致密气藏动态启动压力梯度实验研究

Experimental study on dynamic threshold pressure gradient of tight gas reservoir

针对目前启动压力梯度测试不能准确表征储层真实启动压力梯度的现状,通过引入高精度回压控制系统,建立致密气藏储层条件下的启动压力梯度测试方法。与常压下测试结果相比,在渗透率相同的条件下,新方法得到的启动压力梯度更小,且岩心渗透率越低,2种方法测定结果相差越大。研究结果表明,启动压力梯度在开发过程中并不是定值,而是随着孔隙流体压力的下降而不断变化,呈现动态启动压力梯度。为此,提出并定义启动压力梯度敏感性和启动压力梯度敏感系数的概念,用其描述致密气藏动态启动压力梯度特征,并通过实验对动态启动压力梯度的影响因素进行分析。结果表明,启动压力梯度随孔隙流体压力的下降而线性增大。岩心渗透率越低、含水饱和度越大,在储层条件下的启动压力梯度越大,在开发过程中启动压力梯度的变化幅度也越大,启动压力梯度敏感性越强,动态启动压力梯度现象越明显。

Present threshold pressure gradient test cannot characterize the present situation of real threshold pressure gradient. After back pressure control system with high precision was introduced,experimental approaches were carried out to evaluate the threshold pressure gradient in the tight gas reservoir under reservoir conditions. Compared with the measured result under normal pressure,the threshold pressure gradient obtained by the new method is lower under the condition of same permeability and the difference becomes larger when core permeability is lower. Results show that the threshold pressure gradient is not a constant during the development of reservoir,but varies with the pore fluid pressure,and it is called dynamic threshold pressure gradient. New concepts of threshold pressure gradient sensitivity and threshold pressure gradient sensitive coefficient were proposed and defined to describe the dynamic threshold pressure gradient in the tight gas reservoir. The influencing factors of dynamic threshold pressure gradient were experimentally studied as well. Results indicate that the threshold pressure gradient of the tight cores show a linear increase with the decrease of pore fluid pressure. When the core permeability is lower and the water saturation is larger,the threshold pressure gradient becomes larger under the condition of reservoir and the variation range of the threshold pressure gradient during the development is larger. The threshold pressure gradient sensitivity is stronger and the dynamic variation of threshold pressure gradient is more obvious.