evaluation on subsequent replacement resources for stable production but also a study on the main economic and technical indexes of replacement favorable and replacement areas to finally provide guidance for the gasfield with long-term efficient and stable development.

In southern Sichuan Basin, it’s hard to implement staged fracturing in some shale-gas wells after casing deformation. So, one technology of sand-filling temporary plugging was designed and optimized, and many indoor intra-fracture temporary-plugging experiments on proppant particle were carried on. Then, a few plugging behaviors on high-concentration proppant at narrow fracture opening were investigated. Moreover, combined with statistics, the main factors affecting high-concentration sand plugging inside fractures were discussed. Results show that (1) not only indoor simulation experiment but also grey relation analysis illustrate, the main factor affecting fracture temporary-plugging pressure peak value is the grading of proppant particle. Especially based on large-sized proppant as framework, adding to enough small-sized proppant particle may effectively increase the temporary-plugging pressure; (2) both indoor simulation experiment and grey relation analysis also demonstrate, the main factor impacting on temporary-plugging stable time is the proppant concentration. To improve this concentration is favorable for stabilizing the structure of temporary-plugging horizons. For smooth ceramisite particle, its bridge structure and the formed temporary-plugging horizons are easier to collapse or cause instability than those for quartz sand. Therefore, quartz sand is more suitable to sand-filling temporary plugging; and (3) via certain standards designed for the sand-filling temporary-plugging technology, the minimum concentration to block fracture opening under the given grading of proppant particle can be calculated. It’s deemed that these results are conducive to optimizing the design of key engineering parameters in the development of shale gas in southern Sichuan Basin, such as sand-filling concentration and proppant particle size.

In tight sandstone reservoirs of the Upper Triassic Xujiahe Formation, northeastern Sichuan Basin, there is large amount of gas with huge resource potential. But its productivity varies greatly in different members and areas, and the gas accumulation and enrichment laws are also complicated, bringing about some difficultly in determining the principal exploration targets. In order to clarify the main factors controlling the gas accumulation and enrichment of Xujiahe Formation in different areas of northeastern Basin and determine favorable targets, some Xujiahe Formation in three areas in northeastern basin (including western Yuanba, central-eastern Yuanba, and Tongnanba areas) was taken an objective to analyze the structural difference and its influence on gas accumulation and enrichment based on abundant analysis, test, and seismic data as well as exploration practice. Then, it’s pointed out that tectonic reworking in these different areas plays an important control role in reservoir-forming and hydrocarbon accumulation and enrichment in these tight sandstone reservoirs of Xujiahe Formation. Finally, some favorable exploration targets were determined. Results show that (1) the western Yuanba area is just an area with weaker structural deformation, where faults are not developed, and some calcarenaceous sandstone gas reservoirs of Xujiahe 3 Member with the advantage of intra-source hydrocarbon accumulation should be the principal targets for next exploration; (2) the central-eastern Yuanba area is a transitional deformation area, in which faults are developed in Xujiahe Formation, and Xujiahe 4 gas reservoirs with the advantage of upper reservoir and lower source should be favorable exploration targets; and (3) in the Tongnanba area as a stronger structural deformation area with superimposed marine and continental movement, discordogenic faults connecting marine strata are developed, and this area with the advantage of “marine-terrestrial dual-source hydrocarbon supply” should be the most favorable for exploration.

In order to develop certain drilling technologies with ultra-high temperature suitable for Kunteyi structure in Qaidam Basin, the ultra-deep Kun 1-1 well was taken as an objective to comprehensively analyze its drilling difficulties. Moreover, some support drilling technologies were studied. A series of drilling technologies with ultra-high temperature for matrix strata in Kunteyi structure were also developed by optimizing the 210 ℃ drilling-fluid composition of polyamine organic salt, selecting customized bit and ROP (rate of penetration) improvement tool, and optimizing and screening ultra-high temperature lost-circulation prevention and control formula. Results show that (1) after the ultra-high temperature drilling fluid is aged for 72 h, its high-temperature and high-pressure filtration is still less than 10 mL, and its rheological property is good; (2) the selected Atela bit and torque impactor combination has a remarkable effect on ROP improvement. The ROP in the fourth and fifth sections of Kun 1-1 well is 10% higher than that in its neighboring Kun 2 well; and (3) when the Neotoy high-temperature lost-circulation material is applied to field, the bearing capacity of formation pressure may be improved effectively and the maximum capacity can also be increased to the equivalent density of 2.07 g/cm³. Thus, drilling the first ultra-deep well in Qaidam Basin is smooth. These results may provide powerful technical support for drilling subsequent ultra-high temperature wells in this basin.

