页岩气超临界吸附机理及模型
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摘要
页岩气吸附机理和模型的研究对于页岩气地质储量计算和开发方案编制等具有重要意义.结合低温氮气吸附实验和高压甲烷等温吸附实验,对龙马溪组页岩的微观孔隙结构和超临界吸附特征进行了分析.结果表明,孔隙体积和比表面积主要受中孔(2~200 nm)控制;在压力较大时(>10 MPa),页岩过剩吸附量随着压力增大而降低.4种常用吸附模型的对比分析表明:对于微孔充填方式,D-A模型的拟合效果要优于D-R模型;对于单层吸附方式,L-F模型的拟合效果要优于Langmuir模型.结合孔隙体积分析结果,通过两种假设,证明了甲烷不是以单一的微孔充填或者单分子层吸附方式在页岩中进行吸附的,推测其吸附机理为微孔充填和单分子层吸附并存.并基于该吸附机理,建立了页岩气超临界吸附新模型——DA-LF模型.新模型比4种常用模型具有更好的拟合效果,并可以分别计算出微孔和中孔的吸附量.计算表明微孔充填吸附量比单层吸附量大,占总吸附量的76%左右.这表明页岩气超临界吸附机理可以进一步深化为:微孔充填为主、单分子层吸附并存.
Shale gas is different from conventional natural gas stored in sandstone and carbonate formations because the shale formation is often both the source and the reservoir of the natural gas itself. The adsorbed gas accounts for 20%–85% of the total amount based on current studies in five major shale formations in the United States. Studying on the mechanism and model of shale gas adsorption is of great importance for reserve evaluation and making development plan. The micropore structure and isothermal adsorption curves of Longmaxi shales were analyzed by low-temperature N_2 adsorption and high-pressure CH_4 adsorption experiments. The results show that its specific surface area and pore volume are mainly controlled by mesopores(2–200 nm). The excess adsorption capacity(n_(ex)) increases to a maximum value at the pressure of approximate 10 MPa, and then begins to decline with pressure. This abnormal phenomenon has also been observed in some previous studies, but most researchers stated that the adsorption isotherm of methane in shale monotonically increased with pressure and reached a constant value at a high pressure(i.e., type Ⅰ isotherm). The main reason for these two different types of adsorption isotherms is whether the volume of the adsorbed phase is assumed to be negligible. To simulate the enrichment and production of methane in shale gas reservoirs, an accurate model of gas adsorption is needed. The most commonly used adsorption model for shale gas reservoirs is the classic Langmuir equation and its extended Langmuir-Freundlich(L-F) equation, which assumes that methane is a monolayer adsorbate and the surface of the solid adsorbent is homogeneous with constant adsorption heat. Many physical adsorption phenomena can be described by the Langmuir model, including coalbed methane adsorption, but the assumptions are too ideal to match the complicated situation of shale gas reservoir. Several multilayer adsorption theories also have been proposed and applied, such as the Dubinin-Radushkevich(D-R) and the Dubinin-Astakhov(D-A) equations based on micropore filling mechanism. The D-R and D-A adsorption model have been widely used in methane adsorption on shale because of its quite good fitting quality. The comparative analysis of the four commonly used adsorption models shows that the fitting effect of D-A model is better than D-R model for micropore filling mode, and the fitting effect of L-F model is better than Langmuir model for monolayer adsorption mode. Further through the two hypotheses, it is proved that the adsorption mode of methane is neither single micropore filling nor single monolayer adsorption, and the adsorption mechanism is the coexistence of micropore filling and monolayer adsorption. Based on the adsorption mechanism, a new model of supercritical adsorption of shale gas—DA-LF model was proposed. The new model has better fitting effect than the commonly used models, and can calculate the adsorption amount of micropores and mesopores, respectively. The results show that the adsorption capacity of micropore filling is larger than that of monolayer adsorption, which accounts for about 76% of the total adsorption capacity, indicating that the supercritical adsorption mechanism is mainly micropore filling and monolayer adsorption coexists.
