深煤层水力波及压裂技术及其在沁南地区的应用

为提高深部煤层的煤层气产能,针对其地质特征提出了在深煤层实施多口直井同步水力波及压裂的技术思路。首先基于边界元位移不连续法建立了多裂缝诱导应力数学模型,模拟深煤层诱导应力场分布,分析水力波及压裂复杂缝网形成的可能性,然后采用离散元方法研究应力干扰的裂缝网络延伸情况及其影响因素,最后通过三轴压裂实验和现场应用效果验证了其可行性。结果表明:(1)水力波及压裂技术能增大应力干扰面积和应力干扰强度,促使水平主应力差的减小甚至诱导局部区域的地应力方向发生改变,有利于沟通煤岩中发育的面、端割理,从而形成大规模高效复杂的裂缝网络;(2)水力波及压裂有利于复杂缝网形成的条件包括较小的初始水平主应力差、低泊松比、较小井距、低压裂液黏度、高缝内净压力等;(3)真三轴物理模拟实验结果显示,水力波及压裂技术能够充分沟通煤岩天然裂隙,形成由人工裂缝、面割理和端割理组成的复杂裂缝网络。进而提出了一套深煤层多井同步水力波及压裂工艺优化设计方法,在沁水盆地南部柿庄北地区深煤层选取了5口直井进行先导性试验,裂缝监测及排采数据表明,水力波及压裂井产生的波及体积较大,裂缝网络复杂;较之于常规压裂井,水力波及压裂井不仅见气更早,产量、套压较高且稳定,而且所形成的区域压力降波及邻井,可大幅增加实施井及邻井产量。

Synchronous hydraulic conformance fracturing technology used for deep coal beds and its field application in the Southern Qinshui Basin

For deep CBM productivity improvement, a technical concept of conducting multi-well synchronous hydraulic conformance fracturing was proposed based on its geological characteristics. First, a mathematical model for multi-fracture induced stress was established by using the boundary element displacement discontinuity method, to simulate the distribution of induced stress field in deep coal beds and analyze the possibility of the formation of complex fracture networks induced by the hydraulic conformance fracturing. Then, the propagation situation of fracture networks interfered by stress and its influencing factors were studied by using the discrete element method. And finally, the feasibility of synchronous hydraulic conformance fracturing technology was verified through triaxial fracturing experiment and field application. It is shown that by virtue of synchronous hydraulic conformance fracturing technology, stress interference area and strength are increased, so horizontal major stress difference is decreased and even the direction of earth stress is changed regionally, which is conducive to the connection of developed face cleats and butt cleats in coal rocks, so as to form large, efficient and complex fracture networks. Furthermore, the favorable conditions for the formation of complex fracture networks by hydraulic conformance fracturing include lower initial horizontal major stress difference, low Poisson’s ratio, short well spacing and low fracturing fluid viscosity and high net pressure inside the fractures. Finally, it is shown from the 3D true physical simulation experiments that by virtue of this synchronous hydraulic conformance fracturing technology, natural fractures in coal rocks can be connected sufficiently, and consequently complex fracture networks composed of hydraulic fractures, face cleats and butt cleats are created. Based on these research results, a set of optimization design method for the synchronous hydraulic conformance fracturing of deep coal beds was proposed. Five vertical wells located in the deep coal beds of North Shizhuang Block in the Southern Qinshui Basin were chosen for the pilot test. It is indicated from fracture monitoring and drainage/production data that the stimulated reservoir volume (SRV) of synchronous hydraulic fractured wells is large and its fracture network is complex; and that compared with the conventionally fractured wells, the synchronous hydraulic fractured well is earlier in gas breakthrough, and higher and more stable in production rates and casing pressure and its regional pressure drop even spreads to the adjacent wells, so that their production rates are remarkably raised.