In order to sufficiently clarify the application effect of clean completion to high-temperature and high-pressure (HTHP) wells in Dina 2 gasfield, Kuqa pediment structure, Tarim Basin, some clean completion string equipped with soluble plug screen in D1 well was taken as objectives to analyze both constraints and fishing risks during the late development, which might be caused by insufficient replacement of bottomhole heavy slurry with conventional completion string. Results show that (1) after the completion of D1 well, both gas and oil production during blowout test is 48.6 × 10⁴ m³/d and 46.7 m³/d, respectively, much higher than expected in geological design; and (2) the integrity of clean completion string before acidizing meets design requirement completely, indicating successful application of clean completion to Kuqa pediment structure. In addition, it's concluded that (1) when the clean completion string is run to near perforation bottom boundary, the bottomhole heavy slurry can be replaced sufficiently to realize clean completion as well as uniform reservoir stimulation after completion; (2) a sand-buried cycle during production can be prolonged effectively while the integrity wellbore conditions for the later sand washing of coiled tubing are maintained; (3) during the later workover, segmented efficient fishing can be realized while the clean completion string doesn't have to be backed off, so as to reduce operation risks; and (4) its successful application to Kuqa pediment structure may lay solid technical foundation for further popularization in other ultra-deep HTHP gas wells in this area.

For shale gas, a kind of clean energy, its development receives national attention. At present, the Weiyuan risk operation area has been built up with the average initial gas production per well of 25×10⁴ m³/d, and the gas production of 700×10⁴ m³/d and 20×10⁸ m³/a, respectively. In order to promote its large-scale productivity construction in western Sichuan Basin and realize better economical and social benefits, several stimulated reservoir volume (SRV) wells in this risk operation area were taken as objectives to analyze reservoir geological characteristics, early fracturing measures, and production performance history. Then both fracturing design and construction direction for improving single-well production rate in the late stage were determined. Results show that (1) a sweet spot with high quality is just located at one structural position with natural fractures developed in steep-gentle transition belt (high-quality shale is over 5.5 m in thickness and 1.6 of formation pressure coefficient); (2) at fluid consumption intensity of 25-30 m³/m, the average fluid volume per stage is decreased from 2000 m³ to 1600-1700 m³, little influence on high-yield wells. And the half length of effective hydraulic fracture ranges mainly from 150 m to 180 m; and (3) it is difficult to deal with complexity if only slickwater with viscosity of 2 mPa·s is adopted in the main part. In conclusion, this half length is one of the most important factors influencing on postfrac production and controlling stable production capacity, and the weights of production affecting half length, number of stage, and flow conductivity of fracture are 0.53, 0.41, and 0.06, respectively. In addition, for one variable-viscosity fracturing-fluid technology, its application can effectively deal with some complexities of difficult sand filling and cost control. And this analysis on efficient fracturing parameters of shale-gas horizontal wells in Weiyuan area may provide technological support for the efficient development of shale gas.

For one technology of jet drainage used in the development of shale gas wells, in order to increase and determine both working efficiency and rational working range, a certain group of wells with high and low pressure on the same pad in Fuling shale gasfield of Sichuan Basin was taken as an objective to optimally design the structure of drainage tool and analyze some factors affecting working parameters based on actual drainage pressure and flow rate range, so as to provide a reference for the on-site tool selection and application. Results show that tool parameters including nozzle, nozzle-throat spacing, and throat may be designed by means of Sokolov ejector design method. Via a kind of Fluent fluid simulator, one model of gas-liquid two-phase flow field is established for the jet drainage tool to simulate and analyze the variation of ejector efficiency when the pressure of both ejecting and ejected fluid, and the outlet pressure of mixed fluid change. It can be optimized on the basis of these simulation results. And the optimized jet drainage technology has been applied to Fuling shale gasfield, resulting in a decrease of wellhead pressure with 25 %. What's more, the tested ejector efficiency is of better accordant with theoretical simulation.

