基于施工曲线的页岩气井压后评估新方法

A new post-fracturing evaluation method for shale gas wells based on fracturing curves

基于有限的资料进行压后评估分析,对于压裂方案的持续改进具有重要的意义。为了再认识储层及人工裂缝参数,把压裂施工曲线分为前置液注入和主压裂施工两个阶段。通过统计前置液注入阶段的地层破裂次数、平均压力降幅和平均压力降速,可以定性判断地层脆塑性;根据压裂施工中的能量区域可定量化计算综合脆性指数。真三轴大型物理模拟试验结果显示,主压裂施工阶段曲线压力波动频率和幅度反映了裂缝的复杂程度,结合地层脆塑性可综合诊断远井裂缝形态。以渝东南某页岩气P井为例,进行压后评估分析。结果表明:①水平井筒延伸方向的页岩地层非均质性较强,以自然伽马值260 API作为该区地层脆塑性的界限,偏脆性地层更易形成复杂裂缝系统;②P井有一半裂缝为单一缝,为了进一步改善开发效果,应进一步采取精细分段、转向压裂等措施。该成果为进一步改进该区压裂井设计、提高压裂有效改造体积提供了理论依据。

Post-fracturing evaluation by using limited data is of great significance to continuous improvement of the fracturing programs. In this paper, a fracturing curve was divided into two stages （i.e., prepad fluid injection and main fracturing） so as to further understand the parameters of reservoirs and artificial fractures. The brittleness and plasticity of formations were qualitatively tdentified by use of the statistics of formation fracture frequency, and average pressure dropping range and rate during the prepad fluid injection. The composite brittleness index was quantitatively calculated by using the energy zones in the process of fracturing. It is shown from the large-scale true triaxial physical simulation results that the complexity of fractures is reflected by the pressure fluctuation frequency and amplitude in the main fracturing curve, and combined with the brittleness and plasticity of formations, the fracture morphology far away from the well can be diagnosed. Well P, a shale gas well in SE Chongqing, was taken as an example for post-fracturing evaluation. It is shown that the shale beds are of stronger heterogeneity along the extension directions of horizontal wells, and with GR 260 API as the dividing line between brittleness and plasticity in this area, complex fracture systems tend to form in brittleness-prone formations. In Well P, half of the frac- tures are single fractures, so it is necessary to carry out fine subsection and turnaround fracturing so as to improve development effects. This paper provides a theoretical basis for improving the fracturing well design and increasing the effective stimulated volume in this area.