Deep shale gas（buried depth is larger than 3 500 m） is the potential resource for future exploration in Sichuan Basin. Although the industrial shale gas flows have been obtained at the depth of 3 500~4 000 m in Wufeng-Longmaxi Formation of Sichuan Basin, the commercial development hasn’t been put into practice due to the rapid decline and the low EUR（Estimated Ultimate Recovery）. Based on the analysis of the current status of shale gas exploration and development, the challenges in the deep shale gas development with high efficiency and large scale in Sichuan Basin have been summarized, mainly in the following aspects: the understanding of occurrence mechanism and enrichment law of deep shale gas needs to be deepened, engineering and technology of economical and effective fracturing treatment need to be established, and the organizational operations and management methods of deep shale gas development are difficult to meet the needs of the large-scale and high efficient development. Three countermeasures are proposed to realize the large-scale and highly efficient development of deep shale gas: ①deepening the understanding of deep shale gas enrichment laws, establishing the methods of area selection and evaluation, and forming the prediction and description technologies of “sweet spot” and “sweet window”; ②deepening the research on the geological conditions of deep shale gas, forming an advanced supporting technology of drilling and fracturing and an equipment system to fully release the reservoir capacity; ③promoting the geology-engineering integration, building a new system and mechanism, and greatly reducing the cost to maximize the development benefits of deep shale gas. The industrial gas flows have been obtained in several wells at the depth of more than 3 500 m of Wufeng-Longmaxi Formation in Sichuan Basin and the proven reserves have been submitted. It is the key and priority stratum of the deep shale gas development. By deepening the geological understanding, overcoming the key technical problems, and improving the management system, it can significantly accelerate the speed, reduce the cost, increase the efficiency and achieve a large-scale and highly efficient development in a relatively short period. The output is expected to be higher than that of the middle and shallow shale gas reservoir.

As one of the important economic indicators for the development of shale/tight reservoirs, the production performance and EUR calculation of shale/tight wells is an important subject in unconventional reservoir engineering. This has been challenging for the reservoir engineers from both China and other country for a dozen of years. Based on the understanding of the unique geological characteristics of shale/tight reservoirs, this paper describes in detail the production performance of those wells producing extremely tight reservoirs, from which the inapplicability of using traditional methods has been explained. Further, a new methodology and the workflow has been presented from SPEE recommendations, and one practical example has also been illustrated of implementing the workflow.

In order to study the evaluation standard of deep shale reservoirs, their micro pore characteristics have been studied. Based on the ideas of micro pore structure research with “high precision and cross scale”, and by using nitrogen adsorption, high pressure mercury injection, scanning electron microscope and mineral analysis electron microscope experiments, a series of quantitative characterization techniques of reservoir micro pore structure with full pore size range are formed, and the evaluation standard of deep shale gas reservoir is established. The research results show that: ①The pore types include organic pore, intergranular pore, intragranular pore and microfracture, which are of great difference with each other. The organic pores are dominant with the surface porosity more than 50 %, the pore morphology is complex with the distribution of elliptic, irregular or flat, the shape coefficient is 0.50~0.90, and the fractal dimension（D） is 2.72~2.92. ②Based on the full pore size characterization technology and fractal theory, the pore structure can be divided into four types. The organic pores of type Ⅰ pore structure are developed （organic pore ratio Is greater than or equal to 50 %）, well sorted （sorting coefficient Is greater than or equal to 0.7）, large coefficient of variation （variation coefficient Is greater than or equal to 1.1）, and multi-modal distribution of pore size. ③According to the classification of pore structure and reservoir parameters, shale reservoirs are divided into four types. The key parameters make up the structure of type Ⅰ are TOC≥4 %, total gas content Is greater than or equal to 6 m³/t, porosity Is greater than or equal to 6 %, brittle mineral content Is greater than or equal to 50 %, Young’s modulus Is greater than or equal to 36 GPa, Poisson’s ratio Is less than or equal to 0.225, and type Ⅰ pore structure. This kind of reservoir is mainly distributed in 2—3¹ layers, which is the optimal target window position.

