We introduce a factorized Smith method(FSM)for solving large-scale highranked T-Stein equations within the banded-plus-low-rank structure framework.To effectively reduce both computational complexity and storage requi...We introduce a factorized Smith method(FSM)for solving large-scale highranked T-Stein equations within the banded-plus-low-rank structure framework.To effectively reduce both computational complexity and storage requirements,we develop techniques including deflation and shift,partial truncation and compression,as well as redesign the residual computation and termination condition.Numerical examples demonstrate that the FSM outperforms the Smith method implemented with a hierarchical HODLR structured toolkit in terms of CPU time.展开更多
Pores and fractures and their connectivity play a significant role in coalbed methane production.To investigate the growth characteristics and connectivity of pores and fractures in coal parallel and perpendicular to ...Pores and fractures and their connectivity play a significant role in coalbed methane production.To investigate the growth characteristics and connectivity of pores and fractures in coal parallel and perpendicular to the bedding plane,the pores and fractures of high-rank coal samples collected from the southern Qinshui Basin were measured by low-field nuclear magnetic resonance,X-ray-computed tomography and field emission scanning electron microscopy.Then,the determinants of their connectivity were further discussed.The results show that the high-rank coal samples have similar pore size distributions both parallel and perpendicular to the bedding plane.They primarily contain mesopores(2-50 nm in width),followed by macrospores(> 50 nm in width).The research indicated that the high-rank coal connectivity parallel to the bedding plane is significantly better than that perpendicular to the bedding plane.The connectivity of high-rank coal is mainly determined by throats,and the orientation of the pores and fractures.The two connectivity modes in high-rank coal are "pore connectivity," in which the throats are mainly pores with a low coordination number,and "microfissure connectivity",in which the throats are mainly microfissures with a high coordination number.展开更多
In this paper, influences on the reservoir permeability, the reservoir architecture and the fluid flow pattern caused by hydraulic fracturing are analyzed. Based on the structure and production fluid flow model of pos...In this paper, influences on the reservoir permeability, the reservoir architecture and the fluid flow pattern caused by hydraulic fracturing are analyzed. Based on the structure and production fluid flow model of post fracturing high-rank coal reservoir, Warren-Root Model is improved. A new physical model that is more suitable for post fracturing high-rank coal reservoir is established. The results show that the width, the flow conductivity and the permeability of hydraulic fractures are much larger than natural fractures in coal bed reservoir. Hydraulic fracture changes the flow pattern of gas and flow channel to wellbore, thus should be treated as an independent medium. Warrant-Root Model has some limitations and can’t give a comprehensive interpretation of seepage mechanism in post fracturing high-rank coal reservoir. Modified Warrant-Root Model simplifies coal bed reservoir to an ideal system with hydraulic fracture, orthogonal macroscopic fracture and cuboid matrix. Hydraulic fracture is double wing, vertical and symmetric to wellbore. Coal bed reservoir is divided into cuboids by hydraulic fracture and further by macroscopic fractures. Flow behaviors in coal bed reservoir are simplified to three step flows of gas and two step flows of water. The swap mode of methane between coal matrix and macroscopic fractures is pseudo steady fluid channeling. The flow behaviors of methane to wellbore no longer follow Darcy’s Law and are mainly affected by inertia force. The flow pattern of water follows Darcy’s Law. The new physical model is more suitable for post fracturing high-rank coal reservoir.展开更多
Field geological work, field engineering monitoring, laboratory experiments and numerical simulation were used to study the development characteristics of pore-fracture system and hydraulic fracture of No.3 coal reser...Field geological work, field engineering monitoring, laboratory experiments and numerical simulation were used to study the development characteristics of pore-fracture system and hydraulic fracture of No.3 coal reservoir in Southern Qinshui Basin. Flow patterns of methane and water in pore-fracture system and hydraulic fracture were discussed by using limit method and average method. Based on the structure model and flow pattern of post-fracturing high-rank coal reservoir, flow patterns of methane and water were established. Results show that seepage pattern of methane in pore-fracture system is linked with pore diameter, fracture width, coal bed pressure and flow velocity. While in hydraulic fracture, it is controlled by fracture height, pressure and flow velocity. Seepage pattern of water in pore-fracture system is linked with pore diameter, fracture width and flow velocity. While in hydraulic fracture, it is controlled by fracture height and flow velocity. Pores and fractures in different sizes are linked up by ultramicroscopic fissures, micro-fissures and hydraulic fracture. In post-fracturing high-rank coal reservoir, methane has level-three flow and gets through triple medium to the wellbore; and water passes mainly through double medium to the wellbore which is level-two flow.展开更多
