Based on the summary of deep shale gas breakthrough wells in recent years, four geological characteristics of deep shale gas are put forward. First, the basic evaluation parameters of deep and shallow shale are similar, but gas content and porosity of deep shale are generally higher than those of medium-deep shale. Second, the horizontal stress difference of deep shale is much greater than that of medium-deep shale. Third, deep shale gas in the basin is generally overpressured, and the pressure coefficient is generally between 1.9 and 2.1, while the complex structure area at the edge of the basin is normally pressured. Fourth, the positive structure is still the main factor for high yield of deep shale gas. Based on the analysis of the decline characteristics of single well production curve in Haynesville and Barnett shale gas fields and the geological characteristics of deep shale gas in Sichuan Basin, three main problems of deep shale gas, namely theoretical understanding innovation, engineering process applicability as well as cost and benefit development. The corresponding countermeasures are also pointed out.

The starting pressure gradient and pressure sensitivity effect proposed by Bear（1972） and Fatt（1952） respectively have practical significance for the development of low permeability oil and gas reservoirs. The so-called starting pressure gradient refers to the critical pressure gradient that causes the oil and gas formations to start flowing. The so-called pressure-sensitive effect includes the phenomenon that the formation permeability decreases as the formation pressure drops. Due to the theoretical needs of tight and low-permeability reservoir development analysis in the Ordos Basin, relevant experts and scholars in China have paid attention to the starting pressure gradient and pressure sensitivity effect for nearly 20 years, but their understanding is still in the initial perceptual stage. The reason is the lack of basic derivation. The derivation in this paper shows that the pressure gradient and starting pressure gradient of Darcy linear flow are constant, but the pressure gradient and starting pressure gradient of Darcy plane radial flow both are functions of radial radius. It is incorrect to directly apply the constant starting pressure gradient of linear flow to the plane radial flow equation. Although the pressure sensitivity effect of permeability exists, it can never be used in the flow equation of Darcy’s law. Because constant permeability is the basis for the establishment of Darcy’s law. Otherwise, it will shake the theoretical foundations of Ground Fluid Dynamics, Petroleum Reservoir Engineering and Reservoir Numerical Simulation.

The exploration of the lower Cambrian Qiongzhusi Formation in southern Sichuan Basin shows that the shale is thin with multiple strata series, which restricts the effects of testing and pilot production. In order to further implement the favorable exploration targets and realize the benefit development of the Cambrian Qiongzhusi Formation, systematic coring, core observation and experimental analysis have been carried out. The results show that: ①Shale,siltstone（or argillaceous siltstone）, silty shale and other lithology develop in the lower Cambrian Qiongzhusi Formation in southern Sichuan Basin. Shale is relatively thin with longitudinal discontinuous deposition. ②The thermal evolution degree of shale is too high. The reservoir spaces are mainly inorganic pores, meanwhile, inorganic pores are also dominant in silty reservoir section. ③The reservoir quality of various types of lithologic combination section are similar, and they all have storage capacity. ④Drilling reveals that the siltstone-bearing shale in the middle of Qiongzhusi formation is thick with good physical and gas-bearing property, which is the favorable target interval. The research and understanding is of great significance to broaden the exploration ideas in this area.

In the “dual complex” area of Xindianzi, Yongchuan, all the reasons that the near surface factors vary greatly, the structure is complex, and the signal-to-noise ratio of data is low make the accurate imaging difficult. During the shale gas exploration and development, there are some problems of Xindianzi such as unclear anticline structure, inaccurate imaging of local small faults, and inconsistency of horizontal well design trajectory with actual drilling. The conventional grid tomography inversion method has poor applicability in velocity modeling of low signal-to-noise ratio area. It is difficult not only to obtain the quasi shallow velocity and eliminate the influence of complex near surface on imaging, but also to realize effective data driving in the middle and deep layers and update the velocity accurately in the middle and deep layers. In order to solve the problem of depth domain modeling in “dual complex” area, full offset first break tomography inversion is used to obtain near surface velocity, then the appropriate interface is selected to fuse the models of shallow and medium depth. In the iterative process of grid tomography, the constraints on velocity by the construction model can realize the corresponding between low-frequency velocity and structural distribution, and get high-frequency convergence. The application results show that the joint application of shallow, medium and deep fusion modeling and structural constraint grid tomography technology make up for the shortcomings of the conventional methods, the resulting final velocity field is more accurate, and the imaging of the “dual complex” area is obviously improved. The results strongly support the exploration and development of this area.