With the rapid development of the shale gas industry in China, the shale gas geological evaluation experimental testing technology has been continuously improved. At present, a series of technology, mainly including core pretreatment, mineral composition, geochemistry, pore structure, physical properties, mechanical properties and gas content, has been formed in China. Compared with traditional sandstone, conglomerate, siltstone and other clastic reservoirs, shale reservoir, as a fine-grained deposit, has typical characteristics of low porosity and ultra-low permeability, and the natural fractures and micro nano pores develop. Therefore, it brings four challenges to the experimental technology of shale gas geological evaluation: the sedimentary microfacies are changeable, the diagenetic evolution is complex, the pore structure characterization is difficult and the fluid flow mechanism is diverse. In order to further speed up the progress of shale gas exploration and development in China and solve the bottleneck and problems in the actual production process, four directions are put forward for the experimental testing technology of shale gas geological evaluation, that is, ①the fine description of reservoir urgently needs to be developed from static characterization to dynamic evolution, ②the researches of reservoir pore and fluid occurrence mode in pore develop into simulation experiment under real geological conditions, ③the porosity characterization develops from single scale to macro-micro multi-scale integration, ④experimental data mining based on big data develops into capacity evaluation and prediction. All these are expected to provide reference for the development of the shale gas exploration and development technology and theory in China.

The problems of strong longitudinal heterogeneity and difficulties in fine evaluation exist in shale reservoirs from the Wufeng Formation to the first member of Longmaxi Formation in Weirong Shale Gas Field. Therefore, TOC content, gas content, proportion of organic pores, brittle mineral content, clay mineral content, brittleness index and horizontal stress difference coefficient are selected as the key parameters for the shale gas reservoir evaluation, and the gray correlation analysis method is applied to quantitatively characterize reservoir quality based on the characteristics of Weirong deep shale gas reservoirs and their main influencing factors. The results show that: ①By calculating the correlation between evaluation parameters and reservoir quality, it is clear that gas content is the primary parameter to indicate high-quality reservoir. ②By calculating the comprehensive evaluation index of reservoir quality(Q), the reservoir can be divided into four levels, among which the A-level one is mainly bio-siliceous shale and calcium-bearing-clay-siliceous shale, consisting with the target window position of the current horizontal wells, and the research results are in good agreement with development practices. This method realizes the fine evaluation of reservoir quality under the influence of multiple factors, provides the basis for the objective evaluation of high-quality shale reservoirs, and has reference significance for the evaluation and development of similar gas reservoirs.

The adsorption capacity of shale gas is mainly influenced by the factors such as burial depth, mineral composition, temperature and pressure. The shale gas in South Sichuan has the characteristics of large buried depth, high temperature and high pressure. It has great significance to determine the adsorption and desorption characteristics of this kind of shale reservoir in supercritical state for the implementation of gas reservoir production. By the combination of indoor physical simulation experiments and molecular dynamics simulation, the composite characterization model of Illite and Kerogen is constructed and optimized, the multiple prediction model of whole area adsorption capacity of the deep shale gas in South Sichuan is developed, and the adsorption and desorption characteristics of the gas reservoir are clarified. The experimental results show that: with the increase of formation pressure, the adsorption capacity increases gradually, and when the pressure is higher than 15 MPa, the increase of adsorption capacity gradually slows down. Under the original conditions of gas reservoir, the adsorbed gas volume accounts for 20 % ~ 25 % of the total gas volume. When the formation pressure drops to 20 MPa, the recovery degree of free gas is about 70 % ~ 90 %, and the recovery degree of adsorbed gas is about 25 % ~ 40 %.

The study of geostress field has important theoretical and practical significance for the development of shale gas in Nanchuan area. In order to determine the distribution of the current geostress field of Silurian Longmaxi Formation in Nanchuan area, Eastern Chongqing, the maximum horizontal principal stress direction of the current geostress has been determined according to the direction of borehole collapse of key wells in the area, and the magnitude of the geostress of drilled wells has been collected. The distribution of the rock mechanics parameters in the area has been calculated on the basis of the seismic data, the geological units have been divided based on the tectonic map of the bottom of Silurian Longmaxi Formation in Nanchuan area, and a geological model has been established. Taking the ground stress data of key wells as the constraint, the finite element numerical simulation of the geostress field of Longmaxi Formation in Nanchuan area has been carried out, and the distribution law of the geostress field has been analyzed. The results show that the main stress field is the extrusion stress field. The current maximum principal stress is about NEE25° to SEE20°, 56.12~93.79 MPa while the minimum principal stress is approximately NNW25° to NNE20°, 48.06~71.67 MPa. On the whole, the stress has a distribution trend of low in the east and high in the west. The fault has an influence on the direction and the magnitude of the stress while the mechanical properties of rocks also influence the stress magnitude.