For most tight low-permeability gas reservoirs in China, in order to develop an intensive technology for improving rate of penetration (ROP) and reducing cost, this study analyzed the feasibility of one intensive application of cluster well group and simplified hole structure by taking Daniudi gasfield in Ordos Basin as an objective. In addition, an intensive drilling technique of cluster-type slim hole was put forward and evaluated. Results show that (1) an application of cluster well group can save land acquisition and shorten moving cycle, and an operation mode of integral-trailing "factory like" is applicable to this cluster-type direction well group in Daniudi gasfield; (2) the cutting area of slim-hole bit and the casing size are reduced by 22.46%-23.53% and 18.18%-20.78%, respectively, conducive to ROP improvement and cost reduction; (3) according to the design, there are cone teeth in central position and back row of five-blade PDC bit, which may increase rock-breaking efficiency. An introduction of low-density (1.25 g/cm³) slurry system decreases cementing pressure by 5.5 MPa, so as to ensure one-trip upward cementing of slim hole; and (4) it was tested on site in 19 wells of 4 well groups. The excellent cementing ratio is 100%, the moving time and drilling cycle are individually shortened by 54.49% and 24.54% on average, and the ROP is increased by 15.04%. What's more, the expenses in waste, slurry and casing are reduced, and the expenses in land acquisition, pre-drilling, and moving are also cut down. In conclusion, this intensive drilling technique of cluster-type slim hole may not only remarkably promote ROP improvement and cost reduction for these tight low-permeability gas reservoirs in Daniudi gasfield, but also provide important reference on reducing drilling cost and improving efficiency for those similar tight gas reservoirs in China.

Sichuan Basin was taken an example to accurately predict medium and long term production, reasonably formulate planning scheme, and realize efficient management and development. After status and characteristics of peak prediction methods were summarized, three prediction models were selected, including single-peak Weng's model, single-peak Weibull model, and multi-peak Gauss model. Then, three models with modified algorithm were established to figure out the trend of medium and long term gas production in Sichuan Basin and carry out quantitative whole-life-cycle prediction. Results show that (1) gas production in Sichuan Basin represents a trend of fast increasing in the future 30 years; (2) the annual gas production in 2030 predicted by these three models will be in the range of 808×10⁸ m³~876×10⁸ m³, which is in accordance with gas development situation in Sichuan Basin. It's indicated that these prediction results are reliable; and (3) by modified Weng's model, the peak production predicted is 1453.28×10⁸ m³ and it will occur in 2047. By modified Weibull model, the peak production predicted is 1750.38×10⁸ m³ and it will occur in 2051. By modified multi-peak Gauss model, the peak production predicted is 1423×10⁸ m³ and it will occur in 2051. As a result, they are overall in line with development rules of Sichuan Basin and their predicted peak production results can be taken as the reference of production development trend.

In order to sufficiently and accurately grasp some change trend of H₂S content in the process of gasfield development and to provide the guidance to make measures for similar ones, this paper took one certain high-sulfur gasfield in northeastern Sichuan Basin as an example to carry out statistical analysis on the change trend in various structural positions. It's shown that H₂S content presents a rising trend in the process of field development, and its rising amplitude increases successively as the location of one gas well varies from the high to the middle and then the low. After that, some reasons arousing the change were explored. Finally, a method to quantitatively predict this change was developed. Results show that, (1) with a decrease of formation pressure in the process of development, some H₂S dissolved in formation water is partially desorbed and moves into a gas phase, which may increase its content; and (2) according to two theories of fluid phase equilibrium and material balance, the change trend are clarified and a mathematical model to calculate H₂S content is established. Furthermore, the change trend of this gasfield is predicted. It's indicated that H₂S content is 16% as formation pressure drops to 14 MPa. In conclusion, the change of H₂S content can be predicted accurately by virtue of the model, which is conducive to making anti-corrosive countermeasures effectively in time to avoid an adverse effect of H₂S on field production.