In terms of the coal reservoir permeability of effective stress, coal matrix shrinkage and gas slippage,we conduct the tests of gas permeability under constant confining pressure and effective stress, as well as illus...In terms of the coal reservoir permeability of effective stress, coal matrix shrinkage and gas slippage,we conduct the tests of gas permeability under constant confining pressure and effective stress, as well as illustrate the cumulating method of permeability increment caused by the effects of gas slippage and coal matrix shrinkage.The results show that under the constant confining pressure, gas slippage affecting coal permeability changes to effective stress affecting it mainly. The change point increases with the increase of the confining pressure. The gas slippage effect leads to high permeability under low confining pressure, but coal matrix expansion results in the low value as confining and gas pressures increase. Combined with the drainage process of coalbed methane(CBM)well, the permeability is divided into four change stages based on the above analysis about the three effects, which can improve the change regulation understanding. Four stages are the downward phase under effective stress,the conversion phase of effective stress-coal matrix contraction effect(mainly based on effective stress), the rising stage of the effective stress-coal matrix contraction effect(mainly based on coal matrix contraction effect) and the rising phase of coal matrix contraction-slippage effect(mainly based on slippage effect). Permeability of coal reservoir during the process of drainage and production goes through four stages.展开更多
Coal reservoirs in the Qinshui Basin are characterized by high thermal evolution degree,low permeability,low reservoir pressure,lower gas saturation and strong heterogeneity,so its coalbed methane(CBM)development is q...Coal reservoirs in the Qinshui Basin are characterized by high thermal evolution degree,low permeability,low reservoir pressure,lower gas saturation and strong heterogeneity,so its coalbed methane(CBM)development is quite difficult.In this paper,the development practice of highrank CBM horizontal wells in the Fanzhuang-Zhengzhuang Block was analyzed in terms of geological and engineering factors to clarify the productivity influencing factors,suitable geological conditions and potential tapping countermeasures of multi-lateral horizontal wells.It is shown that the reasons for the low development efficiency of multi-lateral horizontal wells are divided into three types.The first one is geological factor,such as encountering low CBM content areas or faults.The second is engineering factor,such as drilling fluid plugging,drilling collapse,drainage collapse and dust coal blockage.The third is the combination of both factors.It is concluded that encountering low CBM content areas and faults,collapse and blockage are the main reasons for the low production of multi-lateral horizontal wells in the Zhengzhuang-Fanzhuang Block,with the CBM content higher than 20 m^(3)/t,the ratio of critical desorption pressure and reservoir pressure higher than 0.7 and vitrinite reflectance(R_(o))higher than 3.8.The prerequisite for an open-hole horizontal well in this area to produce CBM at a high rate is that the well lies in the tensile stress zone.Finally,the countermeasures to tap the potential of some inefficient wells were put forward.First,the inefficient wells which are blocked with dust coal or collapsed in the later stage should be stimulated based on classifications.And second,it is necessary to explore new types of horizontal wells so as to deal with borehole collapse and continue the operation in the later stage by using tree-like roof horizontal wells,single-lateral horizontal wells with casing or screen completion and fish-bone horizontal wells.展开更多
A desorption simulation experiment with the condition of simulated strata was designed. The experiment, under different depressurizing rates and the same fluid saturation, was conducted on the sample from 3# coal of D...A desorption simulation experiment with the condition of simulated strata was designed. The experiment, under different depressurizing rates and the same fluid saturation, was conducted on the sample from 3# coal of Daning coal mine in Jincheng, Shanxi Province. The gas production rate and pressure change at both ends of the sample were studied systematically, and the mechanisms of some phenomena in the experiment were discussed. The experimental results show that, whether at fast or slow depressurizing rate, the methane adsorbed to high-rank coal can effectively desorb and the desorption efficiency can reach above 90%. There is an obvious inflection point on the gas yield curve during the desorption process and it appears after the pressure on the lump of coal reduces below the desorption pressure. The desorption of methane from high-rank coal is mainly driven by differential pressure, and high pressure difference is conducive to fast desorption. In the scenario of fast depressurization, the desorption inflection appears earlier and the gas production rate in the stage of rapid desorption is higher. It is experimentally concluded that the originally recognized strategy of long-term slow CBM production is doubtful and the economic benefit of CBM exploitation from high-rank coal can be effectively improved by rapid drainage and pressure reduction. The field experiment results in pilot blocks of Fanzhuang and Zhengzhuang show that by increasing the drainage depressurization rate, the peak production of gas well would increase greatly, the time of gas well to reach the economic production shortened, the average time for a gas well to reach expected production reduced by half, and the peak gas production is higher.展开更多