Shale gas is different from conventional natural gas stored in sandstone and carbonate formations because the shale formation is often both the source and the reservoir of the natural gas itself. The adsorbed gas accounts for 20%–85% of the total amount based on current studies in five major shale formations in the United States. Studying on the mechanism and model of shale gas adsorption is of great importance for reserve evaluation and making development plan. The micropore structure and isothermal adsorption curves of Longmaxi shales were analyzed by low-temperature N_2 adsorption and high-pressure CH_4 adsorption experiments. The results show that its specific surface area and pore volume are mainly controlled by mesopores(2–200 nm). The excess adsorption capacity(n_(ex)) increases to a maximum value at the pressure of approximate 10 MPa, and then begins to decline with pressure. This abnormal phenomenon has also been observed in some previous studies, but most researchers stated that the adsorption isotherm of methane in shale monotonically increased with pressure and reached a constant value at a high pressure(i.e., type Ⅰ isotherm). The main reason for these two different types of adsorption isotherms is whether the volume of the adsorbed phase is assumed to be negligible. To simulate the enrichment and production of methane in shale gas reservoirs, an accurate model of gas adsorption is needed. The most commonly used adsorption model for shale gas reservoirs is the classic Langmuir equation and its extended Langmuir-Freundlich(L-F) equation, which assumes that methane is a monolayer adsorbate and the surface of the solid adsorbent is homogeneous with constant adsorption heat. Many physical adsorption phenomena can be described by the Langmuir model, including coalbed methane adsorption, but the assumptions are too ideal to match the complicated situation of shale gas reservoir. Several multilayer adsorption theories also have been proposed and applied, such as the Dubinin-Radushkevich(D-R) and the Dubinin-Astakhov(D-A) equations based on micropore filling mechanism. The D-R and D-A adsorption model have been widely used in methane adsorption on shale because of its quite good fitting quality. The comparative analysis of the four commonly used adsorption models shows that the fitting effect of D-A model is better than D-R model for micropore filling mode, and the fitting effect of L-F model is better than Langmuir model for monolayer adsorption mode. Further through the two hypotheses, it is proved that the adsorption mode of methane is neither single micropore filling nor single monolayer adsorption, and the adsorption mechanism is the coexistence of micropore filling and monolayer adsorption. Based on the adsorption mechanism, a new model of supercritical adsorption of shale gas—DA-LF model was proposed. The new model has better fitting effect than the commonly used models, and can calculate the adsorption amount of micropores and mesopores, respectively. The results show that the adsorption capacity of micropore filling is larger than that of monolayer adsorption, which accounts for about 76% of the total adsorption capacity, indicating that the supercritical adsorption mechanism is mainly micropore filling and monolayer adsorption coexists.
引文
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4 Zhou S W,Yan G,Xue H Q,et al.2D and 3D nanopore characterization of gas shale in Longmaxi formation based on FIB-SEM.Mar Pet Geol,2016,73:174-180