Two distinguished technologies of both drilling under pressure and frozen temporary plugging are commonly used to remove hidden troubles at wellhead. For most gas wells in Sichuan-Chongqing and Changqing areas, some hidden troubles can be removed successfully by means of either technology. However, troubles in many complex wells cannot be removed successfully by only one technology. So, this paper took X well deployed by PetroChina Southwest Oil & Gasfield Company as an example to analyze removal difficulties. And then several corresponding countermeasures were made. Results show that, (1) as for those wells without channels to inject frozen medium, the drilling under pressure can be adopted to establish injection channels; (2) when the frozen medium is injected through the established channels, it may fill in the whole wellhead devices. Therefore, after finishing this operation, some measures on environmental protection shall be made to prevent frozen medium from falling to ground or polluting environment after it is unfrozen; (3) for other wells with internal leakage at valves of Christmas tree, an actual injection volume of frozen medium is higher than a designed one, so the volume shall be increased so as to avoid freezing failure caused by medium leakage; and (4) to develop and successfully apply both drilling under pressure and frozen temporary plugging may provide tools for removing complex hidden troubles at wellhead devices.

In order to develop technologies of drainage gas recovery suitable for gas- and water-producing stage, this paper took the Quaternary unconsolidated sandstone gas reservoirs in Sebei gasfield, Qaidam Basin, as examples to analyzed field development status. Then, gas lift technologies were studied. And diversified supporting technologies of both single-well skid mounted intermittent gas lift and multi-well continuous centralized boosting gas lift were developed from aspects of lift mode selection, conduit flow model optimization, string size, lift parameter, lift equipment, and surface process. Results show that (1) these gas lift technologies have no mechanical parts in the wellbore process, so they are quite available for gas wells with sand and water production; (2) the skid mounted gas lift is of strong flexibility for drainage of gas wells with water-flooded wellbores. And the centralized boosting gas lift is highly automatic and can provide large-scale simultaneous drainage for multiple wells, so it's suitable for those gas wells with water-flooded payzones; and (3) for these two gas lift technologies, certain technical complementation may realize the full coverage of "point and plane" and satisfy the demand of water drainage, so they will be important water-control means during middle to late development stages.

When some technologies of drainage gas recovery are adopted to highly deviated horizontal wells in most gasfield, Sichuan-Chongqing area, both great burial depth of gas reservoirs and complex structure of downhole pipe string may lead to poor adaptability of foaming agent and difficult injection of solid foaming agent into these wells with water breakthrough; moreover, a large difference between threshold pressure of slotting gas lift and pressure of continuous gas injection may decrease an efficiency of ground equipments. So, a method for optimizing the drainage gas recovery technologies was put forward and its application was evaluated. Results show the optimization scheme is composed of two parts, including to develop a liquid foaming agent and optimize its injection mode by some experience on its application to vertical wells, and to introduce a gas lift design optimization method with formation imbibition index to achieve one target to not only increase an injection depth but also maximize a utilization of ground gas-injection equipments; and (2) field tests prove that the optimized foam drainage technologies can improve an effect of foam drainage remarkably, and the optimized deep-well gas lift design technologies with formation imbibition index can increase the lift efficiency, realize fast restoration of gas wells with water breakthrough, and reduce operation cost.

In order to increase dissolving rate of dissolvable bridge plug in shale-gas wells, reduce residuals, alleviate sticking risk during the drifting and plug drilling of coiled tubing, provide fullbore condition, and shorten well construction cycle, an anti-corrosive cosolvent was developed for dissolvable bridge plug by taking this plug which is used extensively in China and mainly composed of magnalium as a research object. The cosolvent chiefly consists of hydrochloric acid and Fe corrosion inhibitor. Results show that, (1) the anti-corrosive cosolvent for dissolvable bridge plug of 10% hydrochloric acid + 2% Fe corrosion inhibitor is selected; (2) this formula can increase the dissolving rate and provide an effective protection for coiled-tubing equipments, wellhead, and wellbore; and (3) a great number of field tests prove that this cosolvent can effectively increase the dissolving rate of dissolvable bridge plug in shale-gas wells and the drifting and plug drilling rate of coiled tubing, shorten the drifting cycle, alleviate the drifting risk, and speed up the commissioning, so as to improve economic benefits in shale-gas development.

