低渗透油气藏水平井分段多簇压裂簇间距优化新方法

水平井分段多簇压裂中簇间距的大小是决定水平井分段多簇压裂成败的关键因素。为提高低渗透油气藏储层压裂改造效果,需建立合理的簇间距优化模型,而现有的优化方法多以应力反转半径作为最佳间距,并未定量化表征压裂后的储层改造效果。为此,基于弹性力学基础理论和位移不连续法建立了考虑水力裂缝干扰模式下的复杂地应力场计算模型,研究了天然裂缝在复杂地应力场条件下发生张开和剪切破裂形成复杂裂缝网络的规律,再以获得最大缝网波及区域面积为优化目标,形成一种新的簇间距优化方法。研究结果表明:(1)张开的水力裂缝会在其周围产生诱导应力,压裂液的滤失则会导致地层孔隙压力变化,相应的地层孔隙弹性应力也会发生变化;(2)天然裂缝剪切破裂区域与张开破裂区域重叠,且前者要远大于后者,可采用天然裂缝剪切破裂区域面积来表征复杂裂缝网络波及区域的大小。采用该方法指导了现场水平井的簇间距优化设计,实验井压裂后取得了理想的增产效果,为低渗透油气藏水平井分段多簇压裂的簇间距优化设计提供了借鉴和指导。

A new method for cluster spacing optimization of multi-cluster staged fracturing in horizontal wells of low-permeability oil and gas reservoirs

Cluster spacing is a key factor for the success of multi-cluster staged fracturing in horizontal wells. In order to improve the fracturing results of low-permeability oil and gas reservoirs, it is necessary to build a rational cluster spacing optimization model. The existing optimization methods, however, use the stress reversal radius as the optimum spacing, so post-fracturing reservoir stimulation results are not characterized quantitatively. In this paper, a calculation model for complex stress field in the mode of hydraulic fracture interference was developed based on elastic mechanics theory and displacement discontinuity method. Then, the formation laws of complex fracture networks when natural fractures suffer opening and shearing fracturing in complex stress fields were investigated. Finally, a new cluster spacing optimization method was developed with the maximum affected area of fracture networks as the optimization objective. In this study, the following conclusions were reached. First, open hydraulic fractures may generate induced stress around them; the leakage of fracturing fluid may lead to the change of formation pore pressure, and correspondingly the change of elastic stress of formation pores. And second, the shearing fracturing area of natural fractures is overlapped with the opening fracturing area, and the former is much larger than the latter, so the size of the area affected by complex fracture networks can be characterized by using the shearing fracturing area of natural fractures. When this method was applied in the cluster spacing optimization design of a horizontal well, satisfactory fracturing results were achieved. Obviously, it provides the reference and instruction for the cluster spacing optimization of multi-cluster staged fracturing in horizontal wells of low-permeability oil and gas reservoirs.