Low average single-well production resulting in low economic benefit has become the main bottleneck of the CBM gas development in China.So it is significant to choose suitable efficient development technologies based ...Low average single-well production resulting in low economic benefit has become the main bottleneck of the CBM gas development in China.So it is significant to choose suitable efficient development technologies based on CBM geological factors for high rank CBM recovery enhancement.In view of this,CBM geological factors were analyzed,different geological models were established and the corresponding models of development engineering technologies were thus put forward.It was proposed that the four main factors affecting high rank CBM recovery from a lower degree to a higher degree respectively include coal texture,rank of coal metamorphism,in-situ stress,and the ratio of critical desorption pressure to initial reservoir pressure.On this basis,four engineering geological models were classified as follows:vertical well,open-hole multilateral horizontal well,U-shaped and roof tree-like horizontal wells,and fish-bone and L-shaped wells.It is concluded that the former two models are more adaptable in such areas with better coal texture and high degree of thermal maturity,while the latter two are commonly applied in a wide range of areas.展开更多
In order to discuss the effect of tectonic stress on the structural evolution of coal, given the importance attached to High-resolution Transmission Electron Microscopy (HTEM), we investigated several aspects of mater...In order to discuss the effect of tectonic stress on the structural evolution of coal, given the importance attached to High-resolution Transmission Electron Microscopy (HTEM), we investigated several aspects of material structures of high-rank Carboniferous period coal, located in the northern foreland basin of the Dabie orogenic belt in eastern China. High powered crystal lattice images of Bright Fields (BF) and Selected Area Diffraction patterns (SAD) of different types of metamorphism in coal were obtained. The results show that the Basic Structural Units (BSU) become increasingly more compact as a function of rising tem-perature and pressure. Under pressure, the local orientation of molecules is strengthened, the arrangement of BSU speeds up and the degree of order is clearly enhanced.展开更多
To study the effects of CBM （coal bed methane） temperature-rising desorption, isothermal adsorption/desorption experiments on three ranks （anthracite, coking coal and lignite） of coal at different temperatures wer...To study the effects of CBM （coal bed methane） temperature-rising desorption, isothermal adsorption/desorption experiments on three ranks （anthracite, coking coal and lignite） of coal at different temperatures were designed based on the traditional CBM decompression desorption. The experimental results indicate that temperature-rising desorption is more effec- tive in high-rank coal, and ever-increasing temperature of high-rank coal reservoir can reduce the negative effects of coal ma- trix shrinkage in the process of production and improve the permeability of the coal reservoir as well. It is also revealed that the technique of temperature-rising desorption applied in higher-rank coal reservoir can enhance CBM recovery ratio. This study provided theoretical support for the application of temperature-rising desorption technique in practical discharging and mining projects, which can effectively tackle the gas production bottleneck problem.展开更多
There are abundant high-rank coal bed methane(CBM)resources in China,accounting for one third of total CBM resources.Its efficient development and utilization is of great significance to guarantee the national energy ...There are abundant high-rank coal bed methane(CBM)resources in China,accounting for one third of total CBM resources.Its efficient development and utilization is of great significance to guarantee the national energy strategic security,diminish the hidden danger of coal mine production and reduce carbon dioxide emission.In order to solve the"four lows"problem(i.e.,low effective utilization ratio of proved reserves,low productivity targeting ratio,low single-well production rate and low development profit)restricting the development of high-rank CBM industry in China,this paper deeply analyzes the core problems restricting the development of high-rank CBM.Based on this,several new methods of production control,area selection and evaluation are put forward by taking multiple measures,such as paying the same attention on theoretical research and technological research&development,carrying out laboratory research and field test in parallel and conducting large scale construction and benefit development simultaneously.And the following research results are obtained.First,the geological difference between CBM and coal mine,the difference in reserves recoverability,the adaptability of engineering technology and the scientificity of production are the main factors restricting CBM development effect.Second,"Four-element"production control theory,methane-leading en gineering transformation method and methane-leading production control theory are proposed,which provides guidance for the development of a series of technologies for the efficient development of high-rank CBM.Third,in practice,the control degree of quality reserves is increased from 32%to 80%,the success ratio of development wells is increased from 60%to 95%,the average single-well daily gas production of vertical wells is increased by about 1100 m^(3),the drilling cost of horizontal wells is reduced by 50%,and the operation cost per cubic meter of gas is reduced by 24%.In conclusion,the established technology series for the efficient development of high-rank CBM actively promote the efficient CBMdevelopment in the Qinshui Basin.The yearly CBM production of PetroChina Huabei Oilfield Company is expected to reach 20108 m^(3) in the middle of the"14th Five-Year Plan",which promotes the strategic development of CBM industry in China.展开更多
