RESEARCH PAPER

PETROLEUM EXPLORATION AND DEVELOPMENT Volume 40, Issue 5, October 2013 Online English edition of the Chinese language journal

Cite this article as: PETROL. EXPLOR. DEVELOP., 2013, 40(5): 615–620.

Received date: 08 Mar. 2013; Revised date: 16 Jul. 2013. * Corresponding author. E-mail: wanghongyan69@petrochina.com.cn Foundation item: Supported by the National Key Basic Research and Development Program (973 Program), China (2013CB228000). Copyright © 2013, Research Institute of Petroleum Exploration and Development, PetroChina. Published by Elsevier BV. All rights reserved.

Scientific issues on effective development of marine shale gas in southern China

WANG Hongyan1,2,3,*, LIU Yuzhang1,2, DONG Dazhong2, ZHAO Qun1,2,3, DU Dong2 1. PetroChina Research Institute of Petroleum Exploration & Development, Beijing 100083, China; 2. National Energy Shale gas Research and Development Center, Langfang 065007, China; 3. CNPC Unconventional Oil and Gas Laboratory, Langfang 065007, China

Abstract: Shale gas resources are abundant in China and have been discovered in some areas. They are widely distributed in the Cam-brian, Ordovician and Silurian strata in Southern China, with technically recoverable resources accounting for 3/4 of the whole country. The Southern China will be the main area for shale gas development. Compared with North America, there are a lot of differences in shale gas exploration and development in Southern China which include intensive tectonic movements in marine shale, complex stress field, deep burial depth, special surface condition, etc. With those, it could be ineffective if the existing theories and techniques of shale gas developed in America are taken for granted. The nano-pore formation effects on shale gas production are unclear; Prediction methods for shale gas production have not been established; In the process of drilling, the horizontal section collapses seriously and the drilling cycle is too long; Stimulation effect is not ideal, with low single well production. In order to effectively develop shale gas in Southern China, three scientific issues should be studied which include quantitative characterization of nano-pore formation and multi-scale storage space, mechanisms of nonlinear flow under multi-field coupling in complex medium, mechanical mechanisms of shale instability and fracture network formation.

Key words: shale gas; effective development; scientific issue; southern marine shale; nano-pore

Introduction

Shale gas is a kind of unconventional gas produced from organic-rich shale [1]. Thanks to the major inroads in explora-tion and production technologies, especially progress in hori-zontal well drilling and fracturing, and their wide application, major breakthroughs have been made recently in shale gas exploration and production in the USA In 2012, shale gas production in the USA reached 2 499×108 m3, accounting for 35% of total gas production in the USA [2]. Shale gas produc-tion on a large scale has reshaped global gas supply setup and led to a "shale gas revolution" around the world. Following this trend, many countries in Europe and Asia have initiated projects of shale gas exploration and production. It is antici-pated that global shale gas production in 2020 will amount to 4 000×108 m3.

Organic-rich mudstones and shales occur widely in various horizons and districts in China. Marine shales mainly distrib-ute in Southern China, North China, the Tarim Basin and Qiangtang Basin, while continental shales extensively distrib-ute in the Junggar Basin, Tuha Basin, Ordos Basin, Bohai Bay Basin and Songliao Basin. Large in resource potential [3], the

technical recoverable resources of shale gas in China have been estimated between 10×1012 m3 and 15×1012 m3. The Cambrian, Ordovician and Silurian System in Paleozoic Erathem in Southern China developed several sets of or-ganic-rich marine shales, such as Silurian Longmaxi Forma-tion and Cambrian Qiongzhusi Formation, wide in distribution, large in thickness (30–50 m), high in total organic carbon (TOC) content and gas content, they are the main part of shale gas resources in China, with technical recoverable resources of 8-11×1012 m3, three fourths of shale gas resources in China. Marine shale gas reservoirs in Southern China have their own features. The paper will discuss some scientific problems re-lated to effective shale gas production based on the compari-son between present shale gas production in China and the US.

1 Current shale gas production in China

Shale gas reservoirs in China have their own unique fea-tures, which poses a number of challenges to production need to be addressed urgently. Compared with the shales in US, shale reservoir conditions in China are more complicated and special (Table 1).