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6 Curtis J B.Fractured shale-gas systems.AAPG Bull,2002,86:1921-1938
7 Ambrose R J,Hartman R C,Diaz-Campos M,et al.Shale gas-in-place calculations Part I:New pore-scale considerations.SPE J,2012,17:219-229
8 Li Y X,Qiao D W,Jiang W L,et al.Gas content of gas-bearing shale and its geological evaluation summary(in Chinese).Geol Bull China,2011,30:308-317[李玉喜,乔德武,姜文利,等.页岩气含气量和页岩气地质评价综述.地质通报,2011,30:308-317]
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11 Liu H L,Wang H Y.Absorptivity and influential factors of marine shales in South China(in Chinese).Nat Gas Ind,2012,32:5-9[刘洪林,王红岩.中国南方海相页岩吸附特征及其影响因素.天然气工业,2012,32:5-9]
12 Xiong W,Guo W,Liu H L,et al.Shale reservoir characteristics and isothermal adsorption properties(in Chinese).Nat Gas Ind,2012,32:113-116[熊伟,郭为,刘洪林,等.页岩的储层特征以及等温吸附特征.天然气工业,2012,32:113-116]
13 Zhang T W,Ellis G S,Ruppel S C,et al.Effect of organic matter type and thermal maturity on methane adsorption in shale gas systems.Org Geochem,2012,47:120-131
14 Tian H,Li T F,Zhang T W,et al.Characterization of methane adsorption on overmature lower Silurian-upper Ordovician shales in Sichuan Basin,southwest China:Experimental results and geological implications.Int J Coal Geol,2016,156:36-49
15 Sakurovs R,Day S,Weir S,et al.Application of a modified Dubinin-Radushkevich equation to adsorption of gases by coals under supercritical conditions.Energy Fuels,2007,21:992-997
16 Clarkson C R,Haghshenas B.Modeling of supercritical fluid adsorption on organic-rich shales and coal.In:Proceedings of SPE Unconventional Resources Conference.The Woodlands:SPE,2013
17 Zhou L,Zhou Y,Li M,et al.Experimental and modeling study of the adsorption of supercritical methane on a high surface activated carbon.Langmuir,2000,16:5955-5959
18 Zhou L,LüC Z,Wang Y L,et al.Physisorption of gases on porous solids at above-critical temperatures(in Chinese).Prog Chem,1999,11:221-226[周理,吕昌忠,王怡林,等.述评超临界温度气体在多孔固体上的物理吸附.化学进展,1999,11:221-226]
19 Wang Z,Li Y,Guo P,et al.Analyzing the adaption of different adsorption models for describing the shale gas adsorption law.Chem Eng Tech,2016,39:1921-1932
20 Chareonsuppanimit P,Mohammad S A,Robinson R L,et al.High-pressure adsorption of gases on shales:Measurements and modeling.Int J Coal Geol,2012,95:34-46
21 Xiong J,Liu X J,Liang L X,et al.Improved Dubibin-Astakhov model for shale gas supercritical adsorption(in Chinese).Acta Pet Sin,2015,36:849-857[熊健,刘向君,梁利喜,等.页岩气超临界吸附的Dubibin-Astakhov改进模型.石油学报,2015,36:849-857]
22 Pini R,Ottiger S,Burlini L,et al.Sorption of carbon dioxide,methane and nitrogen in dry coals at high pressure and moderate temperature.Int J Greenhouse Gas Control,2010,4:90-101
23 Yue G Y.Tectonic characteristics and tectonic evolution of Dabashan orogenic belt and its foreland basin(in Chinese).J Mineral Petrol,1998,18(Suppl):8-15[乐光禹.大巴山造山带及其前陆盆地的构造特征和构造演化.矿物岩石,1998,18(增刊):8-15]
24 Liang F,Bai W H,Zou C N,et al.Shale gas enrichment pattern and exploration significance of Wuxi-2 well in northeast Chongqing,NESichuan Basin.Pet Explor Dev,2016,43:350-358
25 Mikhail R S,Brunauer S,Bodor E E.Investigations of a complete pore structure analysis-I.Analysis of micropores.J Colloid Interface Sci,1968,3:45-53
26 Zhou S W,Wang H Y,Xue H Q,et al.Difference between excess and absolute adsorption capacity of shale and a new shale gas reserve calculation method(in Chinese).Nat Gas Ind,2016,36:12-20[周尚文,王红岩,薛华庆,等.页岩过剩吸附量与绝对吸附量的差异及页岩气储量计算新方法.天然气工业,2016,36:12-20]