The physical characteristics of coal reservoirs are important for evaluating the potential for gas desorption, diffusion, and seepage during coalbed methane （CBM） production, and influence the performance of CBM wel...The physical characteristics of coal reservoirs are important for evaluating the potential for gas desorption, diffusion, and seepage during coalbed methane （CBM） production, and influence the performance of CBM wells. Based on data from mercury injection experiments, low-temperature liquid nitrogen adsorption, isothermal adsorption, initial velocity tests of methane diffusion, and gas natural desorption data from a CBM field, herein the physical characteristics of reservoirs of high-rank coals with different coal-body structures are described, including porosity, adsorption/desorption, diffusion, and seepage. Geometric models are constructed for these reservoirs. The modes of diffusion are discussed and a comprehensive diffusion-seepage model is constructed. The following conclusions were obtained. First, the pore distribution of tectonically deformed coal is different from that of normal coal. Compared to normal coal, all types of pore, including micropores （〈10 nm）, transitional pores （10-100 nm）, mesopores （100-1000 nm）, and macropores （〉1000 nm）, are more abundant in tectonically deformed coal, especially mesopores and macropores. The increase in pore abundance is greater with increasing tectonic deformation of coal; in addition, the pore connectivity is altered. These are the key factors causing differences in other reservoir physical characteristics, such as adsorption/desorption and diffusion in coals with different coal-body structures. Second, normal and cataclastic coals mainly contain micropores. The lack of macropores and its bad connectivity limit gas desorption and diffusion during the early stage of CBM production. However, the good connectivity of micropores is favorable for gas desorption and diffusion in later gas production stage. Thus, because of the slow decline in the rate of gas desorption, long-term gas production can easily be obtained from these reservoirs. Third, under natural conditions the adsorption/desorption properties of granulated and mylonitized coal are good, and the diffusion ability is also enhanced. However, for in situ reservoir conditions, the high dependence of reservoir permeability on stress results in a weak seepage of gas; thus, desorption and diffusion is limited. Fourth, during gas production, the pore range in which transitional diffusion takes place always increases, but that for Fick diffusion decreases. This is a reason for the reduction in diffusion capacity, in which micropores and transitional pores are the primary factors limiting gas diffusion. Finally, the proposed comprehensive model of CBM production under in situ reservoir conditions elucidates the key factors limiting gas production, which is helpful for selection of reservoir stimulation methods.展开更多
During the coalbed methane(CBM)exploitation,the reservoir permeability can be affected by the effective stress that varies with the reservoir fluid pressure,which is a complex,dynamic and significant engineering probl...During the coalbed methane(CBM)exploitation,the reservoir permeability can be affected by the effective stress that varies with the reservoir fluid pressure,which is a complex,dynamic and significant engineering problem.To analyze the response characteristics of the pore-fracture system by the changing stress,this work simulated reservoir and fluid pressures during the exploitation by adjusting confining pressure and displacement pressure.Stress sensitivity experiments under different effective stresses were conducted to systematically study the stage variation characteristics of porosity and permeability of coal.The results show that the permeability decreases exponentially with the increase in effective stress,consistent with previous studies.However,the porosity shows a V-shaped trend,which is different from the traditional understanding that it would decrease continuously with rising effective stress.These variation characteristics(of porosity and permeability above)therefore result in a phased porosity sensitivity of coal permeability(PPS).Moreover,the stress sensitivity of the samples was evaluated using the permeability damage rate method(MPDR)and the stress sensitivity coefficient method(MCSS),both of which showed that it ranges from the degree of strong to extremely strong.When the effective stress is lower than 5–6 MPa,the stress sensitivity of the coal reservoir drops rapidly with effective stress rising;when it is higher than 5–6 MPa,the change in stress sensitivity tends to flatten out,and the stress sensitivity coefficient(CSS)goes down slowly with rising effective stress.Finally,suggestions are proposed for the drainage scheme of CBM wells based on the experimental results.展开更多
Acoustic testing is a widely used technique to measure the coal mechanical properties under high temperature and pressure in situ conditions.This study compared the acoustic wave characteristics of briquette and raw c...Acoustic testing is a widely used technique to measure the coal mechanical properties under high temperature and pressure in situ conditions.This study compared the acoustic wave characteristics of briquette and raw coal under various temperature and pressure conditions.The results show that the longitudinal wave velocity(Vp)decreases with an increasing vitrinite content.A large number of the vitrinite content enhances the process in which the temperature and pressure changed the Vp.The Vp of briquette decreases approximately linearly with the temperature compared to raw coal.The Vp of raw coal experiences initially a rapid,then gradual,and finally the moderate increasing trend with the increase in confining pressure.However,in briquette,the Vp increases approximately linearly with the confining pressure.The results indicate that the Vp is more sensitive to temperature under low confining pressure and peaks at 50℃−60℃ than high confining pressure.However,the Vp is less sensitive to temperature under higher confining pressure,and the positive effect of high confining pressure is dominant.Understanding the mechanical properties of coal under high pressure and temperature develops better insight into coalbed methane(CBM)exploration from deep reservoirs.展开更多
基金Supported partly by NSF of China(Grant No.11801163)NSF of Hunan Province(Grant Nos.2021JJ50032,2023JJ50164 and 2023JJ50165)Degree&Postgraduate Reform Project of Hunan University of Technology and Hunan Province(Grant Nos.JGYB23009 and 2024JGYB210).
