A New Discovery on the Deformation Behavior of Shale Gas Reservoirs Affecting Pore Morphology in the Juhugeng Coal Mining Area of Qinghai Province, Northwest China

Objective The Juhugeng mining area in Qinghai Province of northwest China has attracted wide attention among geologists for it hosts typical coal measure gases.The shale gas reservoirs were reformed by intensive structural movements during geological periods,

Geological characteristics and high production control factors of shale gas reservoirs in Silurian Longmaxi Formation, southern Sichuan Basin, SW China

Marine shale gas resources have great potential in the south of the Sichuan Basin in China.At present,the high-quality shale gas resources at depth of 2000–3500 m are under effective development,and strategic breakthroughs have been made in deeper shale gas resources at depth of 3500–4500 m.To promote the effective production of shale gas in this area,this study examines key factors controlling high shale gas production and presents the next exploration direction in the southern Sichuan Basin based on summarizing the geological understandings from the Lower Silurian Longmaxi Formation shale gas exploration combined with the latest results of geological evaluation.The results show that:(1)The relative sea depth in marine shelf sedimentary environment controls the development and distribution of reservoirs.In the relatively deep water area in deep-water shelf,grade-I reservoirs with a larger continuous thickness develop.The relative depth of sea in marine shelf sedimentary environment can be determined by redox conditions.The research shows that the uranium to thorium mass ratio greater than 1.25 indicates relatively deep water in anoxic reduction environment,and the uranium to thorium mass ratio of 0.75–1.25 indicates semi-deep water in weak reduction and weak oxidation environment,and the uranium to thorium mass ratio less than 0.75 indicates relatively shallow water in strong oxidation environment.(2)The propped fractures in shale reservoirs subject to fracturing treatment are generally 10–12 m high,if grade-I reservoirs are more than 10 m in continuous thickness,then all the propped section would be high-quality reserves;in this case,the longer the continuous thickness of penetrated grade-I reservoirs,the higher the production will be.(3)The shale gas reservoirs at 3500–4500 m depth in southern Sichuan are characterized by high formation pressure,high pressure coefficient,well preserved pores,good pore structure and high proportion of free gas,making them the most favorable new field for shale gas exploration;and the pressure coefficient greater than 1.2 is a necessary condition for shale gas wells to obtain high production.(4)High production wells in the deep shale gas reservoirs are those in areas where Long11-Long13 sub-beds are more than 10 m thick,with 1500 m long horizontal section,grade-I reservoirs penetration rate of over 90%,and fractured by dense cutting+high intensity sand injection+large displacement+large liquid volume.(5)The relatively deep-water area in the deep-water shelf and the area at depth of 3500–4500 m well overlap in the southern Sichuan,and the overlapping area is the most favorable shale gas exploration and development zones in the southern Sichuan in the future.With advancement in theory and technology,annual shale gas production in the southern Sichuan is expected to reach 450×108 m3.

Simultaneously improving ROP and maintaining wellbore stability in shale gas well:A case study of Luzhou shale gas reservoirs

The ROP(rate of penetration)within the horizontal section of shale gas wells in the Luzhou oil field is low,seriously delaying the exploration and development process.It is proved that reducing mud density mitigates the bottom-hole differential pressure(ΔP)and increases the ROP during overbalanced drilling.However,wellbore collapse may occur when wellbore pressure is excessively low.It is urgent to ascertain the optimal equilibrium point between improving ROP and maintaining wellbore stability.The safe mud weight window and the lower limit of mud density in the horizontal section of the Luzhou block are predicted using the piecewise fitting method based on conventional logging data.Then,the accuracy of the collapse pressure prediction was verified using the distinct element method(DEM),and the effect of wellbore pressure,in-situ stress,rock cohesion,and natural fracture density on borehole collapse was investigated.Finally,a fitting model ofΔP and ROP of the horizontal section in the Luzhou block is established to predict ROP promotion potential after mud density reduction.The field application of this approach,demonstrated in 8 horizontal wells in the Luzhou block,effectively validates the efficiency of reducing mud density for ROP improvement.This study provides a useful method for simultaneously improving ROP and maintaining wellbore stability and offers significant insights for petroleum engineers in the design of drilling parameters.