27 Zhou S W,Xue H Q,Guo W,et al.Supercritical isothermal adsorption characteristics of shale gas based on gravimetric method(in Chinese).J China Coal Soc,2016,41:2806-2812[周尚文,薛华庆,郭伟,等.基于重量法的页岩气超临界吸附特征实验研究.煤炭学报,2016,41:2806-2812]
28 Yin S,Ding W L,Liu J J,et al.Adsorption dynamics analysis of shale reservoir based on micro-pore filling model(in Chinese).Geol JChina Univ,2014,20:635-641[尹帅,丁文龙,刘建军,等.基于微孔充填模型的页岩储层吸附动力学特征分析.高校地质学报,2014,20:635-641]
29 Liu S X,Zhong J H,Ma Y S,et al.Study of microscopic pore structure and adsorption isothermal of carboniferous shale,Eastern Qaidam Basin(in Chinese).J China Univ Pet,2015,39:33-42[刘圣鑫,钟建华,马寅生,等.柴东石炭系页岩微观孔隙结构与页岩气等温吸附研究.中国石油大学学报,2015,39:33-42]
30 Sheng M,Li G S,Chen L Q,et al.Mechanisms analysis of shale-gas supercritical adsorption and modeling of isorption adsorption(in Chinese).J China Coal Soc,2014,39:179-183[盛茂,李根生,陈立强,等.页岩气超临界吸附机理分析及等温吸附模型的建立.煤炭学报,2014,39:179-183]
31 Hou X W,Wang M,Liu Y,et al.Supercritical adsorption model of shale gas and its geological significance(in Chinese).J China Univ Min Tech,2016,45:83-90[侯晓伟,王猛,刘宇,等.页岩气超临界状态吸附模型及其地质意义.中国矿业大学学报,2016,45:83-90]
2 Hill R J,Zhang E,Katz B J,et al.Modeling of gas generation from the Barnett shale,Fort Worth Basin,Texas.AAPG Bull,2007,91:501-521
3 Ross D J K,Bustin R M.Characterizing the shale gas resource potential of Devonian-Mississippian strata in the Western Canada sedimentary basin:Application of an integrated formation evaluation.AAPG Bull,2008,92:87-125
4 Zhou S W,Yan G,Xue H Q,et al.2D and 3D nanopore characterization of gas shale in Longmaxi formation based on FIB-SEM.Mar Pet Geol,2016,73:174-180
5 Xia Y,Jin Y,Chen M,et al.Gas flow in shale reservoirs(in Chinese).Chin Sci Bull,2015,60:2259-2271[夏阳,金衍,陈勉,等.页岩气渗流数学模型.科学通报,2015,60:2259-2271]
6 Curtis J B.Fractured shale-gas systems.AAPG Bull,2002,86:1921-1938
7 Ambrose R J,Hartman R C,Diaz-Campos M,et al.Shale gas-in-place calculations Part I:New pore-scale considerations.SPE J,2012,17:219-229
8 Li Y X,Qiao D W,Jiang W L,et al.Gas content of gas-bearing shale and its geological evaluation summary(in Chinese).Geol Bull China,2011,30:308-317[李玉喜,乔德武,姜文利,等.页岩气含气量和页岩气地质评价综述.地质通报,2011,30:308-317]
9 Nie H K,Zhang J C.Shale gas accumulation condition and gas content calculation:A case study of Sichuan Basin and its periphery in the Lower Paleozoic(in Chinese).Acta Geol Sin,2012,86:349-361[聂海宽,张金川.页岩气聚集条件及含气量计算:以四川盆地及其周缘下古生界为例.地质学报,2012,86:349-361]
10 Xue H Q,Wang H Y,Liu H L,et al.Absorptivity capacity and aperture distribution characteristics of shales taking the Longmaxi formation shale of Sichuan basin as an example(in Chinese).Acta Pet Sin,2013,34:826-832[薛华庆,王红岩,刘洪林,等.页岩吸附性能及孔隙结构特征--以四川盆地龙马溪组页岩为例.石油学报,2013,34:826-832]
11 Liu H L,Wang H Y.Absorptivity and influential factors of marine shales in South China(in Chinese).Nat Gas Ind,2012,32:5-9[刘洪林,王红岩.中国南方海相页岩吸附特征及其影响因素.天然气工业,2012,32:5-9]
12 Xiong W,Guo W,Liu H L,et al.Shale reservoir characteristics and isothermal adsorption properties(in Chinese).Nat Gas Ind,2012,32:113-116[熊伟,郭为,刘洪林,等.页岩的储层特征以及等温吸附特征.天然气工业,2012,32:113-116]
13 Zhang T W,Ellis G S,Ruppel S C,et al.Effect of organic matter type and thermal maturity on methane adsorption in shale gas systems.Org Geochem,2012,47:120-131
14 Tian H,Li T F,Zhang T W,et al.Characterization of methane adsorption on overmature lower Silurian-upper Ordovician shales in Sichuan Basin,southwest China:Experimental results and geological implications.Int J Coal Geol,2016,156:36-49