For shale oil and shale gas, to predict their productivity is an important prerequisite for asset assessment and development scheme design. Due to extra-low porosity, low permeability, and multi-stage fracturing of shale reservoirs, however, those methods available for predicting conventional oil and gas productivity are not better suitable for them. In this study, many problems involved predicting shale-oil and shale-gas productivity were analyzed thoroughly, new ideas on this prediction were proposed, and prediction process was recommended. Results show that (1) the main reasons why conventional prediction methods are unsuitable for shale oil and shale gas include that, shale permeability is extremely low and cannot meet productivity test, i.e., pseudo steady state flow; decline laws particularly in the late stage are not made clear; to select a model among multiple prediction ones is difficult; no final conclusion on flow mechanism is made, all leading to great uncertainty in numerical simulation. (2) It's necessary to convert the ideas to predict shale-oil and shale-gas productivity from four aspects, i.e., mining production data and finding out relationships among production rate, and geological and engineering parameters, adopting many methods for comprehensive evaluation and complementary verification, paying attention to uncertainty productivity prediction methods to reduce assessment risk, strengthening production performance analysis, and carrying out reservoir and fracture parameter inversion. And (3) it's essential to establish a workflow to predict productivity for certain situations without production data or with various production data, and to carry out multiple-method comprehensive evaluation selectively so as to achieve one target to predict shale-oil and shale-gas productivity.

As an effective tool for completion and stimulation in Chuanyu shale-gas block, snubbing operation of gas wells has been gradually popularized and applied to other national shale-gas ones, e.g. PetroChina's Changning-Weiyuan block and Sinopec's Fuling block. However, snubbing operation is faced with some technical difficulties of annulus dynamic seal, string internal seal pressure control, and string bending. So, annulus dynamic seal, string internal seal scheme, and string bending resistance calculating formula were established by optimizing an assembly of annulus dynamic seal device, comparing both advantages and disadvantages of string internal seal tools, and building up one model to calculate bending resistance length limit of string. Then, some key technologies for snubbing operation of shale-gas wells were developed. Finally, an application effects of snubbing operation to 4 wells were evaluated. Results show that (1) when wellhead pressure is lower than 13.8 MPa, to run in and out the pipe string under pressure, it's necessary to equip with one annulus dynamic seal assembly and one brake-type dynamic seal assembly at least, and equipped with one annulus dynamic seal assembly and two brake-type dynamic seal assemblies at least while wellhead pressure higher than 13.8 MPa; (2) constant-pressure joint and ceramic plug are currently the most common internal seal tools for running in tubing under pressure. Constant-pressure joint is safe and reliable, and ceramic plug is good in sealing effect, so their collaboration presents the best effect; (3) the effective method to prevent string bending is to calculate the bending resistance length limit of string and ensure the cylinder stroke not exceeding this limit; and (4) the application of snubbing operation to 4 wells has better effect, presenting a remarkable stimulation result.

In Fuling shale-gas field, in order to ensure continuous and stable production for gas wells, Sinopec took the lead in carrying out some field experiments on foam drainage in Y block with low productivity and high to moderate water cut by means of certain foam drainage gas recovery technologies. After analyzing production situations of 4 shale-gas wells before and after an application of these technologies, the dosage of foam drainage agent was adjusted, optimized continuously until better effect was presented, and an expected aim was achieved, so as to provide guidance for their applicability to this block or even in the whole field. Results show that (1) these technologies can better improve shale-gas well production, and reduce water/gas ratio and tubing-casing pressure difference; (2) they can assist other measures to realize production restoration for those gas wells with serious liquid loading; (3) it's necessary to carry out another study on defoaming technology and chemical optimization experiments, and set up a defoaming mode suitable for Fuling gasfield; and (4) their application shall be based on company's development concept to establish technical ideas with “accurate link” and set up a standard process.