After years of exploration and development integration practice, some achievements have been made in Yongchuan Deep Shale Gas Field. However, there is a big difference in productivity of gas well tests in different structural areas, restricting the scale of reserve increment and benefit development. In order to realize a high-yield and enrichment zone in this area and make clear the direction of exploration and development, the key factors of shale gas enrichment in the deep layer of this area, including thick and stable high-quality shale reservoir, favorable structural style, small scale faults without crush, relatively developed natural fractures, relatively large buried depth and good sealing capacity of roof and floor, are summarized through detailed characterization of structural styles and fault distribution rules, detailed description of reservoir characteristics, and analysis of formation pressure characteristics and production rules. In general, this area has high gas abundance and good resource scale.

The lithology, physical property, gas bearing property and other characteristics of shale gas reservoir are obviously different from those of conventional oil and gas reservoir, so the traditional conventional logging technology and the method of log interpretation can not be fully applicable. According to the geological characteristics and logging technology series of Weirong Deep Shale Gas Field, and based on the research on the logging response characteristics and “six-property” relation of the Longmaxi Formation shale reservoir, using core calibration log technique and petrophysical optimization volume model and regression analysis to carry out the research on the logging calculation method of shale reservoir geological and engineering parameters, form the calculation method of shale gas reservoir key parameters and establish the logging identification standard of shale reservoirs. The application shows that the proposed logging evaluation method has achieved good results in the evaluation of Weirong Deep Shale Reservoir.

Significant progress has been made in the exploration and development of shale gas in the Wufeng-Longmaxi Formation of Sichuan Basin, but shale gas content varies greatly among different well locations. Through the analysis of formation water salinity, geochemical index and formation water type in Well-PY1HF, Well-JY1HF, Well-NY1HF, Well-LY1 and other wells in Southeastern Chongqing, it is found that CaCl₂ water type reflects good shale gas preservation conditions, while the water types of NaHCO₃ and Na₂SO₄ reflect formation water and atmospheric water are interconnected to different extent, leading to a relatively poor preservation condition. Based on the characteristics of large-scale faults and residual syncline structures developing in Southeastern Chongqing and the formation water characteristics of Wufeng-Longmaxi Formation, the groundwater seepage mode can be divided into two types: the infiltration type in the large-scale fault development area and the centripetal flow type in the residual syncline area, corresponding to different shale gas preservation conditions. In the area with large-scale faults, the surface water infiltrates deeply along the faults, causing great damage to shale gas reservoir, most of them contain few gases. The formation water in the residual syncline is centripetal flow, and the deep part of the syncline is relatively well preserved. The surface water and the formation water form a free alternating zone at the syncline edge, and the preservation condition of shale gas reservoir is poor, thus the gas bearing property is poor. In the core part of the syncline, the closer to the core part, the weaker the influence of atmospheric water infiltration is, and the better the preservation condition will be.

At present, China’s shale gas is gradually began to change from exploration breakthrough to development and production. The rate of decline of shale gas production mainly depends on the desorption of the adsorbed gas in shale. Therefore, it is of great significance to study the rule of methane desorption in shale for the development of shale gas. In order to study the isothermal desorption characteristics of methane in shale from Longmaxi Formation in South Sichuan Basin, the high-pressure isothermal adsorption and desorption experiments have been carried out with the shale of this formation as the research object. On this basis, the optimized model of methane desorption and the calculated desorption efficiency curve of methane in shale has been used to analyze the desorption characteristics of shale from Longmaxi Formation. The results show that the fitting effects of five models have little difference with low experimental pressure（less than 12 MPa）. While under high pressure, Weibull model and D-A model can describe the methane desorption process better than other models. Based on the experimental test results, the model fitting effect is as follows: Weibull model>D-A model>Desorption model>Langmuir model>Freundlich model. With the decrease of pressure, the shale methane desorption efficiency obtained by Weibull model with the best fitting accuracy tends to increase slowly at first and then rapidly. According to the characteristic pressure points, the methane desorption process is divided into four stages. The methane desorption amount in the rapid desorption stage accounted for 40 % of the total desorption amount, which contributes the most to shale gas production capacity.

