异常高压气藏应力敏感性研究

RESEARCH ON STRESS-SENSITIVITY IN ABNORMAL HIGH PRESSURE GAS RESERVOIR

克拉2异常高压气藏是目前我国探明的最大整装干气气藏,属于异常高压气藏。对于这种具有极高异常压力的气藏,在衰竭式开采过程中,地层压力逐渐下降,作用在岩石颗粒上的有效应力增加。这种效应有可能产生岩石形变,产生应力敏感,使得岩石物性参数孔隙度、渗透率减小,从而影响到气藏流体的流动动态及气井产能,给高效、合理地开发带来许多困难和问题。为此,设计模拟地层条件下储层应力敏感的试验流程与试验方法,以克拉2异常高压气藏的砂岩岩芯为试验对象,进行不同有效应力下储层物性应力敏感性试验以及应力敏感岩芯往复试验研究。试验结果表明,初始渗透率越低,则应力敏感性越强,孔隙度对有效应力的敏感性低于渗透率的应力敏感性。同时,通过三轴高温、高压岩石变形与渗透试验仪得到岩石典型的全应力–应变曲线,定量描述地层压力降低后,岩石出现永久塑性变形的特征,这是储层物性产生应力敏感性的原因。这些研究对该气藏合理开发是非常必要的,对气藏动态储量的计算、产能评价及合理生产工作制度的确定具有重要意义。

Kela 2 gas reservoir is the largest dry gas reservoir so far discovered in China. It has such features as high pressure and pressure coefficient, and gas reservoirs with a pressure coefficient of over 2.0 are not commonly found. The abnormal high-pressure gas reservoirs are quite different in characteristics and performance during the process of depletion exploitation. During the depletion exploitation, the formation pressure depletes slowly, but the effective stress to the rock grain increases. The effective stress may cause rock to deform, resulting in the elastic or plastic deformatiom and this deformation makes the rock physical property parameters such as porosity and permeability become quite sensitive to effective stress. Therefore, it is necessary to know the property of stress sensitivity for this abnormal high-pressure reservoir. The testing chart and method being used to study the property of stress sensitivity are designed. By means of testing porosity and permeability of sand rock core sample of Kela 2 gas reservoir under different effective stresses, the reduction property of porosity and permeability is obtained. The results demonstrate that the rock permeability behaves in an exponential manner with the effective stress variation, and the lower initial permeability would lead to the stronger stress sensibility; the same variation has been found for the porosity, but the magnitude of the effective stress sensibility of porosity is lower than that for permeability.The complete stress-strain curve is obtained through triaxial high temperature and high pressure rock deformation- seepage test. The characteristic of permanence plastic deformation with the dropping of the formation pressure can be quantitatively described~ this is the reason of the stress sensibility of reservoir physical property. These results offer a very important base and guide for researching and designing the production performance of an abnormal high-pressure gas reservoir.