Obtaining both Horizontal Stresses from Wellbore Collapse

Abstract

Abstract One of the most important design parameters in all wellbore stability analysis is the two horizontal stresses. The third principal stress, the overburden stress is determined by integrating the rock bulk density from cuttings or from logs. There exist several approaches to estimate the minimum horizontal stress, σh, but it is more difficult to determine the maximum horizontal stress σH. Inversion methods have been used with success, but usually the maximum horizontal stress is assumed, not measured. A wellbore that collapse during drilling or during production usually assumes an elliptical shape because of anisotropic stress loading. Recently an exact solution for tangential stress of an elliptic wellbore was derived. This model couples the ovality of the wellbore to the stress anisotropy. The rock strength plays an important role, such that a strong consolidated rock will have less ovality than a less consolidated rock. In addition to the solution above, the full 3-dimensional in-situ stress state is investigated to obtain physically relevant solutions. This means the fracture pressure always exceed the collapse pressure for any well orientation. This condition is imposed on the new solution above. Leak-off data are used for calibration. In other words, the solution is based on real caliper data and real fracturing data. This new solution has many practical applications, first of all to develop fracture and collapse curves for deviated wells, but also for sand production, well stimulation and reservoir subsidence. The paper will present several field cases, both from Norway and Brazil, demonstrating improvements in practical wellbore stability analysis.