页岩气藏综合地质建模技术
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摘要
目前,页岩气藏地质建模采用的技术思路和实现方式主要源于常规油气藏,对页岩气藏并不适用,而可资借鉴的国内外相关成果则鲜见。为此,首先针对页岩气藏的特殊性,确定相配套的地质建模技术流程;再结合测录井资料解释、地震叠前叠后资料解释及采样地质实验分析等结果,建立工区构造和页岩小层发育模型;并在此格架体模型下应用地质统计学建模方法建立了页岩气储层厚度、孔隙度、含气饱和度、总有机碳含量、硅质含量、脆性指数等属性模型;综合应用地震AFE属性、构造曲率、应变体积膨胀资料,结合地质认识与钻井显示,采用目标建模方法,建立天然裂缝DFN模型;在人工压裂缝展布模式判断及参数拟合分析的基础上,建立了人工压裂缝模型;最后,采取逐级叠加的方法,建立了页岩气藏综合地质模型并进行气井的生产史拟合与动态预测。研究结果表明:①与常规油气藏相比,页岩气藏地质建模更为复杂,主要表现在小层划分与对比困难、基质参数多且存在着相互约束关系、天然裂缝成因和尺度多样以及天然裂缝干扰和影响下人工压裂缝分布复杂;②天然裂缝模型实现了对裂缝系统几何形态和分布的有效细致描述,人工压裂缝模型能较好地体现人工裂缝分布状况及压裂改造体积,通过逐级融合叠加页岩气藏构造和小层发育模型、多种基质属性参数模型、多尺度天然裂缝模型及其约束下的人工压裂缝模型,可以完成页岩气藏综合地质模型的建立;③气井生产史拟合结果显示,在井底压力误差小于3.3%的情况下,所建立的页岩气藏综合地质模型是可靠的。
At present, the technical ideas and implementation modes adopted in the geological modeling of shale gas reservoirs are mainly derived from those used in conventional oil and gas reservoirs, so they are not applicable to shale gas reservoirs. Moreover, there are few reports on the results of shale gas geological modeling at home and abroad. In view of this, a technical process of geological modeling for shale gas reservoirs was firstly established according to its particularity. Secondly, a structure and shale sublayer development model for the working area was established based on logging interpretation results, pre-stack and post-stack seismic interpretation data and geological test analysis results of samples. Thirdly, property models of shale gas reservoirs, including thickness, porosity, gas saturation, TOC,silicon content and brittleness index, were established using geostatistic modeling method in the frame model. Fourthly, a natural fracture DFN model was established using the object-based modeling method, based on seismic AFE attribute, structural curvature and strain and dilatation data, combined with geological knowledge and drilling display. Fifthly, a hydraulic fracture model was established based on the estimate of hydraulic fracture distribution pattern and the parameter fitting analysis. Finally, a comprehensive geological model for shale gas reservoirs was established by virtue of step-by-step superposition. What's more, it was applied to the production history matching and performance prediction of shale gas wells. And the following research results were obtained. First, the geological modeling of shale gas reservoirs is more complex than that of conventional oil and gas reservoirs, and the complexities are presented as difficult classification and correlation of sublayers, multiple matrix parameters restricting each other, diverse geneses and sizes of natural fractures, and complicated distribution of hydraulic fractures under the interference and influence of natural fractures. Second, the natural fracture DFN model is capable of describing the geometrical shape and distribution of fracture system effectively and finely, and the hydraulic fracture model can better embody the distribution of hydraulic fractures and the stimulated reservoir volume(SRV). The establishment of the comprehensive geological model for shale gas reservoirs can be realized by progressively integrating and superposing the structure and sublayer development model, the multi-matrix property parameter model, the multi-scale natural fracture model and the hydraulic fracture model under the constraint of multi-scale natural fracture model. Third, production history matching results of gas wells show that the error of bottomhole pressure is lower than 3.3%, which indicates that the newly established comprehensive geological model of shale gas reservoirs is reliable. In conclusion, the modeling process and method developed in this paper can be used as reference in the establishment of a comprehensive geological model for shale gas reservoirs.
