Insitu stress determination and its applications

ABSTRACT: Insitu stresses are the most crucial parameters for subsurface engineering design and geomechanical modeling. Therefore, insitu stress determination should be done prior to geomechanical modeling. Accurate determination of insitu stresses is still very challenging. For instance, traditional methods may not give a satisfied solution; however, sophisticated models need more parameters, which may bring more uncertainties. The minimum and maximum horizontal stresses are mainly controlled by vertical stress, lithologies, pore pressures (overpressures and depletion), and tectonic stresses. Our study on depletion-induced stress changes indicates that vertical and horizontal effective stresses increase because of reservoir depletion. These effective stress increments can cause reservoir compaction, permeability reduction, and surface subsidence. Commonly used methods for calculating insitu stresses are reviewed. Practical methods to predict the minimum and maximum horizontal stresses with less uncertainty are presented. These include improved tectonic stress method, modified Daines' method, and anisotropic method for calculating both minimum and maximum horizontal stresses. Accurate calculation of the maximum horizontal stress has more challenges, and improved methods are given for a better estimation of the stress. Poisson's ratio dependent insitu stress polygon method is also proposed for estimating the horizontal stresses. A case study is provided to illustrate how to determine insitu stresses in unconventional resources.