Implication of Water-Rock Interaction for Enhancing Shale Gas Production

Horizontal well drilling and multi-stage hydraulic fracturing technologies are at the root of commercial shale gas development and exploitation.During these processes,typically,a large amount of working fluid enters the formation,resulting in widespread water-rock interaction.Deeply understanding such effects is required to optimize the production system.In this study,the mechanisms of water-rock interaction and the associated responses of shale fabric are systematically reviewed for working fluids such as neutral fluids,acid fluids,alkali fluids and oxidative fluids.It is shown that shale is generally rich in water-sensitive components such as clay minerals,acidsensitive components(like carbonate minerals),alkali-sensitive components(like quartz),oxidative-sensitive components(like organic matter and pyrite),which easily lead to change of rock fabric and mechanical properties owing to water-rock interaction.According to the results,oxidizing acid fluids and oxidizing fracturing fluids should be used to enhance shale gas recovery.This study also indicates that an aspect playing an important role in increasing cumulative gas production is the optimization of the maximum shut-in time based on the change point of the wellhead pressure drop rate.Another important influential factor to be considered is the control of the wellhead pressure considering the stress sensitivity and creep characteristics of the fracture network.

Electrical structure identification of deep shale gas reservoir in complex structural area using wide field electromagnetic method

To fully exploit the technical advantages of the large-depth and high-precision artificial source electromagnetic method in the complex structure area of southern Sichuan and compensate for the shortcomings of the conventional electromagnetic method in exploration depth,precision,and accuracy,the large-depth and high-precision wide field electromagnetic method is applied to the complex structure test area of the Luochang syncline and Yuhe nose anticline in the southern Sichuan.The advantages of the wide field electromagnetic method in detecting deep,low-resistivity thin layers are demonstrated.First,on the basis of the analysis of physical property data,a geological–geoelectric model is established in the test area,and the wide field electromagnetic method is numerically simulated to analyze and evaluate the response characteristics of deep thin shale gas layers on wide field electromagnetic curves.Second,a wide field electromagnetic test is conducted in the complex structure area of southern Sichuan.After data processing and inversion imaging,apparent resistivity logging data are used for calibration to develop an apparent resistivity interpretation model suitable for the test area.On the basis of the results,the characteristics of the electrical structure change in the shallow longitudinal formation of 6 km are implemented,and the transverse electrical distribution characteristics of the deep shale gas layer are delineated.In the prediction area near the well,the subsequent data verification shows that the apparent resistivity obtained using the inversion of the wide field electromagnetic method is consistent with the trend of apparent resistivity revealed by logging,which proves that this method can effectively identify the weak response characteristics of deep shale gas formations in complex structural areas.This experiment,it is shown shows that the wide field electromagnetic method with a large depth and high precision can effectively characterize the electrical characteristics of deep,low-resistivity thin layers in complex structural areas,and a new set of low-cost evaluation technologies for shale gas target layers based on the wide field electromagnetic method is explored.

Low-field NMR application in the characterization of CO_(2)geological storage and utilization related to shale gas reservoirs:a brief review

CO_(2)geological storage and utilization(CGSU)is considered a far-reaching technique to meet the demand of increasing energy supply and decreasing CO_(2)emissions.For CGSUs related to shale gas reservoirs,experimental investigations have attracted variable methodologies,among which low-field NMR(LF-NMR)is a promising method and is playing an increasingly key role in reservoir characterization.Herein,the application of this nondestructive,sensitive,and quick LF-NMR technique in characterizing CGSU behavior in shale gas reservoirs is reviewed.First,the basic principle of LF-NMR for 1H-fluid detection is introduced,which is the theoretical foundation of the reviewed achievements in this paper.Then,the reviewed works are related to the LF-NMR-based measurements of CH_(4)adsorption capacity and the CO_(2)-CH_(4)interaction in shale,as well as the performance on CO_(2)sequestration and simultaneous enhanced gas recovery from shale.Basically,the reviewed achievements have exhibited a large potential for LF-NMR application in CGSUs related to shale gas reservoirs,although some limitations and deficiencies still need to be improved.Accordingly,some suggestions are proposed for a more responsible development of the LF-NMR technique.Hopefully,this review is helpful in promoting the expanding application of the LF-NMR technique in CGSU implementation in shale gas reservoirs.