In order to full understand both accumulation laws and distribution characteristics of remaining gas in Sebei gasfield and provide the guarantee for continuous and stable production in the late development stage, we quantitatively characterized the remaining gas in III-3 Member of this field by means of numerical simulation, and provided potential tapping direction and adjustment ideas for effective development. Results show that (1) the multiple of waterbody in III-3 Member is 30; (2) replacing the displacement relative permeability with the imbibition relative permeability may make numerical simulation models more accurate and reliable; (3) remaining-gas distribution is related to many factors, e.g. original reserve abundance, structural location, development degree, and water invasion degree; (4) remaining-gas reserves can be subdivided into four evaluation classes by combining reserve abundance with water saturation; and (5) according to evaluation results, different adjustment schemes are developed for remaining-gas-accumulated areas. Based on the selected scheme, two new wells are firstly treated, and their commissioning effects are better. In conclusion, the continuous and stable production of Sebei gasfield is inseparable from the potential tapping of remaining oil. The proposed method for quantitative characterization of remaining gas may provide new ideas for its late development stage.

It is found that all methods to evaluate the deliverability of gas wells have their own limitations. The binomial productivity equation, which is especially used to evaluate condensate, high temperature and high pressure, and water-producing gas wells, emerges abnormal. Even for most conventional wells, the productivity equation is also unusually. The reasons for abnormality are complicated. However, the fundamental ones are downhole flowing pressure and an exponent in productivity equation of m. Based on previous study, one formula to calculate additional pressure drop caused by skin factor is inferred from fluid percolation; and some errors in a downhole flowing pressure test are amended by reference of the additional pressure drop and by evidence of the productivity equation. Meanwhile, based on the Reynolds number, the influencing factor of turbulence of λ is presented. And a relevant equation is regressed according to the relationship between λ and m. In fact, m can be easily calculated by adopting λ. It is proven that this method is scientific and reasonable.

In Qaidam Basin, Sebei gasfield belongs to the Quaternary unconsolidated sandstone gas reservoirs, which are composed of cementing and loose rocks easily bringing about sand production. Along with a further development, there are some problems affecting not only a normal production of gas wells but also an efficient development of gas field, containing plenty of sand production, and much water yield even increasing sand production. After long research on Sebei gasfield, the sand-producing mechanisms were made clear and a technology of frac-packing sand control by slotted liner formed. This technology is characterized by simple installation, and a one-trip string is able to implement three procedures of landing sand-control tools, packing, and releasing; moreover, multiple-stage sand barriers of both sand control slotted liner and gravels may effectively stop sand producing from layers; in addition, it may reform those pay zones with poor petrophysical property and increase their flowing capacity. By the end of 2015, there were 47 gas wells implementing frac-packing sand control by slotted liner, and the efficiency reached 83.0 % and the longest effective time was more than 5 a. Generally, the input-output ratio achieves 1:7.5.

In Daniudi gasfield, there are some problems on drainage gas recovery by pre-prepared string. So, we carried out the theoretical study and experiments on velocity-string drainage. And some mature experiences formed, including well selection principles, tubing optimization, parameter design, and industrial drainage process. From 2013 to 2015, there were totally 151 wells implemented velocity-string drainage gas recovery and their success ratio touched more than 98 %. After running velocity string to gas wells, a new connection mode between casing and tubing was rebuilt to fulfill a compound drainage and recovery. In result, the average production efficiency increased from 86.1 % to 96.8 %, and the average water-gas ratio rose to 1.93 m³/10⁴ m³ from 1.73 m³/10⁴ m³, achieving an obvious drainage effect. The field application also shows that this velocity-string drainage may not only decrease the critical liquid-carrying flow rate but also enhance the flowing capacity. In addition, it is featured by no killing operation, short period of drainage operation, and no-pollution payzone.

In western Sichuan Basin, the COD（chemical oxygen demand）of fracturing flowback fluid touched 7 968 mg/L, which is too high to come up the standard of treatment and sewage. So, after an analysis on its affecting factors, it is deemed that the concentration of thickeners is a deciding one. Through the optimization of both thickeners and different additives, the synthesis of multi-hydroxyl complexation crosslinkers, and the research on self-suspending proppant, three fracturing fluids formed, including two guar gum ones with low concentration or with extra-low concentration, and fresh water one, which made COD gradually decrease to 5 599 mg/L, 5 261 mg/L, and 1 250 mg/L, respectively. Then, after an application of both two-stage oxidation and treatment by microwave technology, the COD further decreased to 79 mg/L, less than 100 mg/L newly issued in the Law of Environmental Protection. As a result, this COD comes up the standard for sewage of fracturing flowback fluid.