The types of micro-pores, pore structures and fractal characteristics of shale have been analyzed for the shale reservoir of Wufeng-Longmaxi Formation in Southeastern Chongqing by field emission scanning electron microscope（FESEM）, low temperature liquid nitrogen adsorption and overpressure permeability porosity test. It is found that the macroscopic fracture types are mainly foliated fractures and structural fractures, and most of the fractures in the samples along the basin are arranged in an echelon series, while the fractures in the atmospheric pressure shale gas well samples outside the basin are relatively straight. The positions where the macroscopic fractures develop most are also the places where the microfractures develop well. Usually, it has high gas content and permeability. There are obvious differences between the occurrence characteristics of pore fractures in deep shale and relatively shallow shale. The fractures in the deep samples are less developed compared with those in the shallow samples, and the microfractures are mainly shrinkage and dissolution fractures. The pore type, shape and structure of the shallow samples have similar longitudinal distribution characteristics. The average pore diameter and overburden porosity of shallow samples are obviously larger than that of deep samples. Quartz and clay have a certain influence on the development of micropores and mesopores. The microscopic type is an important influence factor on the development of organic pores. When the burial depth is greater than 3 500 m, depth is the main influence factor on pore morphology and pore size distribution. The deep shale undergoes more complex diagenesis, the pores undergo intense late transformation, and the pore structure is no longer dominated by organic matter. It is difficult to form fracture network when fracturing the deep shale with such pore fracture occurrence characteristics, so further research on the formation mechanism of deep shale fracture network is needed. Through the observation of fractures with different scales, the layer easy to fracture can be quickly determined. Even though such fractures account for a very small proportion of in-situ pore volume and have no effect on the permeability of the reservoir, such pore groups can be stimulated because of fracturing and serve as the main seepage channels of shale reservoir.

Yongchuan deep mountain shale gas block is located in Huayingshan fold belt, which is a low anticline group with broom like spreading to the south. The structural folds are strong and the faults are developed. The target layer of Longmaxi Formation has the characteristics of great difference in burial depth, high ground pressure and ground temperature and large difference in horizontal stress, while the crossing layer has strong abrasiveness, easy instability of well wall and poor drillability. The lack of early geological knowledge leads to low drilling time, long construction period and frequent complex situations, and the fracturing effect of multiple wells is not ideal. In order to solve the main problems faced by Yongchuan work area, researches on the geomechanical parameters, such as drillability, hardness, abrasiveness laboratory experiments, drillability profile establishment, formation three pressure prediction, 3D in-situ stress finite element, simulation and fracture propagation simulation under different horizontal stress difference modes, and horizontal well geological engineering double sweet spot prediction, have been conducted respectively. The results show that the formation drillability level is high, the safe drilling fluid density window is narrow, the difference of in-situ stress direction is large, and the formation of hydraulic fracturing network is affected by high stress difference. Based on the research results of geomechanics, the engineering technology has been adjusted. The drilling speed and fracturing production increase effect are obvious, which has reference significance for the development of deep shale gas in South Sichuan.

Due to the large natural fractures and the single extension of hydraulic fractures, gas channeling occurs easily during fracturing of multi-platform, as a result the instantaneous gas production of adjacent wells is reduced by at most 93 %, and the wellhead pressure is increased by at most 12 MPa, which seriously affects the fracturing development effect of shale gas. For this complex situation, a fracture control and channeling prevention technology for horizontal wells of shale gas is proposed, which mainly includes fracture-length control of multiple fracture and steering fracture. By the measures of increasing the number of hydraulic fractures to reduce the net pressure, or using temporary plugging materials to steer hydraulic fractures, the extension direction of hydraulic fractures has been controlled and the fracture complexity has been increased to reduce the interacting of adjacent wells, and finally make the shale gas effectively develops in the well controlled gas drainage area. The numerical simulation shows that after applying this technology, the effective fracture length is shortened by 11.9 %~24.8 % The field application effect is obvious, the fracture length monitored by real-time micro seismic is reduced by 24 %, and the real-time monitoring pressure of adjacent wells does not change. The fracture control and channeling prevention technology in horizontal wells of shale gas does not only provides theoretical support for field application, but also reduces the probability of complex situations and improves the production of single well.

