零水势面的确定与地层压力体系的划分--以鄂尔多斯盆地延安气田为例

Determination of zero water potential surface and division of formation pressure system: A case study of Yan'an Gas Field in the Ordos Basin

地层静水压力计算和地层压力系统划分通常都采用地表起算的方法,但该方法不适应于地面起伏大、承压面低于地表的地区,而现场采用的水位监测也存在着较多的问题。为了正确认识鄂尔多斯盆地延安气田陆相页岩气藏的压力系统,基于所建立的地层启闭体系识别模型,从流体力学的原理出发,利用开放体系下的实测压力和零水势面海拔计算公式得到了零水势面海拔,计算了地层压力异常的幅度和大小,进而分析了该气田零水势面和地层压力体系的分布特征。研究结果表明:①延安气田上三叠统延长组7段以上地层为静水压力体系;②延安气田零水势面与地表不重合,多分布于地表之下,为一个起伏的曲面,而非传统认知的直线或平面;③以零水势面起算的下二叠统山西组多表现为异常低压,仅在研究区西北部存在着小幅度超压;④由地表起算的异常压力误差介于0.07~3.57 MPa,埋深在2600 m以深时,地表起算形成的压力误差随埋深的增大开始增大;⑤零水势面起算的异常压力在埋深小于2300 m分布着异常低压且随着埋深增加而增大,埋深介于2300~2600 m分布着微小超压,埋深超过2600 m分布着异常低压且随着埋深增加而减小。结论认为,剥蚀厚度与零水势面起算的异常压力相关性更好,零水势面起算的异常压力更加符合于地质实际。

Calculation of formation hydrostatic pressure and division of formation pressure system are commonly performed with the earth surface as the zero point.However,this method is not suitable for the areas where the earth surface relief is great and the pressure-bearing surface is lower than the earth surface.What's more,water level monitoring used on site also faces more problems.In order to accurately understand the pressure system of the continental shale gas reservoir in the Yan'an Gas Field of the Ordos Basin,this paper obtains the altitude of the zero water potential surface by using the pressure measured in the open system and the zero water potential surface altitude calculation formula,based on the stratigraphic opening and closing system identification model and the principle of fluid mechanics.Then,the magnitude and value of the formation pressure anomaly are calculated,and the distribution characteristics of the water potential surface and formation pressure system in the Yan'an Gas Field are analyzed.And the following research results are obtained.First,there is a hydrostatic pressure system in the strata above the 7th Member of the Upper Triassic Yanchang Formation in the Yan'an Gas Field.Second,in the Yan'an Gas Field,the zero water potential surface does not coincide with the earth surface and is mostly distributed below the earth surface.It is a curved surface instead of a straight line or a plane as traditionally recognized.Third,the lower Permian Shanxi Formation calculated from the zero water potential surface is mostly characterized by subpressure except small-amplitude overpressure in the northwest of the study area.Fourth,the error of the abnormal pressure calculated from the earth surface is between 0.07 MPa and 3.57 MPa.And below the burial depth of 2600 m,the pressure error calculated from the earth surface begins to increase with the increase of the burial depth.Fifth,when the burial depth is above2300 m,the abnormal pressure calculated from the zero water potential surface develops subpressure and increases with the increase of burial depth.A slight overpressure develops in the burial depth of 2300-2600 m.And below the burial depth of 2600 m,subpressure develops and decreases with the increase of burial depth.In conclusion,the denudation thickness is better correlated with the abnormal pressure calculated from the zero water potential surface,so the abnormal pressure calculated from the zero water potential surface can reflect the geological reality more accurately.