页岩与致密砂岩气井产气机理及生产动态模拟对比

A comparative study on production mechanism & dynamics simulation of tight sandstone and shale gas well

为揭示页岩和致密砂岩气开发方式差异性的原因,运用页岩与致密砂岩全直径岩心模拟气井全生命周期开发动态,研究2种气藏产气机理。实验结果表明:页岩气生产过程包括高速、中速和低速开发3个阶段,只有中速开发阶段地层视压力与累计产气量呈线性关系,压力降到12 MPa时偏离原有的线性关系;而致密砂岩地层视压力与累计产气量基本呈线性关系,只在压力接近于0.1 MPa时偏离原有的线性关系。致密砂岩高速开采阶段采出程度达90%,低速开发采出程度低;而页岩高速开采阶段采出程度只有17%,中低速开发阶段采出程度可以达到50%,二者低速开发阶段单位压降采气量大幅增加,证明都含有一定量的吸附气,差别在于页岩解吸压力和吸附气量占总气量比例相对较高。最后,根据页岩渗透率和吸附气的认识,建立考虑滑移效应与Langmiur吸附效应的相对简单的页岩全生命周期渗流模型,数值模拟页岩全直径岩心全生命周期生产动态,视压力曲线、日产气递减曲线与实验结果具有很好的一致性,拟合游离气量和吸附气量相对误差不足5%,证明该模型预测页岩气井产能的可行性。

In order to reveal the reasons for the differences between the shale and tight sandstone gas development methods,the full-diameter cores of the shale and tight sandstone are used to simulate the whole life-cycle development dynamics of gas wells and study the gas production mechanism of the two kinds of gas reservoirs.The experimental results show that the shale gas production process includes three stages:high-speed,mediumspeed and low-speed development stage,and only in the medium-speed development stage the stratigraphic apparent pressure has a linear relationship with the cumulative gas production,which deviates from the original linear relationship when the pressure drops to 12 MPa. The tight sandstone formations,on the other hand,have an essentially linear relationship between apparent pressure and cumulative gas production,deviating from the original linear relationship only when the pressure approaches 0.1 MPa. The recovery percent of tight sandstone is 90% in the high-speed stage,and that in low-speed stage is low. While the recovery percent of shale is only17% in the high-speed stage and can reach 50% in the medium-speed and low-speed stage. In the low-speed stage,the gas production per unit pressure drop increases greatly,which proves that both of shale and tight sandstone contain a certain amount of adsorbed gas,and the difference lies in the relatively high proportion of desorption pressure and adsorbed gas in the total gas volume of shale. Finally,this paper builds a relatively simple whole life-cycle percolation model for shale that considers the slippage effect and Langmiur adsorption effect based on the knowledge of shale permeability and adsorbed gas. Numerical simulation of the whole life-cycle production dynamics of the full-diameter shale core,the apparent pressure curve,daily gas production decreasing curve are in good agreement with the experimental results,and the relative error of the fitted free gas volume and adsorbed gas volume is less than 5%,which proves the feasibility of the model in predicting the production capacity of shale gas wells.