The successful development experience at home and abroad shows that fracturing is an important step to realize shale gas development, and testing selection is the most critical step in the preliminary work of fracturing. The quality of the selection determines the effect of fracturing. Taking Weirong Shale Gas Field platform wells as the research object, according to the characteristics of the basic data of the early evaluation wells and the later development wells, and combined with the principle of testing selection, the later development well selection ideas are deepened. And the development well data is rationally utilized from four aspects, which are the trajectory, geology, engineering and fracture prediction, forming a geological-engineering integration technology of shale gas horizontal well testing selection in “five steps” of trajectory evaluation, geological parameter evaluation, engineering parameter evaluation, fracture prediction, and comprehensive segmentation. The test open flow rate of development wells obtained by this technology is increased by about 1~2 times compared with the previous evaluation wells, saving 40 % of the project time. Field tests show that this technology has certain guiding significance for improving the beneficial development of deep shale gas.

The shale gas fields in Southern Sichuan are developed in an integration mode of testing, production and transmission, which has realized the development goals of cost reduction, emission reduction, fast production commissioning and early returns. However, a lot of empirical practices serve as reference of decision-making during implementation of integration practices. There is a lack of general guidance. In order to study the post-frac soaking, the flowback system and the drainage measures, the experiments of overpressured NMR imbibition, permeability stress sensitivity and gas-liquid two phase percolation are conducted, the flow regime and the distribution of pressure profile are simulated, and the discharge and production effect of more than 30 wells in Southern Sichuan shale gas field has been evaluated. The results show that the entry of fracturing fluids into reservoir through imbibition is beneficial to increasing the complexity of shale cracks, and the optimal shut-in time of Southern Sichuan shale gas field is 4~10 days. Meanwhile, a six-staged flowback system is formed, and a post-frac drainage guide chart is established. It is determined that tubing and manual lifting should be implemented when the flow regime changes and the tube should be installed at the well deviation of 70° ~ 85°. Besides, the drainage strategy is also proposed. In general, the research results are of great significance for guiding post-frac production control and drainage technology.

Due to the deep buried depth（3 500~4 200 m）, high ground stress, high ground stress discrepancy（7 to 17 MPa）, low reservoir brittle（< 0.5） and the undeveloped natural fracture, the hydraulic fracture of Weirong deep shale gas face the problems of high fracturing construction pressure, narrower pressure window, low sensitive sand concentration, high fracturing difficulty. Large-scale physical model experiments show that the morphology of Weirong shale fractures are composed of main fracture and branch fracture, within low fracture complexity and forming bedding seam more easily. On the basis of geology-engineering integration, the stratigraphic segmentation and clustering are optimized in combination with geological sweet spot. Through the study of the proppant transport, the placement mode and injection timing of the three-grade particle size proppant have been optimized, which increase the sand loading. The transverse complexity of fractures is improved by the combined temporary plugging steering fracturing technology. The net pressure and complexity of fractures are improved by the temporary plugging in the fractures and the optimization of construction discharge and liquid viscosity, thereby improving the fracturing volume and control reserves. The research results have been successfully applied in Weirong Gas Field. The sand loading has been increased to 1.95 t/m, the average open flow per well is 38.5×10⁴ m³/d, and the single well EUR is 90×10⁸ m³. All those shows a significantly improvement compared with the previous stage. Post-pressure evaluation shows that the fracturing effect is positively correlated with the sand adding strength. Therefore, how to improve the sand adding strength and control the strength of the liquid used in deep shale gas is the key to economic and effective fracturing.