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Table 1 Shale gas geologic features and recovery conditions in China and the USA

E&D level Reservoir conditions Recovery conditions Recovery effect

Shale reservoir Number of wells drilled

Proved reserves/ 1012 m3

Produc-tion/

108 m3

Tech-nology

Reservoir Ro/%

TOC/%

Thick-ness/m

Buried depth

Surface con-ditions

Initial average daily production per well/104 m3

Marine shales in Southern China

<100 0 0.5 Not

matureCambrian and

Silurian 2.5−4.0 2−8 50−400 1 500−4 000

Mountain lands and hills

1~2

Major shale reservoirs in the US

>50 000 1.7 2 499 MatureDevonian and Mississippian

1.1−3.5 2−14 30−300 1 000−3 000 Mainly plains >7

Marine shales in China are characterized by well-developed

fractures and poor preservation conditions due to strong tec-tonic reworking, while those in the US are characterized by relatively few fractures and relatively good preservation con-ditions owing to the mild tectonic activities.

Marine shales in China deposit in old geological time (mainly in the Cambrian, Silurian and Devonian) with high thermal evolution (Ro is generally higher than 3.0%) and low TOC content (Table 1). Compared with the US, shale gas res-ervoirs in China are deeper in buried depth and complicated in surface conditions.

Shale gas industry in China now is still at the stage of re-sources survey and pilot exploration; shale gas recovery just got started. There is no sufficient theoretical and technical support to reservoir characterization, well drilling and com-pletion, reservoir stimulation, gas reservoir engineering, etc., which manifests in the following four aspects.

(1) Lack understanding on shale gas reservoir properties and fluid flow patterns causes uncertainty in core areas. Since some key production parameters such as recoverable shale gas reserves, single well production, development well spacing and well pattern can only reference relevant ones in the US, without considering the uniqueness of marine shales in China, development and evaluation errors are likely to occur.

(2) Serious hole collapse is the major challenge in long-distance horizontal section drilling for shale gas recovery, occuring in most shale gas wells at present, it adversely af-fects drilling cycle and fracturing effect of appraisal wells. An oil-base anti-collapse drilling fluid system was used in Well Wei201-H1, the first horizontal shale gas well in China, and drilling fluid density was increased gradually; but serious hole collapse still could not be avoided at major shale sections.

(3) Copying technologies indiscriminately from the US for shale gas recovery resulted in poor staged fracturing effect. Shale gas output of horizontal wells in the US may reach 10 to 15 times of the output of vertical wells after staged fracturing. Fracturing technology borrowed from American companies has been used in horizontal well gas testing in southern Si-chuan with 7 to 12 stages. The initial output of Well Ning201-H1 in Changning-Weiyuan demo zone is 15×104 m3/d and initial production of three other wells is 1-3×104 m3/d, only 1 to 3 times of vertical well output. The stimulation falls far short of the desired effect, far from high-efficiency recovery.

(4) Shale gas production has not been industrialized in China and study on shale gas reservoir engineering is still a blank. At present some key parameters of productivity ap-praisal and development well pattern in Changning and Wei-yuan block have been defined through the analogy between similar gas reservoirs in China and in the US; there are no theoretical methods for production parameter selection.

2 Current theoretical researches on shale gas production in China

2.1 Shale gas reservoir properties

Shales had been taking as source rocks and cap rocks and we need more information to understand their properties as reservoir rocks. From 2005 to 2010, domestic researchers make use of oil and gas shows (e.g. well kick and circulation loss) during well drilling, geochemical properties of source rocks (organic matter abundance, thermal maturity, hydrocar-bon generating potential, organic-rich shale thickness, gas content, brittle minerals, etc.) and XRD analysis (relative content of clay minerals), to study the properties of shale gas reservoirs and conduct resource potential appraisal and fa-vorable zone optimization [4]. Zou Caineng et al. [5] in 2010 examined microscopic pore structure in shales with advanced techniques such as argon ion polishing, field emission SEM and quantitative description of digital core based on nano-CT, finding there are a large quantity of nano-pores in highly ma-ture marine shales in Southern China, which are mainly or-ganic matter pores, intragranular pores and intercrystalline pores with the diameter between 5nm and 750 nm. At the same time, drawing on the experience of North America, some researchers tried to use sidewall core data, imaging log, adsorption isotherm as well as shale porosity and permeability testing proposed by U.S. Gas Technology Institute and ge-omechanical analysis to study logging responses, geochemical properties and petrophysical properties of shale gas reservoirs for evaluation; it is found that good shale gas reservoirs fea-ture high GR values, low oil and water saturation, well-developed natural micro-fractures, high brittle mineral content, strong heterogeneity and anisotropy [3−6].