Middle Permian Qixia-Maokou Formation is one of the most important series of strata to explore and develop natural gas in Sichuan Basin. Nanchuan area is located in the southeast of Sichuan Basin. The pilot exploration indicates that there is a great development potential for natural gas in Qixia-Maokou Formation. In order to know its reservoir spaces of carbonate rocks, the test methods, such as thin section identification, SEM and phase analysis of X-ray diffraction, have been taken based on the samples of Well-JY205-2. The reservoir spaces are classified into two categories: pores and fractures. Pores include organic pores and inorganic pores, while fractures include stress fractures, grain edge fractures and shrinkage cracks. The types of reservoir spaces are obviously controlled by lithology. In Qixia-Maokou Formation, the dominant reservoir space types of carbonate reservoirs are the intragranular dissolution pores and the dissolution fractures distributed along the particle edge. These dissolution fractures can connect the pores with the fractures better, and are beneficial to improve the permeability of the reservoir. There are abundant intercrystalline pores of clay minerals in the layers with higher argillaceous content. The surface porosity of intragranular dissolution pores and intercrystalline pores of clay minerals in the layer-① of Mao-1 Member are the highest and in good agreement with the higher porosity and permeability in this layer, which indicates that the layer-① of Mao-1 Member is favorable to natural gas exploration.

The evaluation of shale gas development area selection is an important part of shale gas development, and the effectiveness of evaluation methods is conducive to the efficient development of shale gas. Based on 9 candidate blocks of exploration and development mature and immature blocks in Fuling Shale Gas Field, the influence of different geological and engineering parameters on the evaluation of selected areas has been systematically analyzed, and the evaluation index of the blocks have been selected. Among these index, gas content, compressibility and economy are three important aspects of shale gas development area selection evaluation. Thereafter, the analytic hierarchy process and the mean square error method are used to determine the combined weight of the index, which improve the accuracy of the weight coefficients and establish an evaluation method for shale gas development area selection. The research result shows that in the process of selecting shale gas development areas, the main evaluation index are total hydrocarbon value measured by gas logging, fault development degree, formation pressure coefficient, topographic features, fracture development degree of target layer and porosity. According to the development practice in the Fuling Shale Gas Field, the evaluation results are in good agreement with the actual situation, indicating that the evaluation method for shale gas development area selection based on the improved analytic hierarchy process is feasible.

The gas-bearing shale in Wufeng-Longmaxi Formation of Pingqiao anticline in Nanchuan block is thick. Currently, the low vertical producing reserves, underused oil-bearing area remains in the development in the same layer series with the horizontal well spacing of 500 m, leading to the failure of fully exploitation of shale reserves. In order to guide reasonable and efficient development, and improve the reserve production degree and enhance recovery rate, based on the fine description of the reservoir, the description of the rock microfacies and stress characteristics in the vertical direction has been focused on, and the impact on the fracture height and fracture length has been clarified. By the microseismic monitoring, historical fitting of gas well production data and other technical methods, the division of the development layer series and the plane reasonable well spacing optimization have been determined. It is found that: firstly, the gas-bearing shale of Wufeng-Longmaxi Formation in Nanchuan area can be divided into nine small layers and 21 sub layers, and the development layer can be divided into two sets of gas layers. The lower ①—⑤ layer are with the thickness of 35 m, while upper ⑥—⑨ layer are with the thickness of 76 m. Secondly, the reasonable horizontal well spacing of Pingqiao anticline is 250 m.

