A New Method for Predicting the Total Vertical Stress Field and Formation Pore Pressures

Abstract

Abstract Estimation of formation pore pressure requires the knowledge of the overburden load in an area, which is commonly determined by vertical integration of density data. In fields where the total vertical stresses are significantly affected by topography (e.g., onshore valleys, sub-sea canyons) or lateral density variations (e.g., salt domes), the vertical integration can over- or underestimate total vertical stresses which can lead to over- or underprediction of pore pressures. In this paper, to improve the accuracy of the overburden stress estimations in areas of complex topography and lateral density variations and subsequent the pore pressure predictions, we present a new method to calculate total vertical stresses which utilizes an analytical solution to calculate the contribution of each volume element on the vertical stress field. We also perform finite element analysis for two different geographic cases, quantify the error in pore pressure predictions and compare the results of finite element analysis with those of the proposed method. Moreover, we employ the method proposed in this paper to calculate overburden stresses in two case studies which are developed based on field data. Our results show that the effect of topography and lateral density variations on the total vertical stress field can result in significant errors in pore pressure predictions if the total vertical stresses are calculated by vertical integration. On the other hand, the total vertical stress calculated by the proposed method are in good agreement with finite element analysis results and the improved method provides a framework to enhance pore pressure predictions. The proposed method does not require any complex processing of data and calculation times are lower relative to finite element and finite difference analysis. Moreover, the proposed method requires only 3-D density distribution and spatial information about the discrete domain (i.e., dimensions of the volume elements), which are also required for overburden stress estimation using the vertical integration method. In other words, the proposed method improves the pore pressure prediction workflows by refining the overburden calculation algorithm.