2.2 Study on fluid flow patterns in nano-pores and micro-fractures

Compared with conventional reservoirs, shale gas reser-voirs have two distinctive features: (1) reservoirs are tight

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with extremely low permeabilities. Pore and throat diameters are generally less than 1 000 nm and fluid flow is mainly non-linear; (2) the reservoirs contain adsorbed gas. Natural gases in shales exist in the form of adsorbed gas, free gas and dissolved gas, among which adsorbed gas and free gas are in the majority and the minority is dissolved gas. The amount of adsorbed gas in shales is closely related to organic matter content and generally ranges between 20% and 85%. In shale gas reservoirs, gas flow of different scales and phase states can not be described with Darcy's law. At present those theo-ries and methods of conventional gas flow and gas reservoir engineering have been adopted in shale gas reservoir analysis.

2.3 Study on horizontal well stability and mechanisms for forming a premium borehole

Domestic researchers have made a preliminary study on borehole stability at shale intervals. As for borehole stability in isotropic formations, analytical theories and computational methods based on plastoelasticity of porous media are basi-cally mature. But for structured mudstone and shale forma-tions (with bedding and cracks), borehole instability, espe-cially long horizontal section in shale intervals, is a hard nut to crack at the present time.

On theories of drilling fluid to keep borehole stability, do-mestic researchers suggested an approach of adding inhibiting agents and blocking agents in oil-base drilling fluids and wa-ter-base drilling fluids at non-shale gas reservoir beds to lower shale collapse pressure and prolong the period of borehole stability. But there is no clear understanding about the mecha-nism of horizontal borehole stability in marine shale gas res-ervoir beds in Southern China, so there are no corresponding solutions as well.

On logging while drilling and geo-steering drilling, espe-cially theory and technology of rotary steering drilling, there is a big gap in researches between China and those developed countries due to late onset. There are no effective ways to identify shale gas reservoir beds and no theories of steering drilling; therefore it is difficult for well drilling to follow de-signed target points and build a premium borehole for com-pletion. Shale gas reservoir protection has not been started yet. Most horizontal shale gas wells drilled are not good in ce-menting quality and problems such as casing deformation often occur during fracturing.

2.4 Study on basic theories of fracturing for shale gas reservoirs

As Well Wei201, the first shale gas well in China, yielded commercial gas flow after fracturing, domestic researchers set about the study on shale fracturing theories and technologies. In 2010 some researchers carried out the study on fracturing of shale gas wells in China based on the analysis and review of overseas experience. After analyzing fracturing technolo-gies for shale gas reservoirs in the US systematically, re-searchers from Research Institute of Petroleum Exploration &

Development pointed out the direction of fracturing technolo-gies for shale gas reservoirs in China [6]. Researchers from China University of Petroleum (Beijing) presented an ap-proach to make fracture network perpendicular to wellbore through controlling induced stress [7] based on induced stress calculation and analysis. With respect to the evaluation of shale gas reservoir fracturing, China is not capable of moni-toring and interpreting fractures at the same time, and can’t remotely control and guide fracturing in real time. Foreign companies only offer technical support to real-time micro-seismic interpretation; for this technology, Chinese companies must depend on foreign companies.

2.5 Shale gas reservoir engineering theories and methods

Different in quality from conventional reservoirs, theories and methods for conventional gas reservoir development and gas reservoir engineering are not suitable for shale gas pro-duction. Some researchers have made some preliminary ex-ploration on shale gas reservoir engineering theories, while others considering the adsorption and desorption features of shale gas, by using mathematical flow model for fractured wells with dual-medium based on a point source function in accordance with Langmuir's adsorption isothermal equation, analyzed the impacts of Langmuir volume, Langmuir pressure, elastic storativity ratio, interporosity flow coefficient, bound-ary, crack length etc on shale gas well productivity [8−9].