文摘We introduce a factorized Smith method(FSM)for solving large-scale highranked T-Stein equations within the banded-plus-low-rank structure framework.To effectively reduce both computational complexity and storage requirements,we develop techniques including deflation and shift,partial truncation and compression,as well as redesign the residual computation and termination condition.Numerical examples demonstrate that the FSM outperforms the Smith method implemented with a hierarchical HODLR structured toolkit in terms of CPU time.
基金supported by the Fundamental Research Funds for the Central Universities (No.2017XKQY017)。
文摘Pores and fractures and their connectivity play a significant role in coalbed methane production.To investigate the growth characteristics and connectivity of pores and fractures in coal parallel and perpendicular to the bedding plane,the pores and fractures of high-rank coal samples collected from the southern Qinshui Basin were measured by low-field nuclear magnetic resonance,X-ray-computed tomography and field emission scanning electron microscopy.Then,the determinants of their connectivity were further discussed.The results show that the high-rank coal samples have similar pore size distributions both parallel and perpendicular to the bedding plane.They primarily contain mesopores(2-50 nm in width),followed by macrospores(> 50 nm in width).The research indicated that the high-rank coal connectivity parallel to the bedding plane is significantly better than that perpendicular to the bedding plane.The connectivity of high-rank coal is mainly determined by throats,and the orientation of the pores and fractures.The two connectivity modes in high-rank coal are "pore connectivity," in which the throats are mainly pores with a low coordination number,and "microfissure connectivity",in which the throats are mainly microfissures with a high coordination number.
基金supported by the National Natural Science Foundation of China (Nos. 41330638, 41272154, 51325403, 51104148, and 51204162)the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD)Jiangsu Planned Projects for Postdoctoral Research Funds (No. 1302049B)
文摘In this paper, influences on the reservoir permeability, the reservoir architecture and the fluid flow pattern caused by hydraulic fracturing are analyzed. Based on the structure and production fluid flow model of post fracturing high-rank coal reservoir, Warren-Root Model is improved. A new physical model that is more suitable for post fracturing high-rank coal reservoir is established. The results show that the width, the flow conductivity and the permeability of hydraulic fractures are much larger than natural fractures in coal bed reservoir. Hydraulic fracture changes the flow pattern of gas and flow channel to wellbore, thus should be treated as an independent medium. Warrant-Root Model has some limitations and can’t give a comprehensive interpretation of seepage mechanism in post fracturing high-rank coal reservoir. Modified Warrant-Root Model simplifies coal bed reservoir to an ideal system with hydraulic fracture, orthogonal macroscopic fracture and cuboid matrix. Hydraulic fracture is double wing, vertical and symmetric to wellbore. Coal bed reservoir is divided into cuboids by hydraulic fracture and further by macroscopic fractures. Flow behaviors in coal bed reservoir are simplified to three step flows of gas and two step flows of water. The swap mode of methane between coal matrix and macroscopic fractures is pseudo steady fluid channeling. The flow behaviors of methane to wellbore no longer follow Darcy’s Law and are mainly affected by inertia force. The flow pattern of water follows Darcy’s Law. The new physical model is more suitable for post fracturing high-rank coal reservoir.
基金Projects(41330638,41272154)supported by the National Natural Science Foundation of ChinaProject supported by the Priority Academic Program Development of Jiangsu Higher Education Institutions(PAPD),ChinaProject(2014M551705)supported by the China Postdoctoral Science Foundation
文摘Field geological work, field engineering monitoring, laboratory experiments and numerical simulation were used to study the development characteristics of pore-fracture system and hydraulic fracture of No.3 coal reservoir in Southern Qinshui Basin. Flow patterns of methane and water in pore-fracture system and hydraulic fracture were discussed by using limit method and average method. Based on the structure model and flow pattern of post-fracturing high-rank coal reservoir, flow patterns of methane and water were established. Results show that seepage pattern of methane in pore-fracture system is linked with pore diameter, fracture width, coal bed pressure and flow velocity. While in hydraulic fracture, it is controlled by fracture height, pressure and flow velocity. Seepage pattern of water in pore-fracture system is linked with pore diameter, fracture width and flow velocity. While in hydraulic fracture, it is controlled by fracture height and flow velocity. Pores and fractures in different sizes are linked up by ultramicroscopic fissures, micro-fissures and hydraulic fracture. In post-fracturing high-rank coal reservoir, methane has level-three flow and gets through triple medium to the wellbore; and water passes mainly through double medium to the wellbore which is level-two flow.