At present, the technical ideas and implementation modes adopted in the geological modeling of shale gas reservoirs are mainly derived from those used in conventional oil and gas reservoirs, so they are not applicable to shale gas reservoirs. Moreover, there are few reports on the results of shale gas geological modeling at home and abroad. In view of this, a technical process of geological modeling for shale gas reservoirs was firstly established according to its particularity. Secondly, a structure and shale sublayer development model for the working area was established based on logging interpretation results, pre-stack and post-stack seismic interpretation data and geological test analysis results of samples. Thirdly, property models of shale gas reservoirs, including thickness, porosity, gas saturation, TOC,silicon content and brittleness index, were established using geostatistic modeling method in the frame model. Fourthly, a natural fracture DFN model was established using the object-based modeling method, based on seismic AFE attribute, structural curvature and strain and dilatation data, combined with geological knowledge and drilling display. Fifthly, a hydraulic fracture model was established based on the estimate of hydraulic fracture distribution pattern and the parameter fitting analysis. Finally, a comprehensive geological model for shale gas reservoirs was established by virtue of step-by-step superposition. What's more, it was applied to the production history matching and performance prediction of shale gas wells. And the following research results were obtained. First, the geological modeling of shale gas reservoirs is more complex than that of conventional oil and gas reservoirs, and the complexities are presented as difficult classification and correlation of sublayers, multiple matrix parameters restricting each other, diverse geneses and sizes of natural fractures, and complicated distribution of hydraulic fractures under the interference and influence of natural fractures. Second, the natural fracture DFN model is capable of describing the geometrical shape and distribution of fracture system effectively and finely, and the hydraulic fracture model can better embody the distribution of hydraulic fractures and the stimulated reservoir volume(SRV). The establishment of the comprehensive geological model for shale gas reservoirs can be realized by progressively integrating and superposing the structure and sublayer development model, the multi-matrix property parameter model, the multi-scale natural fracture model and the hydraulic fracture model under the constraint of multi-scale natural fracture model. Third, production history matching results of gas wells show that the error of bottomhole pressure is lower than 3.3%, which indicates that the newly established comprehensive geological model of shale gas reservoirs is reliable. In conclusion, the modeling process and method developed in this paper can be used as reference in the establishment of a comprehensive geological model for shale gas reservoirs.
引文
[1]Fisher MK,Wright CA,Davidson BM,Goodwin AK,Fielder EO,Buckler WS,et al.Integrating fracture mapping technologies to optimize stimulations in the Barnett Shale[C]//paper 77441 presented at the SPE Annual Technical Conference and Exhibiton,29 September-2 October 2002,San Antonio,Texas,USA.
[2]Du C,Zhang X,Melton B,Fullilove D,Suliman B,Gowelly S,et al.A workflow for integrated Barnett Shale gas reservoir modeling and simulation[C]//paper 122934 presented at the 2009 SPELatin American and Caribbean Petroleum Engineering Conference,31 May-3 June 2009,Cartagena,Colombia.
[3]Gale JFW,Reed RM&Holder J.Natural fractures in the Barnett Shale and their importance for hydraulic fracture treatments[J].AAPG Bulletin,2007,91(4):603-622.
[4]Jugal KG,Richard AA,Matias GZ,Yao Y,Elizabeth T,Shalawn KJ,et al.Integration for fracture,reservoir,and geomechanics modeling for shale gas reservoir development[C]//paper 164018presented at the SPE Unconventional Gas Conference and Exhibition,28-30 January 2013,Muscat,Oman.
[5]Kudapa VK,Kumar S,Gupta DK,Sharma P&kunhAbdulla R.Modelling of gas production from shale matrix to fracture network[C]//paper 187473-MS presented at the SPE Intelligent Oil and Gas Symposium,9-10 May 2017,Abu Dhabi,UAE.
[6]Diaz de Souza OC,Sharp AJ,Martinez RC,Foster RA,Simpson MR,Piekenbrock EJ,et al.Integrated unconventional shale gas reservoir modeling:A worked example from the Haynesville Shale,De Soto parish,North Lousiana[C]//paper 154692 presented at the Americas Unconventional Resources Conference,5-7June 2012,Pittsburgh,Pennsylvania,USA.
[7]John H,Civan F,Michel-Villazon G,Devegowda D&Sigal R.Modeling multiple-porosity transport in gas-bearing shale formation[C]//paper 153535 presented at the SPE Latin American and Caribbean Petroleum Engineering Conference,16-18 April 2012,Mexico City,Mexico.