3 Advancement in overseas shale gas development

3.1 Study on shale gas reservoir features

Since U.S. Department of Energy initiated its "eastern gas shales project" in the 1970s, shale gas production in North America has gone through three stages, i.e. evaluation in early exploration, early production and large-scale production, which all concentrate on reservoir properties. Researchers at the early stage thought organic matter maturity, abundance and type are the major factors controlling good reservoirs. Later on researchers come to understand the correlation among natural fractures, stress field, horizontal wellbore tra-jectory and fracturing modes, which has then been used in individual well design, drilling and completion to promote shale gas production.

Curtis [1] believes that pore structures in shales are deter-mined by the arrangement of all mineral components and are related to shale sedimentary environment and diagenesis. In accordance with mineral compositions in shales, shales may be further classified into siliceous shale, calcareous shale, dolomitic shale, phosphorus-rich shale, fossil-rich shale [10], etc. Jarvie et al. and Rickman et al. [11−12], by examining the correlation between Barnett shale brittleness and mineralogy, found a dependence of rock brittleness on the relative propor-tion between quartz, carbonate and clay. Higher quartz content corresponds to better brittleness, higher clay content corre-sponds to poor brittleness, and the effect of carbonate content

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on brittleness falls between the former two. They also intro-duced the concept of brittleness index. Grieser and Hallibur-ton [13] used full wave sonic logging data to calculate Poisson's ratio and Young's modulus and classified those shales with Young's modulus above 34.5 MPa and Poisson's ratio below 0.25 into brittle shale. In order to understand shale gas reser-voir properties on a macro-scale, integrated core, logging and 3D seismic analysis has been widely used in fault interpreta-tion and petrophysical property analysis, which would then be used in the design of individual well drilling and completion and guiding horizontal wellbore trajectory and positioning as well as selecting intervals to be fractured.

3.2 Fluid flow patterns in nano-pores and microcracks

Although overseas study on multi-field coupled non-linear flow in shale gas reservoirs has made great progress, it is still at a preliminary stage. For example, Javadpour et al. [14] pointed out that gas flow in nano-pores in shale gas reservoir beds cannot be simply described with linear flow, and ran some tests to measure the average free path, and Knudsen number, etc. of gases in shale reservoirs. Based on the study of pore structures and fluid flow in shale beds, Wang et al. [15] concluded that single-phase gas flow follows the non-Darcy flow law in nano-pores due to Klinkenberg effect and follows Darcy's law in natural and fractured cracks. Schepers et al. [16] pointed out that shale gas flow in nano-pores and microcracks are mainly non-Darcy flow (Klinkenberg effect, Forchheimer flow). Freeman et al. [17] analyzed methane flow and its pres-sure curves in shale reservoirs. But problems related to mathematical expression of shale reservoir characteristic pa-rameter dependence on stress and flow patterns of gas and water phases have not been touched upon.

3.3 Study on horizontal borehole stability and how to build a premium borehole

Overseas study on borehole instability at mudstone and shale sections began in 1940, which gives rise to the drilling and completion theories and technologies for shale gas wells represented by the US. The studies mainly involve borehole stability mechanism, field case and handling, and ways of acquiring borehole stability parameters, which concern me-chanical instability mechanism, physical chemical instability mechanism and coupling of these two mechanisms. Mechan-ics in borehole stability at mudstone and shale intervals in-cludes uniform formation study based on continuum theory and borehole stability study for laminated formations and fractured formations, which gives birth to a series of anti-collapse agents for high-performance water-base drilling fluids and drilling fluid systems as well as oil-base drilling fluid systems.

Vertical wells were used for shale gas production in the US at an early stage. After 2000, the advantages of horizontal wells in shale gas production stood out gradually and rotary geo-steering drilling is adopted in horizontal well drilling to

guarantee accurate positioning and wellbore smoothness, which is of great benefit to horizontal well extension and borehole cleaning as well as well completion. Meanwhile, underbalanced drilling and managed pressure drilling are used to prevent reservoir damage.