The casing damage of horizontal wells for shale gas, especially for deep shale gas, has always been a problem that troubles the staged fracturing development of shale gas. The conventional methods, such as casing patching and chemical plugging, cannot simultaneously meet the demands of high pressure, high flow rate and small bridge plug passing during fracturing, resulting in the difficult implementation of the staged fracturing technology through pumping of bridge plugs. Besides, the staged fracturing technology with multi-packer is difficult to meet the requirements of pumping rate and scale due to the limited number of segments and limited displacement, resulting from the limited range of the ball-drop sliding sleeve. Taking Well-WY9-2HF as an example, by conducting the researches such as risk demonstration, pipe string mechanical analysis and structural optimization, tool development and improvement, and wellbore simulation test, the following conclusions are drawn: the string with suspension packer is suitable for isolating the casing-damaged sections; the combination of the upper casing and the lower tubing for injection can raise the pump rate; the full bore infinite sliding sleeve can increase the number of segments and the pump rate; together with multi-packer, well control safety measures and high-rate segmented fracturing technique, the full bore infinite sliding sleeve is used to realize a large-scale and high-rate fracturing with 31 stages in Well-WY9-2HF, which achieves the expected targets and obtains a commissioning production of 8.75×10⁴ m³/d. This technology can be used as a reference for the fracturing and commissioning of such wells.

Casing deformation occurs frequently during the development of the Weirong Shale Gas Field in the Sichuan Basin. In order to solve this problem, analysis on load and deformation degree of three casing deformation has been carried out by finite element method to discuss the deformation type. Meanwhile, by means of theoretical analysis, finite element simulation and equivalent test evaluation, the quantitative analysis is carried out on the parameters such as the overall angle change rate, cementing quality, wellbore temperature-pressure change and instantaneous stop pump. The influence of different engineering factors on casing strength weakening shows that the engineering factors are not the main factors of casing deformation. According to the analysis of comprehensive factors in the transformation process such as formation anisotropy, wellbore temperature-pressure coupling and reservoir lithologic interface, it is considered that the casing strength decreases under engineering conditions, while in the process of fracturing, the slip of weak strata or the non-uniform compression load of reservoir on casing leads to casing stress concentration and deformation. The relevant understanding provides a reference for the formulation of casing deformation prevention and control measures of Weirong Shale Gas Field.

After large-scale volume fracturing of deep shale gas wells, there comes the difficulty of wellbore plugging, which affects long-term production. The common methods to remove the wellbore plugging by coiled tubing and drill pipe are simple. It lacks of the pertinence measures to treat the diameter shaft and the mixed stemming. For example, Well-WY23-1HF can not produce normally due to the fracturing of adjacent well. The formation collapses and the cuttings enter the wellbore, causing serious plugging. The trajectory of its plugging section is of the complex situation of warped, casing deformation and so on. The measures of plugging removal and resuming production are drill-grinding string, drill-grinding process, well killing fluid design, etc. The field application shows that the same type of bridge plug should be used to reduce the impact of tapering, ensure the smooth trajectory, prevent the accumulation of plugging material, and avoid the complication of wellbore conditions for multi-stage fracturing of horizontal wells. At the same time, combined with the comprehensive application of soluble bridge plug and large diameter non-easy-to-drill bridge plug, the combination of pipe string with oil flushing with nibs+drill pipe with bit （or milling shoe） is adopted to remove plug, and the recovery of production is successfully achieved. This technology provides a reference for similar shale gas wells.

The marl gas reservoir of the Permian Maokou Formation in the Nanchuan area is characterized by low porosity and low permeability, high carbonate content, irregular fracture development, poor connectivity and strong heterogeneity. The conventional acid fracturing technology used to transform the reservoir directly has the problems such as the rapid reaction of acid under the reservoir conditions, and limited dissolution distance. Based on the typical geological characteristics of the Maokou Formation in this area, experimental testing and evaluation of the gelling acid system has been selected. A gelling acid system with strong viscosity, good compatibility stability and degradability in the mode of “multistage alternating injection of acid fracturing+closed acidizing” is screened. The gelling acid system has been successfully applied in Well-DS1HF in Nanchuan area for the first time, and the stable production is 23.05×10⁴ m³/d under the production system of 10 mm nozzle.The good results verify that the application of this technology in marl gas reservoirs is feasible. It has a good reference and application for the development of similar gas reservoirs in China.

