致密-低渗透油藏两相启动压力梯度变化规律

致密-低渗油藏油水两相渗流时存在启动压力梯度，导致基质中剩余油和残余油的驱动非常困难，水驱开采效果普遍较差。为了确定致密-低渗储层两相启动压力梯度变化规律，通过物理模拟方法研究了一维水驱油过程中两相启动压力梯度与平均含水饱和度之间的关系，并分析了两相启动压力梯度的形成原因。结果表明，两相启动压力梯度随平均含水饱和度增大先升高后降低，在油水前缘位于出口端附近时达到最大值；两相启动压力梯度最大值与岩心气测渗透率呈良好的幂函数关系，渗透率越低两相启动压力梯度最大值上升速度越快；同一岩心中两相启动压力梯度远大于原油启动压力梯度。可见致密-低渗油藏水驱过程中两相启动压力梯度随着油水前缘运移先增大后减小，同时油水间毛管力是两相启动压力梯度形成的主要原因。

Change law of two-phase threshold pressure gradient in tight - low permeability reservoir

The pressure gradient exists in the oil-water two-phase percolation in tight and low-permeability reservoirs, which makes it very difficult to drive the remaining oil and residual oil in the matrix, and the water flooding production effect is generally poor. In this paper, the threshold pressure gradient and critical pressure gradient of water flooding with different permeability were experimentally measured, and the relationship between average water saturation and critical pressure gradient of water flooding during one-dimensional water flooding was analyzed. Besides, the formation reason of critical pressure gradient of water flooding oil was analyzed. The results show that the critical pressure gradient of water flooding increases first and then decreases with the increase of the average water saturation, and reaches a maximum value when the oil-water front is near the outlet end. The relationship between the maximum critical pressure gradient of water flooding and core gas permeability is a good power function, and the lower the permeability, the faster the maximum value of the critical pressure gradient of water flooding increases. It can be seen that the two-phase threshold pressure gradient first increases and then decreases with the migration of oil-water leading edge during water drive in tight and low-permeability reservoirs, and the capillary force between oil and water is the main reason for the formation of the two-phase starting pressure gradient.