15 Sakurovs R,Day S,Weir S,et al.Application of a modified Dubinin-Radushkevich equation to adsorption of gases by coals under supercritical conditions.Energy Fuels,2007,21:992-997
16 Clarkson C R,Haghshenas B.Modeling of supercritical fluid adsorption on organic-rich shales and coal.In:Proceedings of SPE Unconventional Resources Conference.The Woodlands:SPE,2013
17 Zhou L,Zhou Y,Li M,et al.Experimental and modeling study of the adsorption of supercritical methane on a high surface activated carbon.Langmuir,2000,16:5955-5959
18 Zhou L,LüC Z,Wang Y L,et al.Physisorption of gases on porous solids at above-critical temperatures(in Chinese).Prog Chem,1999,11:221-226[周理,吕昌忠,王怡林,等.述评超临界温度气体在多孔固体上的物理吸附.化学进展,1999,11:221-226]
19 Wang Z,Li Y,Guo P,et al.Analyzing the adaption of different adsorption models for describing the shale gas adsorption law.Chem Eng Tech,2016,39:1921-1932
20 Chareonsuppanimit P,Mohammad S A,Robinson R L,et al.High-pressure adsorption of gases on shales:Measurements and modeling.Int J Coal Geol,2012,95:34-46
21 Xiong J,Liu X J,Liang L X,et al.Improved Dubibin-Astakhov model for shale gas supercritical adsorption(in Chinese).Acta Pet Sin,2015,36:849-857[熊健,刘向君,梁利喜,等.页岩气超临界吸附的Dubibin-Astakhov改进模型.石油学报,2015,36:849-857]
22 Pini R,Ottiger S,Burlini L,et al.Sorption of carbon dioxide,methane and nitrogen in dry coals at high pressure and moderate temperature.Int J Greenhouse Gas Control,2010,4:90-101
23 Yue G Y.Tectonic characteristics and tectonic evolution of Dabashan orogenic belt and its foreland basin(in Chinese).J Mineral Petrol,1998,18(Suppl):8-15[乐光禹.大巴山造山带及其前陆盆地的构造特征和构造演化.矿物岩石,1998,18(增刊):8-15]
24 Liang F,Bai W H,Zou C N,et al.Shale gas enrichment pattern and exploration significance of Wuxi-2 well in northeast Chongqing,NESichuan Basin.Pet Explor Dev,2016,43:350-358
25 Mikhail R S,Brunauer S,Bodor E E.Investigations of a complete pore structure analysis-I.Analysis of micropores.J Colloid Interface Sci,1968,3:45-53
26 Zhou S W,Wang H Y,Xue H Q,et al.Difference between excess and absolute adsorption capacity of shale and a new shale gas reserve calculation method(in Chinese).Nat Gas Ind,2016,36:12-20[周尚文,王红岩,薛华庆,等.页岩过剩吸附量与绝对吸附量的差异及页岩气储量计算新方法.天然气工业,2016,36:12-20]
27 Zhou S W,Xue H Q,Guo W,et al.Supercritical isothermal adsorption characteristics of shale gas based on gravimetric method(in Chinese).J China Coal Soc,2016,41:2806-2812[周尚文,薛华庆,郭伟,等.基于重量法的页岩气超临界吸附特征实验研究.煤炭学报,2016,41:2806-2812]
28 Yin S,Ding W L,Liu J J,et al.Adsorption dynamics analysis of shale reservoir based on micro-pore filling model(in Chinese).Geol JChina Univ,2014,20:635-641[尹帅,丁文龙,刘建军,等.基于微孔充填模型的页岩储层吸附动力学特征分析.高校地质学报,2014,20:635-641]
29 Liu S X,Zhong J H,Ma Y S,et al.Study of microscopic pore structure and adsorption isothermal of carboniferous shale,Eastern Qaidam Basin(in Chinese).J China Univ Pet,2015,39:33-42[刘圣鑫,钟建华,马寅生,等.柴东石炭系页岩微观孔隙结构与页岩气等温吸附研究.中国石油大学学报,2015,39:33-42]
30 Sheng M,Li G S,Chen L Q,et al.Mechanisms analysis of shale-gas supercritical adsorption and modeling of isorption adsorption(in Chinese).J China Coal Soc,2014,39:179-183[盛茂,李根生,陈立强,等.页岩气超临界吸附机理分析及等温吸附模型的建立.煤炭学报,2014,39:179-183]
31 Hou X W,Wang M,Liu Y,et al.Supercritical adsorption model of shale gas and its geological significance(in Chinese).J China Univ Min Tech,2016,45:83-90[侯晓伟,王猛,刘宇,等.页岩气超临界状态吸附模型及其地质意义.中国矿业大学学报,2016,45:83-90]