基金the National Basic Research Program(973)of China(No.2009cb219605)
文摘In terms of the coal reservoir permeability of effective stress, coal matrix shrinkage and gas slippage,we conduct the tests of gas permeability under constant confining pressure and effective stress, as well as illustrate the cumulating method of permeability increment caused by the effects of gas slippage and coal matrix shrinkage.The results show that under the constant confining pressure, gas slippage affecting coal permeability changes to effective stress affecting it mainly. The change point increases with the increase of the confining pressure. The gas slippage effect leads to high permeability under low confining pressure, but coal matrix expansion results in the low value as confining and gas pressures increase. Combined with the drainage process of coalbed methane(CBM)well, the permeability is divided into four change stages based on the above analysis about the three effects, which can improve the change regulation understanding. Four stages are the downward phase under effective stress,the conversion phase of effective stress-coal matrix contraction effect(mainly based on effective stress), the rising stage of the effective stress-coal matrix contraction effect(mainly based on coal matrix contraction effect) and the rising phase of coal matrix contraction-slippage effect(mainly based on slippage effect). Permeability of coal reservoir during the process of drainage and production goes through four stages.
文摘Coal reservoirs in the Qinshui Basin are characterized by high thermal evolution degree,low permeability,low reservoir pressure,lower gas saturation and strong heterogeneity,so its coalbed methane(CBM)development is quite difficult.In this paper,the development practice of highrank CBM horizontal wells in the Fanzhuang-Zhengzhuang Block was analyzed in terms of geological and engineering factors to clarify the productivity influencing factors,suitable geological conditions and potential tapping countermeasures of multi-lateral horizontal wells.It is shown that the reasons for the low development efficiency of multi-lateral horizontal wells are divided into three types.The first one is geological factor,such as encountering low CBM content areas or faults.The second is engineering factor,such as drilling fluid plugging,drilling collapse,drainage collapse and dust coal blockage.The third is the combination of both factors.It is concluded that encountering low CBM content areas and faults,collapse and blockage are the main reasons for the low production of multi-lateral horizontal wells in the Zhengzhuang-Fanzhuang Block,with the CBM content higher than 20 m^(3)/t,the ratio of critical desorption pressure and reservoir pressure higher than 0.7 and vitrinite reflectance(R_(o))higher than 3.8.The prerequisite for an open-hole horizontal well in this area to produce CBM at a high rate is that the well lies in the tensile stress zone.Finally,the countermeasures to tap the potential of some inefficient wells were put forward.First,the inefficient wells which are blocked with dust coal or collapsed in the later stage should be stimulated based on classifications.And second,it is necessary to explore new types of horizontal wells so as to deal with borehole collapse and continue the operation in the later stage by using tree-like roof horizontal wells,single-lateral horizontal wells with casing or screen completion and fish-bone horizontal wells.
基金Supported by the China National Science and Technology Major Project(2017ZX05064)
文摘A desorption simulation experiment with the condition of simulated strata was designed. The experiment, under different depressurizing rates and the same fluid saturation, was conducted on the sample from 3# coal of Daning coal mine in Jincheng, Shanxi Province. The gas production rate and pressure change at both ends of the sample were studied systematically, and the mechanisms of some phenomena in the experiment were discussed. The experimental results show that, whether at fast or slow depressurizing rate, the methane adsorbed to high-rank coal can effectively desorb and the desorption efficiency can reach above 90%. There is an obvious inflection point on the gas yield curve during the desorption process and it appears after the pressure on the lump of coal reduces below the desorption pressure. The desorption of methane from high-rank coal is mainly driven by differential pressure, and high pressure difference is conducive to fast desorption. In the scenario of fast depressurization, the desorption inflection appears earlier and the gas production rate in the stage of rapid desorption is higher. It is experimentally concluded that the originally recognized strategy of long-term slow CBM production is doubtful and the economic benefit of CBM exploitation from high-rank coal can be effectively improved by rapid drainage and pressure reduction. The field experiment results in pilot blocks of Fanzhuang and Zhengzhuang show that by increasing the drainage depressurization rate, the peak production of gas well would increase greatly, the time of gas well to reach the economic production shortened, the average time for a gas well to reach expected production reduced by half, and the peak gas production is higher.
基金Project supported by the National Major Science and Technology Project“Demonstrative horizontal well development of CBM in Qinshui Basin,Shanxi”(No.:2011ZX05061)PetroChina Major Science and Technology Project“Demonstrative exploration and development of CBM in Qinshui Basin”(No.:2010E-2208)PetroChina Huabei Oilfield Science and Technology Project“Research on efficient development technologies for Mabe East block”(No.:2016-HB-M06).