Characteristics of shale gas reservoirs in Wufeng Formation and Longmaxi Formation in Well WX2,northeast Chongqing

To further understand shale reservoir characteristics of Wufeng Formation and Longmaxi Formation in the Wuxi area,northeast Chongqing,based on drilling data of Well WX2,and taking the graptolite biostratigraphy as the standard marker of stratigraphic division and comparison,the geochemistry,petrology,reservoir space and properties of organic-rich black shale were well investigated,and its gasbearing capacity and controlling factors were also analyzed.The result shows that in the Wufeng Formation and Longmaxi Formation of Well WX2,the organic-rich shale is 89.8 m thick and is characterized by good kerogen type,high organic abundance,moderate maturity and favorable hydrocarbongeneration condition,and the graptolite sequence is developed completely and continuously;the organic abundance is influenced by depositional rate,and the slow depositional rate is favorable for accumulation of organic matter in the black graptolite shale;from top to bottom,content of siliceous minerals increases and content of clay minerals decrease,therefore the brittleness increases;the organicrich siliceous shale and clay siliceous shale are favorable lithofacies for development of shale reservoirs;the nanopore is dominated by the parallel-plate pore with four open sides and has good connectivity;the pore size distribution curve has the multimodal characteristic,and the pore diameter mainly is in the range of 0.42e0.62 nm and the range of 3e5 nm;organic pores and interlayer pores of clay minerals make the greatest contribution to the total pore volume,while pores of brittle minerals have the least contribution;from top to bottom,organic pores gradually increase while interlayer pores of clay minerals gradually decrease;the on-site core gas content exceeds 8 m3/t,and the gas-bearing capacity is jointly controlled by hydrocarbon generation,reservoir and preservation conditions;and the WF2-LM6 biozone of the Katian to the early Aeronian is the high-quality shale reservoir,where the LM1 biozone of the Hirnantian was the best“sweet spot”which is the target of horizontal well drilling.

Characterizing hydraulic fractures in shale gas reservoirs using transient pressure tests

Hydraulic fracturing combined with horizontal drilling has been the technology that makes it possible to economically produce natural gas from unconventional shale gas or tight gas reservoirs.Hydraulic fracturing operations,in particular,multistage fracturing treatments along with horizontal wells in unconventional formations create complex fracture geometries or networks,which are difficult to characterize.The traditional analysis using a single vertical or horizontal fracture concept may be no longer applicable.Knowledge of these created fracture properties,such as their spatial distribution,extension and fracture areas,is essential information to evaluate stimulation results.However,there are currently few effective approaches available for quantifying hydraulic fractures in unconventional reservoirs.This work presents an unconventional gas reservoir simulator and its application to quantify hydraulic fractures in shale gas reservoirs using transient pressure data.The numerical model incorporates most known physical processes for gas production from unconventional reservoirs,including two-phase flow of liquid and gas,Klinkenberg effect,non-Darcy flow,and nonlinear adsorption.In addition,the model is able to handle various types and scales of fractures or heterogeneity using continuum,discrete or hybrid modeling approaches under different well production conditions of varying rate or pressure.Our modeling studies indicate that the most sensitive parameter of hydraulic fractures to early transient gas flow through extremely low permeability rock is actually the fracture-matrix contacting area,generated by fracturing stimulation.Based on this observation,it is possible to use transient pressure testing data to estimate the area of fractures generated from fracturing operations.We will conduct a series of modeling studies and present a methodology using typical transient pressure responses,simulated by the numerical model,to estimate fracture areas created or to quantity hydraulic fractures with traditional well testing technology.The type curves of pressure transients from this study can be used to quantify hydraulic fractures in field application.

