页岩气藏缝网压裂裂缝间距优化研究

Optimization of fracture spacing in fracture network of shale gas reservoir

页岩气藏一般具有低孔隙、低渗透等特征,对其实施缝网压裂是高效开发页岩气的最佳途径。采用位移不连续法建立线弹性二维均质地层诱导应力场分布数学模型,通过水平应力差异系数对顺序压裂和交替压裂的裂缝间距进行优化研究。结果表明,水平应力差异系数受到裂缝净压力、裂缝缝长、原地应力场等因素的影响;裂缝净压力越大、缝长越长,水平应力差异系数越小;随着与裂缝距离的增加,水平应力差异系数呈现先减小后增加的趋势,因此,存在后续裂缝形成复杂网络的最佳裂缝间距;顺序压裂裂缝间距不宜过大,且后续压裂裂缝间距应适当减小;交替压裂裂缝间距最优时,缝间水平应力差异系数最小,对中间裂缝形成缝网最有利。

Shale gas reservoirs have low porosity and low permeability characteristics. One of the best ways to produce gas from shale gas reservoir efficiently is using hydraulic fracturing to create complicated fracture networks. Displacement discontinuity method is employed to set up a mathematical model of two-dimensional induced stress field distribution based on the assumption that the formation is homogeneous and elastic. Through the horizontal stress difference coefficient, fracture spacing has been researched. The results show that the horizontal stress difference coefficient is affected by fracture net pressure, fracture length and in-situ stress field etc. The greater the net stress is, the longer the fracture length is, the smaller the difference coefficient of horizontal stress is. With the increase of fracture spacing, the horizontal stress difference coefficient firstly decreases and then increases. Therefore, there is a best fracture spacing for subsequent fracture to form complicated fracture network. The fracture spacing of consecutive multi-stage hydraulic fracturing should not be too large and subsequent fracture spacing should be suitably reduced. When the fracture spacing of alternate multi-stage hydraulic fracturing is optimal, the horizontal stress difference coefficient between the first two fractures is the minimum, and it provides the most advantages for the third fracture to form more complex fracture networks in the altered stress region.