In the later stage of heavy oil reservoir development by steam injection, the steam channeling is serious and the development effect gets worse year by year. So it is urgent to apply relevant technologies to improve steam flooding performance. According to the characteristics of the heavy oil reservoir in Z32 Qigu Formation in Fengcheng Oilfield, the feasibility of steam flooding assisted by CO₂ injection to improve the development effect has been studied. In the process of steam injection in heavy oil reservoir, the injection of CO₂, N₂ or flue gas can make crude oil swell, increase formation elastic energy, reduce viscosity of crude oil and improve oil flow capacity of crude. The swell and viscosity reduction effects of different types of injected gas on heavy oil have been measured by high temperature and high pressure crude oil sampler and core flow experiment device. The volume expansion of crude oil is about 10 % ~ 40 %, 2 % ~ 10 % and 4 % ~ 16 % by injecting CO₂, N₂ and simulated flue gas respectively, the viscosity is reduced by 27.3 % ~ 82.9 %, 4.3 % ~ 18.5 % and 10.2 % ~ 45.8 %, respectively, and the oil recovery is enhanced by 5.83 %, 3.08 % and 3.75 %, respectively. Therefore, CO₂ is recommended as the injection medium. Field application shows that after the new technology is implemented, the daily oil production of the well group increases from 26.6 t to 36.2 t, the oil steam ratio increases from 0.174 to 0.206, and the total oil production increases by 1 158 t. Remarkable economic benefits are obtained, which provides references for the scale application of this new technology.

Taking the sandy beach-bar deposit in the 3rd member of Funing formation（Ef₃） in the west slope zone of Qintong sag as the research object, based on core, logging and various analysis test data, the sedimentary characteristics and sedimentary dynamic conditions of the sandy beach-bar are analyzed comprehensively, the genetic types of sandy beach-bar have been divided and the main controlling factors under spatiotemporal conditions have been summarized. The research suggests that the sandy beach-bar deposits can be further divided into beach sand and bar sand, and they have great differences in lithology, color, thickness, particle size and sedimentary structure. According to the analysis of the sedimentary dynamic conditions for the formation of sandy beach-bar, the sedimentary genetic types of sandy beach-bar in the western slope belt can be divided into two types: ①the redeposited sandy beach-bar in front or side margin of far source delta, ②near-shore sandy beach-bar controlled by near-source alluvial fan. The former is found in each sand group of Ef₃, and most widely distributed in Ef₃⁴ sand group. The latter is only found in Ef₃⁴ sand group, which is restricted to the southeast coast of the west slope and developed in belts. The study of sedimentary characteristics of sandy beach-bar in continental faulted lacustrine basin is of theoretical significance for the exploration and development of lithologic oil and gas reservoirs in thin interbedded lacustrine facies.

At present, the leakage of shale gas drilling is quite common in China, and there are few plugging methods suitable for oil-based drilling fluid. The total loss of drilling fluid is still a difficult problem for technicians in the industry. Taking the field operation of Well-WY-1HF as an example, during the drilling of its horizontal well interval by using oil-based drilling fluid, the conventional plugging technology of bridge plug and cement plugging technology of oil well can not work efficiently. Based on the analysis of the characteristics of the loss zones, the effect of different plugging material of oil-base drilling fluid performance have been studied to determine the formula for successful plugging, that is high density （1.89 g/cm³） cement solidified plugging combined with low density （1.17 g/cm³） inorganic gel curing plugging, cooperating with the use of bridge plug with high concentration and large particle. It ensures the subsequent drifting, well logging, the casing and cementing operation smoothly. The successful implementation of this method provides certain reference for the application of shale gas plugging technology in the same area.

SD Oilfield’s progressive exploration and development is getting more and more difficult under low oil price. In order to tap its potential and extend the peripheral trap results, by the means such as: ①fine structural interpretation to identification of low sequence faults, ②strengthen the basic geological researches to deepen the understanding of the main control factors of oil and gas reservoir formation, ③deploy the development and evaluation of wells to improve utilization of single wells, ④combined dynamic and static analyses to rolling expand the oil-bearing area of old area, ⑤comprehensive evaluation of upper and lower layers to reduce the risk of the progressive exploration, two oil-bearing layers and six oil-bearing fault blocks have been added in SD Oilfield during the period from the 12th Five-Year Plan to the 13th Five-Year Plan and the newly added proved reserves are 1 011×10⁴ t, which strongly support the persistent increase of oil production in Sinopec East China Oil and Gas Company and establish the accumulation mode of SD Oilfield. It is clear that the key direction of the progressive exploration are the subsurface reservoir and igneous related reservoir of Daiyi Formation in the future.