致密砂岩气井的应力敏感效应渗流模型

 常规气藏的压裂气井生产时一般包括线性流动和径向流动阶段,椭圆流动被认为是过渡阶段而往往被忽略.致密砂岩气藏的渗透率极低,具有较强的应力敏感性,压力传播速度较慢,各个流动阶段在时间尺度上被延长,椭圆流动将持续很长一段时间,成为致密砂岩气井的重要流动阶段.目前试井分析中,常规模型与实际压力双对数曲线往往有较大差距.为精确描述致密砂岩气井的流动特征,运用保角变换方法建立椭圆系坐标下的渗流模型,采用应力敏感系数表示储层的应力敏感特征,通过摄动方法和变量分离,将拉普拉斯空间下的控制方程转换为马丢方程和变型马丢方程从而求解.通过分析理论曲线发现:应力敏感效应增加了拟井底流压降,并使得拟压力导数曲线末端上翘,产生假边界现象.该模型能够更加准确表达致密砂岩气井的渗流特征,对致密气试井具有较好的研究意义.

Analytical Model in Elliptical Flow Period of Tight Sand Gas Well Considering Stress Sensitivity

The flow phases of fractured gas well in a conventional reservoir usually cover a linear flow period and a radial flow period. The elliptical flow period is usually regarded as a transition phase and thus ignored. The tight gas is the most realistic unconventional gas in China. Tight gas reservoirs are featured with low permeability and strong stress sensitivity. The pressure transmits slowly and every flow period is fairly extended. The elliptical flow phase at a tight gas well keeps a quit long time, even lasts for a couple of years. Therefore, the elliptical flow stage can't be ignored. When it comes to well test analysis, the conventional model is usually hard to match the double log curve in the field. To describe the flow phases more accurately, a flow model in elliptical coordinates is built through conformal transformation. The stress sensitivity coefficient is utilized in the model to consider the stress- sensitivity of tight gas reservoirs. After perturbation transforming and variable separation, the controlling equation is transformed into the Mathieu equation. Then it can be solved with a corresponding method. It's found that the stress sensitivity makes a higher pseudo-BHP drop and an end of derivative curve up warp. This model can simulate the flow period of tight sand gas well more accurately, and is significant to well testing of tight gas reservoir.