The potential impact of shale gas development on regional environmental quality include the risk of groundwater pollution during hydraulic fracturing, the risk of surface water pollution due to the backflow drainage, and the influences of methane leakage on regional atmospheric environmental quality. The way of tracking, monitoring and evaluating the environmental quality of shale gas production areas is effective to confirm the effects of pollution prevention measures and understand the long-term cumulative environmental impact. The pollution prevention measures in Nanchuan shale gas production area have been summarized. The groundwater environment, surface water environment, atmospheric environment, acoustic environment, and soil environment in the production area have been monitored. According to the most unfavorable principle, the single factor indexes with the largest monitoring value have been used to evaluate the current environmental quality, and the inter-annual changes of the environmental quality of each element has been assessd. The results show that the monitoring factors of surface water environmental quality, groundwater environmental quality, atmospheric environmental quality, acoustic environmental quality, and soil environmental quality in Nanchuan shale gas production area meet the environmental quality standards of the corresponding functional area. There is no significant inter-annual changes in the evaluation indexes of each monitoring factor, especially the characteristic pollution factor of shale gas development such as chloride. The regional shale gas development has not caused pollution of various environmental elements and significant adverse changes in environmental quality. Although the current pollution prevention measures are effective, the continuous and cumulative environmental impacts still require longer-term follow-up monitoring and evaluation.

The geometry size of fractures and the time of phase change in chemical fracturing by self-generated solid are closely related to the temperature field of cracks. Based on the pseudo-three-dimensional crack propagation model, a coupled temperature field model of the cracks is established. And its coupling solution is carried out. This model takes into account the variation of viscosity of self-generated solid chemical fracturing fluids with temperature, and applies homogeneous treatment to two-phase fracturing fluids. The practical calculation results show that mutual influence among temperature field, viscosity of fracturing fluid system and fracture geometry. The increase of non-phase-change fracturing fluid accumulation with low specific heat capacity and high heat conduction coefficient is conducive to the improvement of the temperature in the cracks and the rapid phase-change of phase-change fracturing fluid to form stable support. According to the temperature distribution in the cracks, the fracturing fluid system with different phase change temperature can be selected to achieve the purpose of “phase transition in a short time and with effective support”. The research result helps to improve the pertinence of self-generated solid chemical fracturing working fluid system and operation technology design.

At present, there is no particularly effective method to predict the degree of production at the end of the stable production period of gas reservoirs with active edge water. The production often declines rapidly without warning. Based on the physical model of long core of gas reservoirs by water flooding in Yakela area, the characteristics of gas-water two-phase seepage during water invasion are simulated by the experiment of gas recovery by water flooding. Taking the material balance equation of water flooding as the theoretical method, the concept of water invasion coefficient is introduced to describe the water invasion characteristics of gas reservoirs. As the hydrocarbon pore volume（HPV） of the injected water is consistent with the concept of water invasion coefficient, it is feasible to simulate the effect of water invasion on gas reservoir by water flooding experiments. The experimental results show that when the injected water reached 0.3 ~ 0.45 times of HPV, the waterflood front breaks through, and the gas production enters the rapid decline period from the stable production period. The production history shows that when the water invasion coefficients of the middle-lower gas reservoir reach 0.33 ~ 0.36, the gas production decreases rapidly, and the experiment is basically consistent with the production practice. By calculating the water invasion coefficient of upper gas reservoir, when the water invasion coefficients reach 0.33 ~ 0.36 from August 2019 to February 2020, the end of stable production period will come. It is basically consistent with the results of numerical simulation and necessary to adjust the high risk wells in time.

Sidetracking horizontal wells can be used to control reserves, digging potentials and longitudinal residual oil and gas reservoirs. The researches at home and broad show that both restart production of long-time shutdown wells and increasing oil and gas production in low production wells need to imply sidetracking horizontal wells. The Sulige Gas Field, located in the Ordos Basin, is a giant gas field with reserves exceeded trillion, and in there an annual production capacity of 249×10⁸ m³ has been built. However, its single well yield is low, the rate of decline is fast, and the number of wells with daily production less than 3 000 m³ accounts for 50 %. To explore side drilling horizontal wells and to improve gas recovery are the technical directions to achieve the steady production, cost reduction and efficiency increase in development of Sulige Gas Field. In view of the difficulties of sidetracking horizontal wells with small holes in the research block, the researches are carried out on safe drilling, completion technology and economic benefit. Twelve sidetracking horizontal wells are tested in site, with an average horizontal segment length of 672 m and an average drilling cycle of 49 d. The initial average daily gas production of a single well after sidetracking was 2.7×10⁴ m³. The practice shows that it is technically feasible for the casing sidetracking horizontal well to improve gas recovery of low production and low efficiency wells in Sulige Gas Field.