3.4 Study on basic theories of shale gas reservoir fracturing

The development mode of combined horizontal well and multi-stage fracturing is used for about 85% of shale gas wells in the US at present, for which maximizing the contact be-tween fracture network and matrix would increase gas yield significantly. For example, U.S. Newfield Exploration Com-pany adopted 5-7-stage fracturing in some development wells for Woodford shale gas production with remarkable stimula-tion effect [18]. In 2005, hydraulic jetting was used in 53 Bar-nett shale gas wells for the first time, among which 26 wells ended with a technical and an economic success [19]. In 2006, simultaneous fracturing was used for the first time in Barnett shales, to fracture two or more wells at the same time to in-crease fracture concentration and area, which can enhance shale gas yield remarkably[20]. In 2009, a kind of foam frac-turing with extremely light proppant and extremely high gaseous mass succeeded in multi-stage horizontal shale well fracturing [21].

With the progress in shale gas reservoir fracturing, come the inroads in design theory, fluid flow and effectiveness evaluation of shale gas reservoir fracturing. Rickman et al. [12] used brittleness index to characterize the difficulty of fracture network buildup in reservoir rocks, and developed two for-mulas to calculate brittleness index based on mineral compo-sition, Young's modulus and Poisson's ratio; the higher the brittleness index, the more likely a complicated fracture net-work to create. Terrestrial stress and natural cracks must be taken into account in fracturing design. The smaller the dif-ference of horizontal principal stress the reservoir has, the more developed natural cracks, the more complicated the fracture network could create by stimulation.

3.5 Study on shale gas reservoir engineering theories and methods

The US has achieved a number of research findings in shale gas reservoir engineering, providing effective evaluation means for high efficient development of shale gas. Gatens [22] established typical production curves and a dual-porosity de-sorption model through automatic fit based on the analysis of production data of over 800 shale gas wells in Devonian. Carlson [23] presented a simple and fast method to predict fracture spacing and continuity in fractured vertical wells. Javadpour et al. [14] believed that gas flow in shales is domi-nated by a network of nano-pores and micro-pores, and built a diffusion-controlled migration model to predict shale gas well productivity. Freeman et al. [17] described the flow pattern variation in fractured horizontal wells from initial linear flow

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to complex linear flow and finally to elliptical flow, and also analyzed the impacts of fracture flow conductivity, desorption, fracture spacing and stimulated reservoir volume on gas pro-duction. Based on the traditional dual-porosity model, Hasan [24], considering different properties of cracks in frac-tured reservoirs, set up a tri-porosity (2 kinds of cracks) model to derive the spatial Laplace solution to a linear flow. Craig et al. [25] developed a numerical simulator to study various shale gas production behaviors in fractured horizontal wells, and demonstrated that fluid flow behavior in shales is affected by fracture structure; this understanding is helpful for the evalua-tion of petrophysical properties and reserves in shale reser-voirs. Economides and Wang [26] thought that recovery capac-ity of shale gas wells depends on the maximum fracture sys-tem connected by fracturing, i.e. stimulated volume. Fazeli-pour [27] presented a concept of hydraulic fracturing stimula-tion zone based on microseismic inversion, also known as dominant flow field caused by fracturing, i.e. SRV (Stimu-lated Reservoir Volume).

4 Key scientific problems need to be addressed

Overseas theories and technologies could not be copied blindly for the shale gas development in China due to the complexity and particularity of marine shales in China. There are no criteria of good reservoirs, confirmed core areas, and any applicable flow theories and gas reservoir engineering methods for shale gas production in China to date. Horizontal borehole collapse and and poor multi-stage fracturing results are two other problems haunting the development of shale gas. Key scientific problems to be addressed include the genesis and characterization of nano-pores and microcracks, mecha-nism of multi-field coupled non-linear flow in complicated medium and gas reservoir engineering methods, and mechan-ics of shale instability and fracture buildup.

4.1 Genesis and characterization of nano-pores and microcracks

Shale gas reservoirs are fine-grained sedimentary rocks rich in organic matter, which are quite different in petrophysical properties from conventional gas reservoirs, because their reservoir space mainly comprises organic matter-hosted pores and intergranular pores of nano-scale and micro-scale and natural cracks, with special pore structures, wettability, gas occurrences and flow patterns; in addition, matrix permeabil-ity is extremely low. Organic matter-hosted pore distribution is affected by kerogen types, organic matter abundance and maturity, and rock composition; shale porosity evolution, natural crack assemblages and scales are controlled by sedi-mentary environment, diagenesis, geomechanical properties, hydrocarbon generation, in-situ stress, tectonic activities, etc. Existing reservoir evaluation is incapable of describing nano-pore genesis and multi-scale pore-crack structures due to the diversity and complexity of shale reservoir spaces and flow patterns. It is difficult to quantitatively characterize po-

rosity, gas-bearing properties, permeability and fracability of shale gas reservoirs and satisfy the need of shale gas produc-tion with high efficiency. Therefore the problem of the genesis of nano-pores and micro-pores in shales and quantitative characterization of multi-scale reservoir spaces is one of the key problems in shale gas geological research, which focuses on the genesis and occurrences of pores in shale gas reservoirs in addition to natural cracks so as to answer the question of reservoir space development and distribution.

