Inversion Technique To Determine the In-Situ, Stress Field From Fracturing Data

Abstract

Abstract Typically one assumes that the two horizontal in-situ stresses are equal in borehole stability studies. This simplification has been necessary due to lack of information. The new method presented here distinguishes between the in-situ stresses and estimates the magnitude and the direction of each of the two horizontal in-situ stresses. The input data used are the leak-off fracturing information. The new method uses stress transformation equations to take advantage of the directional characteristics of offshore boreholes. By including the hole inclination and the azimuth, one can solve for the two unequal horizontal in-situ stresses. The model results in finding the inverse of an overdetermined system of equations. A field study demonstrates the method. Introduction Experience from geotechnical and mining engineering shows that the in-situ stress field is typically nonhydrostatic. This means that all three principal in-situ stresses have different magnitudes, a reasonable assumption for deep boreholes as well. Knowledge of the in-situ stress field is a key factor in any work related to rock mechanics. Realizing its importance, the oil industry is using novel techniques like mini fracturing testing and strain recovery of cores to an increasingly larger extent. The overburden stress is readily obtained from logs and, if the horizontal stresses are known, the principles of mechanics can be used to analyze principles of mechanics can be used to analyze borehole problems like fracturing, lost circulation, collapse and sand production. Also, a potential exists to minimize hole problems by planning the trajectory and direction of the hole and the directions of the perforations. Hydraulic fracturing is an important method to stimulate tight reservoirs. The fractures will propagate at a ninety degree angle to the least principal in-situ stress. Therefore, it is also important to know the in-situ stress field to plan a proper field drainage. plan a proper field drainage. P. 55