文摘Low average single-well production resulting in low economic benefit has become the main bottleneck of the CBM gas development in China.So it is significant to choose suitable efficient development technologies based on CBM geological factors for high rank CBM recovery enhancement.In view of this,CBM geological factors were analyzed,different geological models were established and the corresponding models of development engineering technologies were thus put forward.It was proposed that the four main factors affecting high rank CBM recovery from a lower degree to a higher degree respectively include coal texture,rank of coal metamorphism,in-situ stress,and the ratio of critical desorption pressure to initial reservoir pressure.On this basis,four engineering geological models were classified as follows:vertical well,open-hole multilateral horizontal well,U-shaped and roof tree-like horizontal wells,and fish-bone and L-shaped wells.It is concluded that the former two models are more adaptable in such areas with better coal texture and high degree of thermal maturity,while the latter two are commonly applied in a wide range of areas.
基金support for this work, provided by the National Natural Science Foundation of China (No40872105)the Scientific Research Foundation of the North China Institute of Science Technology (NoA08002)
文摘In order to discuss the effect of tectonic stress on the structural evolution of coal, given the importance attached to High-resolution Transmission Electron Microscopy (HTEM), we investigated several aspects of material structures of high-rank Carboniferous period coal, located in the northern foreland basin of the Dabie orogenic belt in eastern China. High powered crystal lattice images of Bright Fields (BF) and Selected Area Diffraction patterns (SAD) of different types of metamorphism in coal were obtained. The results show that the Basic Structural Units (BSU) become increasingly more compact as a function of rising tem-perature and pressure. Under pressure, the local orientation of molecules is strengthened, the arrangement of BSU speeds up and the degree of order is clearly enhanced.
文摘To study the effects of CBM （coal bed methane） temperature-rising desorption, isothermal adsorption/desorption experiments on three ranks （anthracite, coking coal and lignite） of coal at different temperatures were designed based on the traditional CBM decompression desorption. The experimental results indicate that temperature-rising desorption is more effec- tive in high-rank coal, and ever-increasing temperature of high-rank coal reservoir can reduce the negative effects of coal ma- trix shrinkage in the process of production and improve the permeability of the coal reservoir as well. It is also revealed that the technique of temperature-rising desorption applied in higher-rank coal reservoir can enhance CBM recovery ratio. This study provided theoretical support for the application of temperature-rising desorption technique in practical discharging and mining projects, which can effectively tackle the gas production bottleneck problem.
基金Scientific and Technological Project of China National Petroleum Exploration and Production Corporation"Research on North China Coalbed Methane Reserve Utilization and Beneficial Development Technology"(kt2021-10-07)China National Petroleum North China Technology Project of Oilfield Company"Geological Design Technology of High Coalbed Methane Horizontal Well and Implementation Tracking Research"(2022-HB-M03).
文摘There are abundant high-rank coal bed methane(CBM)resources in China,accounting for one third of total CBM resources.Its efficient development and utilization is of great significance to guarantee the national energy strategic security,diminish the hidden danger of coal mine production and reduce carbon dioxide emission.In order to solve the"four lows"problem(i.e.,low effective utilization ratio of proved reserves,low productivity targeting ratio,low single-well production rate and low development profit)restricting the development of high-rank CBM industry in China,this paper deeply analyzes the core problems restricting the development of high-rank CBM.Based on this,several new methods of production control,area selection and evaluation are put forward by taking multiple measures,such as paying the same attention on theoretical research and technological research&development,carrying out laboratory research and field test in parallel and conducting large scale construction and benefit development simultaneously.And the following research results are obtained.First,the geological difference between CBM and coal mine,the difference in reserves recoverability,the adaptability of engineering technology and the scientificity of production are the main factors restricting CBM development effect.Second,"Four-element"production control theory,methane-leading en gineering transformation method and methane-leading production control theory are proposed,which provides guidance for the development of a series of technologies for the efficient development of high-rank CBM.Third,in practice,the control degree of quality reserves is increased from 32%to 80%,the success ratio of development wells is increased from 60%to 95%,the average single-well daily gas production of vertical wells is increased by about 1100 m^(3),the drilling cost of horizontal wells is reduced by 50%,and the operation cost per cubic meter of gas is reduced by 24%.In conclusion,the established technology series for the efficient development of high-rank CBM actively promote the efficient CBMdevelopment in the Qinshui Basin.The yearly CBM production of PetroChina Huabei Oilfield Company is expected to reach 20108 m^(3) in the middle of the"14th Five-Year Plan",which promotes the strategic development of CBM industry in China.