[8]Porta GD,Kenter JAM,Immenhauser A&Bahamonde JR.Lithofacies character and architecture across a Pennsylvanian inner-platform transect(Sierra De Cuera,Asturias,Spain)[J].Journal of Sedimentary Research,2002,72(6):898-916.
[9]Falivene O,Arbues P,Gardiner A,Pickup G,Munoz JA&Cabrera L.Best practice stochastic facies modeling from a channel-fill turbidite sandstone analog(the Quarry outcrop,Eocene Ainsa Basin,Northeast Spain)[J].AAPG Bulletin,2006,90(7):1003-1029.
[10]Wang G&Carr TR.Organic-rich Marcellus Shale lithofacies modeling and distribution pattern analysis in the Appalachian Basin[J].AAPG Bulletin,2013,97(12):2173-2205.
[11]乔辉,贾爱林,位云生.页岩气水平井地质信息解析与三维构造建模[J].西南石油大学学报(自然科学版),2018,40(1):78-88.Qiao Hui,Jia Ailin&Wei Yunsheng.Geological information analysis of horizontal wells and 3D modeling of shale gas reservoir[J].Journal of Southwest Petroleum University(Science&Technology Edition),2018,40(1):78-88.
[12]石浩.涪陵页岩气田P区块储层三维地质建模及应用浅析[J].江汉石油职工大学学报,2017,30(6):1-4.Shi Hao.Application of three-dimensional geological reservoir modeling in Block P in Fuling Shale Gasfield[J].Journal of Jianghan Petroleum University of Staff and Workers,2017,30(6):1-4.
[13]郑海桥,陈义才.涪陵地区焦石坝构造龙马溪组页岩气地质建模[J].重庆科技学院学报(自然科学版),2016,18(2):5-9.Zheng Haiqiao&Chen Yicai.Geological modeling of Longmaxi Shale gas in Jiaoshiba,Fuling area[J].Journal of Chongqing University of Science and Technology(Natural Science Edition),2016,18(2):5-9.
[14]马成龙,张新新,李少龙.页岩气有效储层三维地质建模——以威远地区威202H2平台区为例[J].断块油气田,2017,24(4):495-499.Ma Chenglong,Zhang Xinxin&Li Shaolong.3D geological modeling of effective shale-gas reservoirs:Taking Wei 202H2platform of Weiyuan Area as an example[J].Fault-Block Oil&Gas Field,2017,24(4):495-499.
[15]Donahoe T&Gao D.Application of 3D seismic attribute analysis to structure interpretation and hydrocarbon exploration Southwest Pennsylvania,Central Appalachian Basin:A case study[J].Interpretation,2016,4(3):291-302.
[16]Gao D&Duan T.Seismic structure and texture analyses for fractured reservoir characterization:An integrated workflow[J].Interpretation,2017,5(4):623-639.
[17]Cather M,Guo B&Schechter DS.An integrated geology-reservoir description and modeling of the naturally fractured Spraberry Trend area reservoirs[C]//The 49th Annual Technical Meeting of the Petroleum Society,8-10 June 1998,Calgary,Alberta,Canada.
[18]Koesoemadinata A,El-Kaseeh G,Banik N,Dai JC,Egan M,Gonzalez A,et al.Seismic reservoir characterization in Marcellus Shale[C]//SEG San Antonio 2011 Annual Meeting,New York City,New York,USA.
[19]徐康泰,李江飞,毛重超.页岩气DFN离散裂缝网格地质建模研究[J].周口师范学院学报,2017,34(2):70-73.Xu Kangtai,Li Jiangfei&Mao Chongchao.Study of DFN geological model of discrete crack grid for shale gas[J].Journal of Zhoukou Normal University,2017,34(2):70-73.
[20]Chuprakov DA,Akulich AV,Siebrits E&Thiercelin M.Hydraulic-fracture propagation in a naturally fractured reservoir[C]//paper 128715 presented at the SPE Oil and Gas India Conference and Exhibition,20-22 January 2010,Mumbai,India.
[21]Mhiri A,Blasingame TA&Moridis GJ.Stochastic modeling of a fracture network in a hydraulically fractured shale-gas reservoir[C]//Paper 175021-MS presented at the SPE Annual Technical Conference and Exhibition,28-30 September 2015,Houston,Texas,USA.
