深部煤及顶底板不同射孔位置条件下的压裂模拟以鄂尔多斯盆地某气田8号深部煤层为例

Fracturing simulation with different perforation positions at deep coal seam and roof/ floor rock — case study at the No.8 deep coal seam of a gas field in the Ordos basin

  • 摘要: 鄂尔多斯8号深部煤层埋深超过2 000 m,地质结构复杂,目标煤层的顶部与底部分别存在灰岩层与泥岩层,现场采用水平井压裂作为8号深部煤层气的开发手段。为明确8号深部煤及顶底板不同射孔位置条件下的水力裂缝扩展路径规律,综合考虑储层地质结构、井眼轨迹以及储层岩石力学特性,形成了深部煤层水力压裂有限元模拟实现流程。首先,加工了井下实际岩心试样,基于三轴压缩试验明确了岩石力学特性,并利用CT扫描表征了基质孔隙与裂隙结构特征;然后,采用带孔隙压力节点的黏聚力模型等效表征了深部煤层中的弱面与孔裂隙对流体渗流的影响,并建立了考虑渗流−应力−损伤耦合的深部煤层水力压裂三维有限元模型,并基于现场压裂测试施工工艺进行了有限元模型的参数反演与验证;最终,进行了深部煤及顶底板不同射孔位置条件下的压裂测试模拟计算。结果表明:与顶底板岩石相比,煤岩弹性模量低、泊松比高、孔裂隙发育;煤层中射孔时水力裂缝被完全限制在煤层中扩展,水力裂缝难以穿过煤层与顶底板界面进入灰岩层与泥岩层中,水力裂缝在达到峰值排量前完全闭合;灰岩层中射孔时水力裂缝穿过灰岩层与煤层界面,进入煤层中扩展,煤层中的水力裂缝长度大于灰岩层中的水力裂缝长度;泥岩层中射孔时水力裂缝穿过泥岩层与煤层界面,进入煤层中扩展,泥岩层与煤层中水力裂缝长度均得到了扩展;顶板灰岩层与底板泥岩层中射孔时的起裂压力大于煤层中射孔时的起裂压力;由于大量压裂液在煤层中滤失,在顶板灰岩层与底板泥岩层中射孔时,煤层中的水力裂缝最先发生闭合。研究成果可以为深部煤层的水力裂缝扩展建模提供思路,并指导深部煤层气的高效开发。

     

    Abstract: The No.8 deep coal seam in the Ordos basin is situated at a burial depth of over 2 000 meters and has a complex geological structure. The roof and floor layers of the target coal seam are composed of limestone and mudstone, respectively. Horizontal well fracturing is used on-site as a development method for the No.8 deep coalbed methane. A finite element simulation implementation process has been developed to help clarify the propagation path law of hydraulic fractures in the No.8 deep coal seam. This process takes into account the reservoir geological structure, wellbore trajectory, and mechanical characteristics of reservoir rock to simulate hydraulic fracturing with different perforation positions at the coal seam and roof/ floor rock. Firstly, the rock specimens from the reservoir are processed, and their mechanical properties are tested using triaxial compression experiments. The characteristics of the matrix pores and fracture structures are analyzed by using CT scanning. Then, the cohesive model with pore pressure nodes is used to understand the impact of weak surfaces and pore fractures on the fluid seepage within deep coal seam. A three-dimensional finite element model of deep coal seam hydraulic fracturing is established which considers the seepage - stress - damage coupling. The parameters of the model are inverted and verified based on on-site fracturing testing and construction technology. Finally, the simulation of fracturing are conducted with different perforation positions at deep coal and roof/ floor rock. According to the research findings, the following results are obtained: The coal rock has a lower elastic modulus and a higher Poisson’s ratio than the limestone and mudstone rocks, and the pores and fractures are developed. When perforating in the coal seam, the hydraulic fractures are completely limited to the expansion of the coal seam. It is difficult for hydraulic fractures to pass through the interface to enter the limestone and mudstone. The hydraulic fractures are wholly closed before reaching the peak injection rate. When perforating in the limestone, the hydraulic fractures propagate into the coal seam by passing through the interface between the coal and limestone, and they expand further into the coal seam. It is worth noting that the length of hydraulic fractures in the coal is greater than that in the limestone. When perforating in the mudstone, the hydraulic fractures propagate into the coal seam by passing through the interface between the coal and mudstone. The hydraulic fractures in both mudstone and coal are extended in length. The perforation in limestone and mudstone requires a higher initiation pressure compared to the perforation in coal. During the perforation in the limestone and the mudstone, the hydraulic fractures in the coal tend to close first, due to the filtration of a significant amount of fracturing fluid in the coal. The research results provide some ideas for modeling the expansion of hydraulic fractures in deep coal seams and guide the efficient development of deep coalbed methane.

     

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