For gas reservoirs, the length of horizontal section is one of the most important technological indexes in horizontal-well development. Previous study mostly focused on the validity of both open flow capacity and cumulative gas production. But now, under the tendency of prolonged and low oil and gas price, economical benefits should be taken into consideration in a design of horizontal section. Taking Cn gas reservoirs in offshore Country A as examples and also serving numerical simulation as a prediction tool, we first developed the numerical simulation of production profile for different lengths of horizontal section, and then established the model considering different drilling lengths to evaluate economic benefits of single well; thirdly analyzed the net present value（NPV）under different horizontal-section lengths; finally, proposed that the optimum and economical horizontal-section length should vary from 200 m to 250 m.

Downhole gas-liquid separation becomes more and more important in further exploitation of oilfields. At present, a separator called column gas-liquid cylindrical cyclone is hugely popular in oil and gas industry due to its simple technical process and low manufacture cost. In this study, this typical separator was applied to numerical simulation; moreover, a 3D turbulence model based on another mixture model was also applied to describing mixture flowing within the separator. The gas-liquid separating efficiency under various parameters including length and outlet diameter of separator was obtained from this numerical simulation. With an increase of the length, the efficiency decreased. And along with an enlargement of outlet diameter, the efficiency at first increased and then fell. For inlet shape, the rectangle was more suitable than the round, and the efficiency increased from 66.45 % to 79.04 %. Finally, an optimum geometry was proposed for the separator; as a result, the gas-liquid separating efficiency reached 86.15 %.

In Longnvsi structure, only a few wells obtained high-yield commercial gas from Longwangmiao Formation. Moreover, an overall effect of reservoir stimulation was not apparent. So, it’s very critical to develop and optimize more effective stimulation technologies. Some factors impacting on the stimulation effect were analyzed on the basis of geological features and theory of reservoir stimulation. And some stimulation technologies available for these features were made clear. Results show that (1) fracture growth is the key factor influencing on the stimulation effect of Longwangmiao reservoirs; (2) among four stimulation technologies containing gelled acid fracturing, diversion of acid fracturing, gelled + weighted acid fracturing, and acidizing + sanding fracturing, the second one is the most suitable for Longwangmiao Formation; and (3) the most effective stimulation measures in the next exploration and development include implementing technologies of high-angle or horizontal wells, and adopting diversion of acid fracturing.

Tubing conveyed hydraulic jet with seal drag, as one technology of stimulated reservoir volume fracturing, is often used for horizontal wells. However, there are many problems during its site use, including low efficiency, sticking, and blockage. So, in order to find out safer and more reliable fracturing way for horizontal wells, we carried out the study and experiment on tubing conveyed hydraulic jet sand-packed fracturing. Results show that the hydraulic jet sand-packed fracturing is feasible because sand-packed sealing may substitute packer to realize an effective packoff between intervals. In addition, it is demonstrated from field test that the mode of casing annular sand-packed is featured by safety operation, and high efficiency.

In some gas-gathering stations, there are some problems which may bring about much difficulty in well production, such as low rate of mixed transmission between high- and low-pressure gas wells, and input pressure close to pipeline pressure. So, an experiment on jet drainage mix transmission was implemented for high- and low-pressure wells in Daniudi gasfield at first time. For low-pressure wells, the input pressure decreased to 1 Mpa very successfully, less 1.9 Mpa than pipeline pressure, and realized a negative-pressure production of lower than pipeline pressure. Meanwhile, for these low-pressure wells, an increase of both flow and liquid-carrying capability results in an improvement of liquid-drained effect. This technology, obtained an experimental success, provides a new view to solve these mentioned-above problems.

SEM is often used for reservoir study to obtain high-resolution rock structure features, mineral type and occurrence, reservoir space attribute, and other diagenesis characteristics. It is featured by high magnification and long depth of field. Combined SEM with spectrum analysis, we can not only observe microscopic features of rock samples but also analyze qualitatively and semi-quantitatively different components among these samples. So, we carried out a study on three items for clastic-rock and carbonate-rock reservoirs, including, to identify mineral and to calculation its content by combining backscatter and image analysis software, to study the features of porous structure by combining backscatter and image analysis software, and to identify mineral, to calculation its content, and to make its distribution clear by integrating spectrum surface scanning with image analysis software.