[22]时贤,程远方,蒋恕,李友志,孙元伟,王欣.页岩储层裂缝网络延伸模型及其应用[J].石油学报,2014,35(6):1130-1137.Shi Xian,Cheng Yuanfang,Jiang Shu,Li Youzhi,Sun Yuanwei&Wang Xin.Simulation of complex fracture network propagation and its application for shale gas reservoir[J].Acta Petrolei Sinica,2014,35(6):1130-1137.
[23]赵金洲,李勇明,王松,江有适,张烈辉.天然裂缝影响下的复杂压裂裂缝网络模拟[J].天然气工业,2014,34(1):68-73.Zhao Jinzhou,Li Yongming,Wang Song,Jiang Youshi&Zhang Liehui.Simulation of a complex fracture network influenced by natural fractures[J].Natural Gas Industry,2014,34(1):68-73.
[24]Olorode OM,Freeman CM,Moridis GJ&Blasingame TA.High-resolution numerical modeling of complex and irregular fracture patterns in shale gas and tight gas reservoirs[C]//Paper152482 presented at the SPE Latin American and Caribbean Petroleum Engineering Conference,16-18 April 2012,Mexico city,Mexico.
[25]Huang J,Safari R,Mutlu U,Burns K,Geldmacher I,McClure MW,et al.Natural-hydraulic fracture interaction:microseismic observations and geomechanical predictions[C]//Unconventional Resources Technology Conference,25-27 August 2014,Denver,Colorado,USA.
[26]Wu K&Olson JE.Mechanics analysis of interaction between hydraulic and natural fractures in shale reservoirs[C]//Unconventional Resources Technology Conference,25-27 August 2014,Denver,Colorado,USA.
[27]Cipolla CL.Modeling production and evaluating fracture performance in unconventional gas reservoirs[J].Journal of Petroleum Technology,2009,61(9):84-90.
[28]Gong B,Qin G,Towler BF&Wang H.Discrete modeling of natural and hydraulic fractures in shale-gas reservoirs[C]//Paper146842 presented at the SPE Annual Technical Conference and Exhibition,30 October-2 November 2011,Denver,Colorado,USA.
[29]刘洪,廖如刚,李小斌,胡昌权,肖晖,黄园园,等.页岩气“井工厂”不同压裂模式下裂缝复杂程度研究[J].天然气工业,2018,38(12):70-76.Liu Hong,Liao Rugang,Li Xiaobin,Hu Changquan,Xiao Hui,Huang Yuanyuan,et al.A comparative analysis on the fracture complexity in different fracking patterns of shale gas"well factory"[J].Natural Gas Industry,2018,38(12):70-76.
[30]尹丛彬,李彦超,王素兵,熊雨然,何封,秦俐.页岩压裂裂缝网络预测方法及其应用[J].天然气工业,2017,37(4):60-68.Yin Congbin,Li Yanchao,Wang Subing,Xiong Yuran,He Feng&Qin Li.Methodology of hydraulic fracture network prediction in shale reservoirs and its application[J].Natural Gas Industry,2017,37(4):60-68.
[31]Rickman R,Mullen M,Petre E,Grieser B&Kunder D.A practical use of shale petrophysics for stimulation design optimization:All shale plays are not clones of the Barnnett Shale[C]//Paper115258 presented at the SPE Annual Technical Conference and Exhibition,21-24 September 2008,Denver,Colorado,USA.
[2]Du C,Zhang X,Melton B,Fullilove D,Suliman B,Gowelly S,et al.A workflow for integrated Barnett Shale gas reservoir modeling and simulation[C]//paper 122934 presented at the 2009 SPELatin American and Caribbean Petroleum Engineering Conference,31 May-3 June 2009,Cartagena,Colombia.
[3]Gale JFW,Reed RM&Holder J.Natural fractures in the Barnett Shale and their importance for hydraulic fracture treatments[J].AAPG Bulletin,2007,91(4):603-622.
[4]Jugal KG,Richard AA,Matias GZ,Yao Y,Elizabeth T,Shalawn KJ,et al.Integration for fracture,reservoir,and geomechanics modeling for shale gas reservoir development[C]//paper 164018presented at the SPE Unconventional Gas Conference and Exhibition,28-30 January 2013,Muscat,Oman.
