The Importance of In-Situ-Stress Profiles in Hydraulic-Fracturing Applications

Abstract

Technology Today Series articles provide useful summary information on both classic and emerging concepts in petroleum engineering. Purpose: To provide the general reader with a basic understanding of a significant concept, technique, or development within a specific area of technology. Summary In-situ stresses define the local forces acting on lithologic layers in the subsurface. Knowledge of these stresses is important in drilling, wellbore-stability, and, especially, hydraulic-fracturing applications. The measurement of in-situ stress is not straightforward and, therefore, often goesunmeasured. As such, we often assume values of in-situ stress or estimate in-situ stresses from logging parameters. This article illustrates the importance of in-situ-stress estimates as they relate to hydraulic fracturing and outlines several techniques for estimating in-situ-stress magnitudes. In-Situ Stress The in-situ stresses acting on a formation can be decomposed into three principal compressive stresses, one vertical and two horizontal. The two horizontal compressive stresses are usually not equal. The vertical stress is caused by the overburden weight acting on the top of a formation. The horizontal stresses are the result of the poroelastic deformation of the rocks plus externally applied tectonic forces. The parameters that affect the magnitude of the in-situ stresses include overburden weight, fluid porepressure, porosity, anomalies in the rock fabric (i.e., natural fractures), rock mechanical properties (such as Poisson's ratio), and tectonic activity. Knowledge of the in-situ-stress magnitude and direction can impact decisions and designs throughout the drilling and completion of a well. During drilling, in-situ stress may affect the mud and cement densities required to prevent unwanted fracturing of openhole strata in the wellbore. For wells that will be stimulated at high pressures, casing design must account for the maximum anticipated stresses. Wellbore-stability calculations, particularly for horizontal wells, require knowledge of in-situ-stress magnitude and direction. For hydraulic-fracture treatment applications, the in-situ stresses control fracture azimuth and orientation (vertical and horizontal), fracture-height growth, fracture width, treatment pressures, and fracture conductivity. As Fig.1 illustrates, fractures grow perpendicular to the minimum in-situ-stress direction; thus, stress direction can affect well-placement and -spacing decisions.