等效孔隙结构模型在鄂尔多斯致密含气砂岩中的应用

摘要

Core samples from a well in Ordos basin are used to study the elastic properties of tight gas sands in lower Shihezi Formation. Elastic parameters are measured at in-situ physical conditions. The total pore space is divided into three space types based on the microscopic analyses. Soft porosity model (SPM) is modified to construct a new model which could represent the tight gas sands reservoir. A modeling example using the well-log data from Daniudi gas field is completed using self-consistent theory. Model predicted results include Vp, Vs, Poisson’s ratio, acoustic impedance of P wave and acoustic impedance of S wave. Comparison of those elastic properties derived from the model, well-log, and core sample measurement is conducted to evaluate the new model. Predicted Vp, Vs are overall in agreement with well-log data. The same agreement also appears in Poisson’ s ratio curve. But the model predicts a slightly different behavior of rocks from the core samples in the impedance of P wave (AIp) and acoustic impedance of S wave (AIs). Predicted AIp and AIs are slightly higher than those of core samples. Pressure may be the main factor responsible for the difference between the model and core samples. The AIp and AIs are calculated from velocity and density, which are derived from the well-log data. The density in-situ condition is higher than what is measured in laboratory because of the pressure produced by upper strata. So the predicted AIp and AIs show a slight rise in comparison to the core samples. This article provides a good example of applying SPM to tight gas sands of Ordos, and show that the model will be very helpful to predict elastic properties of tight gas reservoirs.%该文作者采集了鄂尔多斯大牛地气田二叠系下石盒子组钻井岩芯，在60 Mpa围压和50℃温度下进行了岩芯弹性参数测试。通过岩石薄片分析孔隙结构特征，按照孔隙的纵横比值（<0.01,0.01~0.9,>0.9）将孔隙划分为三类，进而对柔性孔隙模型理论进行了修改，建立了孔隙结构模型。建立的模型应用于常规测井数据，通过自洽算法进行了纵波、横波速度和弹性参数的估算。模型计算结果与测井曲线、岩芯实验测试结果进行了对比。结果显示模型与测井的速度曲线和泊松比值曲线能很好地吻合，但模型预测的波阻抗值稍大于岩芯的测试结果。其原因在于利用模型计算声波阻抗时使用的密度值来自于测井数据，而在储层状态下,由于上覆地层的压力，会使得岩石的密度增大,导致波阻抗值上升。研究成果表明了柔性孔隙模型在鄂尔多斯致密砂岩弹性参数的预测中能取得很好的效果。