Tortuosity is an important parameter to describe a bending deformation degree of seepage path flowing in porous media for fluids. However, it’s more difficult to accurately determine this parameter. Two methods of both electrical conductivity and molecular diffusion are widely used for tortuosity calculation in lab. Molecular diffusion is more suitable for development of gasfield because tortuosity obtained from electrical conductivity method is a debatable point. For molecular diffusion, the testing theory points out that tortuosity reflects complex attributes of porous media structure whereas the ratio of self-diffusion coefficient vs. effective diffusion coefficient characterizes an obstruction effect of porous media on gas diffusion; and tortuosity has a necessarily close connection with the ratio. In this study, based on a method of pressure attenuation to calculate diffusion coefficient from reference, and we improved this calculation equation, such as adopting oil sand saturated with crude oil to simulate sandstone reservoirs at first, and then measuring the diffusion coefficient of CH4 in oil and the effective diffusion coefficient of oil sand with different permeability and porosity, thirdly analyzing the relationship among diffusion coefficient, effective diffusion coefficient, porosity, and tortuosity, finally calculating tortuosity of oil sand. Results show that the effective coefficient of CH₄ positively correlates with both porosity and permeability whereas tortuosity negatively correlates with permeability and porosity.

Yakela condensate gasfield is a principal gas source for Sinopec Northwest Company implementing the West-East Gas Pipeline Project. Since its development from 2005, this field has maintained a stable production for over ten years. However, there are some problems accompanying with its further development, especially serious water invasion resulting in a largely decreasing productivity. So, some measures, including to monitor water production, to make an adjustment while abnormal water production, and to prolong flowing period to the utmost, are made to keep the stable production and increase the recovery for gas wells.

There are many coal seams developed in Shouyang block, Qinshui Basin. Among the main ones of No. 3, No. 9 and No. 15, the last is the dominantly developed one at present. In order to achieve the aim of cutting the cost to increase the profit, a technology of multiple-zone production in CBM reservoirs was used. In addition, integrated previous exploration acknowledge and actual production, some factors affecting multiple-zone production and the feasibility were discussed. Results show that (1) in this block, the reservoir pressure, pressure gradient, desorption characteristic parameter, original permeability are heterogeneous; (2) a large-scale multiple-zone production is not available for this block; and (3) the multiple-zone production can be only selected for some coal wells with better conditions after full assessment, however, we should emphasize fracturing and controlling production pressure.

For many gas wells with higher water production in western Sichuan Basin, a kind of velocity string with ∅50.8 mm diameter has been used for drainage. However, another common-used string of ∅38.1 mm diameter is just a little larger not available for these wells because it may in advance weaken the liquid-carrying capacity when production declines. Moreover, for those wells with higher gas production, it brings about some degree of yield reduction, which is not conducive to a stable production. So, through the gas-liquid two-phase pressure-drop model and the IPR inflow performance analysis, an optimization of gas production passage is implemented for the wells with higher gas and water production based on the effect of small-diameter string, annulus pressure loss, and critical flow capacity on the gas production; and a new idea on conversion of gas production passage by stages is presented according to different production features. Meanwhile, if small-diameter string is used in these wells, gas production will decline and fall degree is relative to production features.

In gas reservoirs of Longwangmiao Formation, pressure relief and blowout pipeline should be installed at wellhead. But during this installation, thread nipples cannot been manufactured on the spot. Moreover, it's difficult to connect these nipples, resulting in some problems of long installation time and low efficiency. So, a simulation platform of adjustable wellhead was developed. It can finish an indoor process, and realize the factory prefabrication, integration installation, and standardized construction. This paper illustrates the platform laws, and the flow process in terms of platform structure, development method, and platform parameter. In addition, Though this platform, the locations of throttle valve, casing valve, and technical casing valve and surface casing valve are simulated. One connection of both throttle and casing valves and another connection of both technical and surface casing valves are installed on the spot. Consequently, operation period shortens and working efficiency increases.