4.2 Mechanism of multi-field coupled non-linear flow in complicated medium and gas reservoir engineering methods

Shale pores mainly range in nano-scales and micro-scales and cracks may also occur. Compared with conventional res-ervoirs, shale gas flow may take on many forms such as de-sorption, diffusion, flow in pores, flow in microcracks, flow in fractured cracks with fluid-structure coupling effect, etc.; it is necessary to conduct mathematical description of adsorbed gas desorption, diffusion flow of free gas, fluid dynamics of various flows and fluids, and various behaviors. On account of the coupling effects of flow field, stress field and tempera-ture field, shale deformation would have a great impact on pore structures and flow features; flow mechanism and pat-terns in shale reservoirs are quite different from those of con-ventional low-permeability reservoirs and Darcy's law is no longer applicable. It is necessary to build mathematical mod-els to reflect the coupling effects of flow field, stress field and temperature field as well as effects of rock matrix and frac-tured cracks so as to reveal those patterns of flow in nano-pores, flow in micro-pores, flow in cracks, flow in frac-tured cracks, non-linear flow with fluid-structure interaction in multi-stage fractured horizontal wells, and non-linear sin-gle-phase and dual-phase flow with matrix-fracture coupling effect in fractured shale gas wells. Studies should be carried out on coupled flow mechanism in complicated shale medium to lay the theoretical foundation for shale gas production. The significance of this scientific problem lies in finding out the fluid transport patterns in pores and cracks, so as to answer the question of recoverable shale gas reserves and productiv-ity appraisal.

4.3 Mechanics of shale instability and fracture creation

According to shale gas production in North America, the most effective technologies are horizontal well drilling and multi-stage fracturing. The drilling of long-distance horizontal wellbores break the original mechanical equilibrium of forma-tion; formation instability would be further intensified greatly due to shale hydration, secondary crack extension and effects of weak plane, leading to circulation loss or borehole collapse finally, seriously restricting the progress of shale gas explora-tion and production. Therefore the mechanics of shale insta-bility is a key question needs to be answered for safe and ef-fective shale gas production.

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It is proved by a lot of field tests that conventional reservoir fracturing mainly generate single fracture, while shale frac-turing may give birth to complicated fractures or a fracture network due to its complex laminated structures and crack features. Whether or not an effective fracture network can be built is critical to high shale gas yield. The mechanics and geometry of fracture buildup in marine shales in Southern China is of great importance to the effective fracturing of shales. The subject on mechanics of shale instability and frac-ture buildup focuses on the mechanism of borehole stability and complicated fracture network buildup at shale intervals.

5 Conclusions

Marine shale gas production in Southern China involves very complicated basic theories and methods. It is necessary to concentrate on those key issues and conduct integrated study on development geology, fluid mechanics in porous medium, geomechanics, gas reservoir engineering, well drill-ing and completion, and gas production engineering. It is necessary to characterize marine shale reservoir space based on shale gas provinces in Southern China so as to put forward feasible approaches to quantitatively characterize marine shale gas reservoirs in China, reveal microscopic flow mechanism in shales and the flow patterns in nano-pores and microcracks, look into the mechanism of long-distance horizontal borehole instability and put forward solutions to this problem, figure out the mechanism of fracture initiation and extension and interaction between fracturing fluids and shale gas reservoirs to establish theories and methods of fracturing suitable for shale gas reservoirs in China; find out the mechanism of shale gas production decline and production performance variation so as to predict fractured horizontal well productivity under multi-medium coupling effects and characterize recoveries in different zones, and set up a parameter optimization system for shale gas reservoir engineering. It is necessary to combine scientific researches with production practice and combine fundamental researches with technical breakthrough.

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