A New Approach to Calculate Effective Stimulated Reservoir Volume in Shale Gas Reservoir
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摘要: 页岩气藏矿场压裂实践表明,储层有效改造体积(effective stimulated reservoir volume,简称ESRV)是影响页岩气藏体积压裂水平井生产效果的关键因素,ESRV的准确计算对页岩气藏压裂方案评价与体积压裂水平井产量预测具有重要作用.基于页岩储层改造体积(stimulated reservoir volume,简称SRV)多尺度介质气体运移机制,建立了SRV区域正交离散裂缝耦合双重介质基质团块来表征单元体渗流模型(representation elementary volume,简称REV),并结合北美页岩储层实例研究了次生裂缝间距、宽度等缝网参数对页岩气藏气体运移规律的影响.在此基础上根据SRV区域次生裂缝分布特征,采用分形质量维数定量表征裂缝间距分布规律,结合页岩气藏次生裂缝间距对基质团块内流体动用程度的影响规律,得到了页岩气藏体积压裂ESRV计算方法.结果表明SRV区域次生裂缝间距对基质团块内吸附及自由气影响较大,次生裂缝间距小于0.20 m时可以实现SRV区域基质团块内流体向各方向裂缝的"最短距离"渗流.选取北美典型页岩储层生产井体积压裂数据进行ESRV计算,页岩气藏目标井ESRV占体积压裂SRV的37.78%.因此ESRV受改造区域次裂缝分布规律及SRV有效裂缝间距界限的影响,是储层固有性质及人工压裂因素综合作用的结果.Abstract: Hydraulic fracturing practices in shale reservoirs show that effective stimulated reservoir volume (ESRV) significantly affects the production of hydraulic fractured well. Therefore, estimating ESRV is an important prerequisite for the evaluation and production prediction of hydraulic fracturing wells in shale reservoirs. This paper introduces a representation elementary volume (REV) of orthogonal discrete fracture coupled dual-porosity matrixflow model to predict the volumetric flux of gas in shale reservoirs. The influence of fracture space and fracture width on gas migration was studied. Considering fractal characteristics of the fracture network in stimulated reservoir volume (SRV), fractal dimension was used to quantitatively evaluate the fracture space distribution. Combining the effective fracture space and fractal characteristic of fracture network, a new approach was proposed to evaluate the ESRV in shale reservoirs. The approach was used in Eagle Ford shale gas reservoir and the results show that the fracture space has a great influence on migration of adsorbed gas. Fracture network has a contribution to enhance absorbed and free gas recovery ratio when the fracture space is less than 0.20 m. The ESRV was evaluated in this paper and the results indicate that the ESRV accounts for 37.78% of the total SRV in shale gas reservoir. The ESRV was influenced by both secondary fracture distribution and effective fracture space, as a result of reservoir intrinsic property and hydraulic fracturing practices.
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图 1 页岩储层SRV区域表征单元体渗流模型
Fig. 1. Representative elementary volume flow model of SRV region in shale reservoirs
表 1 Barnett页岩气藏储层参数
Table 1. Reservoir parameters of Barnett shale gas reservoir
参数 数值 次生裂缝宽度(m) 0.001 气体压缩系数(MPa-1) 0.05 干酪根孔径(nm) 50 无机基岩孔隙度 0.1 REV水平方向裂缝条数 5 REV垂直方向裂缝条数 5 气体粘度(mPa·s) 0.018 4 气体摩尔质量(kg·mol-1) 0.016 储层温度(K) 338 单位岩心体积中干酪根固体体积 0.5 无机基岩孔隙迂曲度 5 干酪根表面朗格缪尔最大吸附浓度(m3·kg-1) 3.1×10-3 REV入口压力(MPa) 15 REV压力梯度(MPa·m-1) 0.05 干酪根孔隙度 0.2 无机基岩孔径(nm) 100 次生裂缝孔隙度 0.02 次生裂缝间距(m) 0.05 干酪根表面扩散系数(10-4 m2·s-1) 5 朗格缪尔压力(MPa) 13.78 干酪根固体形状因子(m-2) 0.5 角动量调节系数 0.8 干酪根孔隙迂曲度 5 REV初始压力(MPa) 50 -
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