基金supported by the National Natural Science Foundation of China(Grant No.41372162)the Science and Technology Innovation Team Support Plan of Henan Province(Grant No.14IRTSTHN002)
文摘The physical characteristics of coal reservoirs are important for evaluating the potential for gas desorption, diffusion, and seepage during coalbed methane （CBM） production, and influence the performance of CBM wells. Based on data from mercury injection experiments, low-temperature liquid nitrogen adsorption, isothermal adsorption, initial velocity tests of methane diffusion, and gas natural desorption data from a CBM field, herein the physical characteristics of reservoirs of high-rank coals with different coal-body structures are described, including porosity, adsorption/desorption, diffusion, and seepage. Geometric models are constructed for these reservoirs. The modes of diffusion are discussed and a comprehensive diffusion-seepage model is constructed. The following conclusions were obtained. First, the pore distribution of tectonically deformed coal is different from that of normal coal. Compared to normal coal, all types of pore, including micropores （〈10 nm）, transitional pores （10-100 nm）, mesopores （100-1000 nm）, and macropores （〉1000 nm）, are more abundant in tectonically deformed coal, especially mesopores and macropores. The increase in pore abundance is greater with increasing tectonic deformation of coal; in addition, the pore connectivity is altered. These are the key factors causing differences in other reservoir physical characteristics, such as adsorption/desorption and diffusion in coals with different coal-body structures. Second, normal and cataclastic coals mainly contain micropores. The lack of macropores and its bad connectivity limit gas desorption and diffusion during the early stage of CBM production. However, the good connectivity of micropores is favorable for gas desorption and diffusion in later gas production stage. Thus, because of the slow decline in the rate of gas desorption, long-term gas production can easily be obtained from these reservoirs. Third, under natural conditions the adsorption/desorption properties of granulated and mylonitized coal are good, and the diffusion ability is also enhanced. However, for in situ reservoir conditions, the high dependence of reservoir permeability on stress results in a weak seepage of gas; thus, desorption and diffusion is limited. Fourth, during gas production, the pore range in which transitional diffusion takes place always increases, but that for Fick diffusion decreases. This is a reason for the reduction in diffusion capacity, in which micropores and transitional pores are the primary factors limiting gas diffusion. Finally, the proposed comprehensive model of CBM production under in situ reservoir conditions elucidates the key factors limiting gas production, which is helpful for selection of reservoir stimulation methods.
基金This research was funded by the National Natural Science Foundation of China (Grant No. 41872170)the National Major Special Project of Science and Technology of China (No. 2016ZX05044)the Fundamental Research Funds for the Central Universities (No. 2020CXNL11).
文摘During the coalbed methane(CBM)exploitation,the reservoir permeability can be affected by the effective stress that varies with the reservoir fluid pressure,which is a complex,dynamic and significant engineering problem.To analyze the response characteristics of the pore-fracture system by the changing stress,this work simulated reservoir and fluid pressures during the exploitation by adjusting confining pressure and displacement pressure.Stress sensitivity experiments under different effective stresses were conducted to systematically study the stage variation characteristics of porosity and permeability of coal.The results show that the permeability decreases exponentially with the increase in effective stress,consistent with previous studies.However,the porosity shows a V-shaped trend,which is different from the traditional understanding that it would decrease continuously with rising effective stress.These variation characteristics(of porosity and permeability above)therefore result in a phased porosity sensitivity of coal permeability(PPS).Moreover,the stress sensitivity of the samples was evaluated using the permeability damage rate method(MPDR)and the stress sensitivity coefficient method(MCSS),both of which showed that it ranges from the degree of strong to extremely strong.When the effective stress is lower than 5–6 MPa,the stress sensitivity of the coal reservoir drops rapidly with effective stress rising;when it is higher than 5–6 MPa,the change in stress sensitivity tends to flatten out,and the stress sensitivity coefficient(CSS)goes down slowly with rising effective stress.Finally,suggestions are proposed for the drainage scheme of CBM wells based on the experimental results.
基金This research was financially supported by the National Natural Science Foundation of China(Grant Nos.42072191 and 42130802)the Shanxi Science and Technology Plan Announced Bidding Project(No.20201101003)+2 种基金the China Huaneng Group Scicence and Technology Project(No.HNKJ20-H87)the Qing Lan Project of Jiangsu ProvincePriority Academic Program Development of Jiangsu Higher Education Institutions(PPAD).
文摘Acoustic testing is a widely used technique to measure the coal mechanical properties under high temperature and pressure in situ conditions.This study compared the acoustic wave characteristics of briquette and raw coal under various temperature and pressure conditions.The results show that the longitudinal wave velocity(Vp)decreases with an increasing vitrinite content.A large number of the vitrinite content enhances the process in which the temperature and pressure changed the Vp.The Vp of briquette decreases approximately linearly with the temperature compared to raw coal.The Vp of raw coal experiences initially a rapid,then gradual,and finally the moderate increasing trend with the increase in confining pressure.However,in briquette,the Vp increases approximately linearly with the confining pressure.The results indicate that the Vp is more sensitive to temperature under low confining pressure and peaks at 50℃−60℃ than high confining pressure.However,the Vp is less sensitive to temperature under higher confining pressure,and the positive effect of high confining pressure is dominant.Understanding the mechanical properties of coal under high pressure and temperature develops better insight into coalbed methane(CBM)exploration from deep reservoirs.