[5]Kudapa VK,Kumar S,Gupta DK,Sharma P&kunhAbdulla R.Modelling of gas production from shale matrix to fracture network[C]//paper 187473-MS presented at the SPE Intelligent Oil and Gas Symposium,9-10 May 2017,Abu Dhabi,UAE.
[6]Diaz de Souza OC,Sharp AJ,Martinez RC,Foster RA,Simpson MR,Piekenbrock EJ,et al.Integrated unconventional shale gas reservoir modeling:A worked example from the Haynesville Shale,De Soto parish,North Lousiana[C]//paper 154692 presented at the Americas Unconventional Resources Conference,5-7June 2012,Pittsburgh,Pennsylvania,USA.
[7]John H,Civan F,Michel-Villazon G,Devegowda D&Sigal R.Modeling multiple-porosity transport in gas-bearing shale formation[C]//paper 153535 presented at the SPE Latin American and Caribbean Petroleum Engineering Conference,16-18 April 2012,Mexico City,Mexico.
[8]Porta GD,Kenter JAM,Immenhauser A&Bahamonde JR.Lithofacies character and architecture across a Pennsylvanian inner-platform transect(Sierra De Cuera,Asturias,Spain)[J].Journal of Sedimentary Research,2002,72(6):898-916.
[9]Falivene O,Arbues P,Gardiner A,Pickup G,Munoz JA&Cabrera L.Best practice stochastic facies modeling from a channel-fill turbidite sandstone analog(the Quarry outcrop,Eocene Ainsa Basin,Northeast Spain)[J].AAPG Bulletin,2006,90(7):1003-1029.
[10]Wang G&Carr TR.Organic-rich Marcellus Shale lithofacies modeling and distribution pattern analysis in the Appalachian Basin[J].AAPG Bulletin,2013,97(12):2173-2205.
[11]乔辉,贾爱林,位云生.页岩气水平井地质信息解析与三维构造建模[J].西南石油大学学报(自然科学版),2018,40(1):78-88.Qiao Hui,Jia Ailin&Wei Yunsheng.Geological information analysis of horizontal wells and 3D modeling of shale gas reservoir[J].Journal of Southwest Petroleum University(Science&Technology Edition),2018,40(1):78-88.
[12]石浩.涪陵页岩气田P区块储层三维地质建模及应用浅析[J].江汉石油职工大学学报,2017,30(6):1-4.Shi Hao.Application of three-dimensional geological reservoir modeling in Block P in Fuling Shale Gasfield[J].Journal of Jianghan Petroleum University of Staff and Workers,2017,30(6):1-4.
[13]郑海桥,陈义才.涪陵地区焦石坝构造龙马溪组页岩气地质建模[J].重庆科技学院学报(自然科学版),2016,18(2):5-9.Zheng Haiqiao&Chen Yicai.Geological modeling of Longmaxi Shale gas in Jiaoshiba,Fuling area[J].Journal of Chongqing University of Science and Technology(Natural Science Edition),2016,18(2):5-9.
[14]马成龙,张新新,李少龙.页岩气有效储层三维地质建模——以威远地区威202H2平台区为例[J].断块油气田,2017,24(4):495-499.Ma Chenglong,Zhang Xinxin&Li Shaolong.3D geological modeling of effective shale-gas reservoirs:Taking Wei 202H2platform of Weiyuan Area as an example[J].Fault-Block Oil&Gas Field,2017,24(4):495-499.
[15]Donahoe T&Gao D.Application of 3D seismic attribute analysis to structure interpretation and hydrocarbon exploration Southwest Pennsylvania,Central Appalachian Basin:A case study[J].Interpretation,2016,4(3):291-302.
[16]Gao D&Duan T.Seismic structure and texture analyses for fractured reservoir characterization:An integrated workflow[J].Interpretation,2017,5(4):623-639.
[17]Cather M,Guo B&Schechter DS.An integrated geology-reservoir description and modeling of the naturally fractured Spraberry Trend area reservoirs[C]//The 49th Annual Technical Meeting of the Petroleum Society,8-10 June 1998,Calgary,Alberta,Canada.
[18]Koesoemadinata A,El-Kaseeh G,Banik N,Dai JC,Egan M,Gonzalez A,et al.Seismic reservoir characterization in Marcellus Shale[C]//SEG San Antonio 2011 Annual Meeting,New York City,New York,USA.