Shale gas reservoirs: Theoretical, practical and research issues

Shale gas reservoirs are found all over the world.Their endowment worldwide is estimated at 10,000 tcf by the GFREE team in the Schulich School of Engineering at the University of Calgary.The shale gas work and production initiated successfully in the Unites States and extended to Canada will have application,with modifications,in several other countries in the future.The‘modifications’qualifier is important as each shale gas reservoir should be considered as a research project by itself to avoid fiascos and major financial losses.Shale gas reservoirs are best represented by at least quadruple porosity models.Some of the production obtained from shale reservoirs is dominated by diffusion flow.The approximate boundary between viscous and diffusion-like flow is estimated with Knudsen number.Viscous flow is present,for example,when the architecture of the rock is dominated by mega pore throat,macro pore throat,meso pore throat and sometimes micro pore throat.Diffusion flow on the other hand is observed at the nano pore throat level.The process speed concept has been used successfully in conventional reservoirs for several decades.However,the concept discussed in this paper for tight gas and shale gas reservoirs,with the support of core data,has been developed only recently,and permits differentiating between viscous and diffusion dominated flow.This is valuable,for example,in those cases where the formation to be developed is composed of alternating stacked layers of tight sands and shales,or where there are lateral variations due to facies changes.An approach to develop the concept of a super-giant shale gas reservoir is presented as well as a description of GFREE,a successful research program for tight formations.The paper closes with examples of detailed original gas-in-place(OGIP)calculations for 3 North American shale gas reservoirs including free gas in natural fractures and the porous network within the organic matter,gas in the non-organic matter,adsorbed gas,and estimates of free gas within fractures created during hydraulic fracturing jobs.The examples show that the amount of free gas in shale reservoirs,as a percent of the total OGIP,is probably larger than considered previously in the literature.

Simulation of the Production Performances of Horizontal Wells with a Fractured Shale Gas Reservoir

The production performances of a well with a shale gas reservoir displaying a complex fracture network are simulated.In particular,a micro-seismic cloud diagram is used to describe the fracture network,and accordingly,a production model is introduced based on a multi-scale flow mechanism.A finite volume method is then exploited for the integration of the model equations.The effects of apparent permeability,conductivity,Langmuir volume,and bottom hole pressure on gas well production are studied accordingly.The simulation results show that ignoring the micro-scale flow mechanism of the shale gas leads to underestimating the well gas production.It is shown that after ten years of production,the cumulative gas production difference between the two scenarios with and without considering the micro-scale flow mechanisms is 19.5%.The greater the fracture conductivity,the higher the initial gas production of the gas well and the cumulative gas production.The larger the Langmuir volume,the higher the gas production rate and the cumulative gas production.With the reduction of the bottom hole pressure,the cumulative gas production increases,but the growth rate gradually decreases.

Geochemical characteristics and genetic mechanism of the high-N2 shale gas reservoir in the Longmaxi Formation, Dianqianbei Area, China

As an important pilot target for shale gas exploration and development in China,the Longmaxi Formation shale in the Dianqianbei Area is characterized by high content of nitrogen,which severely increases exploration risk.Accordingly,this study explores the genesis of shale gas reservoir and the mechanism of nitrogen enrichment through investigating shale gas compositions,isotope features,and geochemical characteristics of associated gases.The high-nitrogen shale gas reservoir in the Longmaxi Formation is demonstrated to be a typical dry gas reservoir.Specifically,the alkane carbon isotope reversal is ascribed to the secondary cracking of crude oil and the Rayleigh fractionation induced by the basalt mantle plume.Such a thermogenic oil-type gas reservoir is composed of both oil-cracking gas and kerogen-cracking gas.The normally high nitrogen content(18.05%-40.92%) is attributed to organic matter cracking and thermal ammoniation in the high-maturity stage.Specifically,the high heat flow effect of the Emeishan mantle plume exacerbates the thermal cracking of organic matter in the Longmaxi Formation shale,accompanied by nitrogen generation.In comparison,the abnormally high nitrogen content(86.79%-98.54%) is ascribed to the communication between the atmosphere and deep underground fluids by deep faults,which results in hydrocarbon loss and nitrogen intrusion,acting as the key factor for deconstruction of the primary shale gas reservoir.Results of this study not only enrich research on genetic mechanism of high-maturity N_@ shale gas reservoirs,but also provide theoretical guidance for subsequent gas reservoir resource evaluation and well-drilling deployment in this area.