[19]徐康泰,李江飞,毛重超.页岩气DFN离散裂缝网格地质建模研究[J].周口师范学院学报,2017,34(2):70-73.Xu Kangtai,Li Jiangfei&Mao Chongchao.Study of DFN geological model of discrete crack grid for shale gas[J].Journal of Zhoukou Normal University,2017,34(2):70-73.
[20]Chuprakov DA,Akulich AV,Siebrits E&Thiercelin M.Hydraulic-fracture propagation in a naturally fractured reservoir[C]//paper 128715 presented at the SPE Oil and Gas India Conference and Exhibition,20-22 January 2010,Mumbai,India.
[21]Mhiri A,Blasingame TA&Moridis GJ.Stochastic modeling of a fracture network in a hydraulically fractured shale-gas reservoir[C]//Paper 175021-MS presented at the SPE Annual Technical Conference and Exhibition,28-30 September 2015,Houston,Texas,USA.
[22]时贤,程远方,蒋恕,李友志,孙元伟,王欣.页岩储层裂缝网络延伸模型及其应用[J].石油学报,2014,35(6):1130-1137.Shi Xian,Cheng Yuanfang,Jiang Shu,Li Youzhi,Sun Yuanwei&Wang Xin.Simulation of complex fracture network propagation and its application for shale gas reservoir[J].Acta Petrolei Sinica,2014,35(6):1130-1137.
[23]赵金洲,李勇明,王松,江有适,张烈辉.天然裂缝影响下的复杂压裂裂缝网络模拟[J].天然气工业,2014,34(1):68-73.Zhao Jinzhou,Li Yongming,Wang Song,Jiang Youshi&Zhang Liehui.Simulation of a complex fracture network influenced by natural fractures[J].Natural Gas Industry,2014,34(1):68-73.
[24]Olorode OM,Freeman CM,Moridis GJ&Blasingame TA.High-resolution numerical modeling of complex and irregular fracture patterns in shale gas and tight gas reservoirs[C]//Paper152482 presented at the SPE Latin American and Caribbean Petroleum Engineering Conference,16-18 April 2012,Mexico city,Mexico.
[25]Huang J,Safari R,Mutlu U,Burns K,Geldmacher I,McClure MW,et al.Natural-hydraulic fracture interaction:microseismic observations and geomechanical predictions[C]//Unconventional Resources Technology Conference,25-27 August 2014,Denver,Colorado,USA.
[26]Wu K&Olson JE.Mechanics analysis of interaction between hydraulic and natural fractures in shale reservoirs[C]//Unconventional Resources Technology Conference,25-27 August 2014,Denver,Colorado,USA.
[27]Cipolla CL.Modeling production and evaluating fracture performance in unconventional gas reservoirs[J].Journal of Petroleum Technology,2009,61(9):84-90.
[28]Gong B,Qin G,Towler BF&Wang H.Discrete modeling of natural and hydraulic fractures in shale-gas reservoirs[C]//Paper146842 presented at the SPE Annual Technical Conference and Exhibition,30 October-2 November 2011,Denver,Colorado,USA.
[29]刘洪,廖如刚,李小斌,胡昌权,肖晖,黄园园,等.页岩气“井工厂”不同压裂模式下裂缝复杂程度研究[J].天然气工业,2018,38(12):70-76.Liu Hong,Liao Rugang,Li Xiaobin,Hu Changquan,Xiao Hui,Huang Yuanyuan,et al.A comparative analysis on the fracture complexity in different fracking patterns of shale gas"well factory"[J].Natural Gas Industry,2018,38(12):70-76.
[30]尹丛彬,李彦超,王素兵,熊雨然,何封,秦俐.页岩压裂裂缝网络预测方法及其应用[J].天然气工业,2017,37(4):60-68.Yin Congbin,Li Yanchao,Wang Subing,Xiong Yuran,He Feng&Qin Li.Methodology of hydraulic fracture network prediction in shale reservoirs and its application[J].Natural Gas Industry,2017,37(4):60-68.
[31]Rickman R,Mullen M,Petre E,Grieser B&Kunder D.A practical use of shale petrophysics for stimulation design optimization:All shale plays are not clones of the Barnnett Shale[C]//Paper115258 presented at the SPE Annual Technical Conference and Exhibition,21-24 September 2008,Denver,Colorado,USA.