Numerical simulation of hydraulic fracture propagation in laminated shale reservoirs

The main area of the Jiaoshiba anticline of the Fuling shale gas field was taken as the research object,laboratory rock mechanical experiments and direct shear experiments were conducted to clarify the mechanical anisotropy characteristics and parameters of rock samples with rich beddings.Based on the experimental results,a 3D fracture propagation model of the target reservoir taking mechanical anisotropy,weak bedding plane and vertical stress difference into account was established by the discrete element method to analyze distribution patterns of hydraulic fractures under different bedding densities,mechanical properties,and fracturing engineering parameters(including perforation clusters,injection rates and fracturing fluid viscosity).The research results show that considering the influence of the weak bedding plane and longitudinal stress difference,the interlayer stress difference 3–4 MPa in the study area can control the fracture height within the zone of stress barrier,and the fracture height is less than 40 m.If the influence of the weak bedding plane is not considered,the simulation result of fracture height is obviously higher.Although the opening of high-density bedding fractures increases the complexity of hydraulic fractures,it significantly limited the propagation of fracture height.By reducing the number of clusters,increasing the injection rate,and increasing the volume and proportion of high-viscosity fracturing fluid in the pad stage,the restriction on fracture height due to the bedding plane and vertical stress difference can be reduced,and the longitudinal propagation of fractures can be promoted.The fracture propagation model was used to simulate one stage of Well A in Fuling shale gas field,and the simulation results were consistent with the micro-seismic monitoring results.

Shale Gas Characterization of Sembar Formation, Khipro Area, Pakistan

This study pertains to the evaluation of shale gas and rock physics properties of this area with respect to its total organic content of Sember Formation, Khiproarea, Pakistan. We use well logs data for this study. The Khipro area is prominent in the Lower Indus Basin for its hydrocarbon (oil and gas) structural traps. In shale gas evaluation, TOC of Sember Formation is estimated. The analysis has been done with the help of the wire line data of the well Bilal North-01. The presence of shale gas in the study area is analyzed with the help of different techniques. Rock physics and petrophysical analysis have been done in order to get the properties of the area related to the shale gas evaluation.

hale gas reservoir modeling and production evaluation considering complex gas transport mechanisms and dispersed distribution of kerogen

Stimulated shale reservoirs consist of kerogen,inorganic matter,secondary and hydraulic fractures.The dispersed distribution of kerogen within matrices and complex gas flow mechanisms make production evaluation challenging.Here we establish an analytical method that addresses kerogen-inorganic matter gas transfer,dispersed kerogen distribution,and complex gas flow mechanisms to facilitate evaluating gas production.The matrix element is defined as a kerogen core with an exterior inorganic sphere.Unlike most previous models,we merely use boundary conditions to describe kerogen-inorganic matter gas transfer without the instantaneous kerogen gas source term.It is closer to real inter-porosity flow conditions between kerogen and inorganic matter.Knudsen diffusion,surface diffusion,adsorption/desorption,and slip corrected flow are involved in matrix gas flow.Matrix-fracture coupling is realized by using a seven-region linear flow model.The model is verified against a published model and field data.Results reveal that inorganic matrices serve as a major gas source especially at early times.Kerogen provides limited contributions to production even under a pseudo-steady state.Kerogen properties’influence starts from the late matrix-fracture inter-porosity flow regime,while inorganic matter properties control almost all flow regimes except the early-mid time fracture linear flow regime.The contribution of different linear flow regions is also documented.

Reservoir space of the Es_3~3–Es_4~1shale in Dongying sag

The Es3/3-Es1/4 shales in Dongying sag are source rocks with large reserves of shale oil and gas. For the iden- tification of development characteristics and geological significance of the reservoir space, FM1 logging, core observation, thin section analysis, X-ray diffraction, fluorescence microscopy, scanning electron microscopy, mercury porosimetry, low-temperature nitrogen adsorption, atomic force microscopy, and conventional physical property testing were used to study the petrology and reservoir space of the Es3/3-Es1/4 shale in Dongying sag. The results suggest that the shale is rich in carbonate minerals. Phanero- crystalline stratiform and lamellar argillaceous limestone and calcareous claystone are the oil- and gas-bearing lithofacies. The oil in the micropores is mainly present as membranes and clots. The shale reservoir space has a network structure with veins, carbonate and clay minerals, and micropores among pyrite and the matrix. The results provide the geological framework for future shale oil and gas explora- tion in Dongying sag.

Will the future of shale reservoirs lie in CO2 geological sequestration?

CO2 geological sequestration in a depleted shale gas reservoir is a promising method to address the global energy crisis as well as to reduce greenhouse gas emissions. Though improvements have been achieved by many researchers, the carbon sequestration and enhanced gas recovery(CS-EGR) in shale formations is still in a preliminary stage. The current research status of CO2 sequestration in shale gas reservoirs with potential EGR is systematically and critically addressed in the paper. In addition, some original findings are also presented in this paper. This paper will shed light on the technology development that addresses the dual problem of energy crisis and environmental degradation.

Characteristics and controlling factors of shale gas reservoir in the Fuling area,Sichuan Basin

Statistical analysis of massive laboratory analytical data shows that for intervals in the Upper Ordovician Wufeng Formation and the Lower Silurian Longmaxi Formation in the Fuling area,Sichuan Basin,the total organic carbon content is 0.55%-5.89%with an average of 2.54%,presenting a gradual increase trend from top to bottom.Based on whole-rock X-ray diffraction data,the content of clay minerals of the shale is 16.6%-62.8%with an average of 40.9%,gradually decreasing from top to bottom.The content of brittle minerals gradually increases from top to bottom,ranging from 37.2%to 83.4%with an average of 59.1%.Porosity is 1.17%-7.98%with an average of 4.61%through the helium injection method.The horizontal permeability is 0.002-335.209 mD with an average of 23.785 mD by the steady-state method.Massive analytical data from the high-pressure mercury injection method show the micropores are well developed in the shale,and mainly provide the primary specific surface area of pores,while the micropores,transitional pores and mesopores primarily contribute to permeability.The pore types were observed using SEM in combination with argon ion polishing technology.From top to bottom of the shale interval,the organic pores increase and the inorganic pores decrease.Gas desorption data show that the total gas content is 0.44-5.19 m^(3)/t with an average of 1.97 m^(3)/t,and gradually increase from top to bottom of the shale interval.The development of marine shale gas reservoir in Well A is controlled by mineral compositions,the development characteristics of organic matter.TOC is not only a major intrinsic factor controlling the lower reservoir interval,but also an important material to provide the reservoir space for the shale gas.In the late diagenetic stage,the clay mineral assemblages changed from montmorillonite to illite,forming new micropores and leading to an increase of the porosity and great influences on the upper reservoir interval.The content of brittle minerals is more than 50%,facilitating formation of fractures,and the permeability performance can be enhanced significantly.In general,the Wufeng Formation and the bottom interval of the Longmaxi Formation are both high-quality reservoirs and primary pay intervals.

Diagenesis of shale and its control on pore structure, a case study from typical marine, transitional and continental shales

Due to discrepancies in pore structure,the productivity of shale gas reservoirs under different diagenesis stages varies greatly.This study discussed the controlling of sedimentation and diagenesis on shale pore structure in typical marine,transitional,and continental shales,respectively.Continental shale samples from the Shuinan Formation,Jiaolai Basin,transitional shale samples from the Taiyuan,Shanxi and Xiashihezi Formations,Ordos Basin,and marine shale samples from the Longmaxi Formation,Sichuan Basin,were collected.Scanning electron microscope with argon ion polishing,high-pressure mercury injection,and low-temperature nitrogen adsorption experiments were conducted to acquire pore structure parameters.And the diagenetic stage of the reservoir was classified according to thermal maturity,organic geochemical parameters,and mineral composition.Our results exhibit that continental,transitional,and marine shales are period A,period B of the middle diagenetic stage,and the late diagenetic stage,respectively.For pore structure,micropore(0–2 nm)and mesopore(2–50 nm)controlled pore volume and specific surface area of transitional and marine shales,and specific surface area of continental shale have similar results,while micropore,mesopore,and macropore(>50 nm)all have a significant proportion of pore volume in continental shale.The pore structure characteristics and controlling factors exhibit a pronounced difference in different diagenesis stages,the compaction and cementation in period A of the middle diagenesis stage is relatively weak,intergranular pore and interlayer pore of clay minerals are well preserved,and moldic pore and dissolved pore developed as well;organic matter is in high maturity in period B of the middle diagenesis stage,organic matter pore developed correspondingly,while the intergranular pore developed poorly affected by compaction,notably,the carbonate is negligible in transitional shale,and the interlayer pore of clay minerals are well preserved with weak cementation;while dissolution and metasomatism controlled the pore structure in the late diagenesis stage in marine shale,the primary pores were poorly preserved,and the organic matter pore and carbonate dissolved pore developed.Results from this work are of a specific reference for shale gas development under different diagenesis stages.