Relationship Between Horizontal Stress and Depth in Sedimentary Basins

Abstract

Summary Previous work on establishing a relationship between in-situ stress and depth is reviewed. On the basis of data from literature, supplemented by available field data, equations are proposed to describe the trend of horizontal stress with depth for normally pressured formations in the U.S. gulf coast region. A separate correlation between horizontal stress and pore pressure allows this trend to be extended to formations containing abnormal pore pressures. Having achieved a good correlation for data from this well-documented region, we repeat the process for other parts of the world, using data from hydraulic fracture treatments and formation integrity (leakoff) tests. Similar relationships between horizontal stress and depth are found to hold for normally pressured areas that are geologically and tectonically similar to the U.S. gulf coast. Coupled with regional correlations between horizontal stress and pore pressure, these relationships enable horizontal stress levels to be estimated for a given depth, provided the pore pressure also is known. Introduction The instantaneous shut-in pressure (ISIP) recorded during or after a fracturing job or during a minifracture test provides a good approximation to the minimum principal in-situ total stress components sHmin. The vertical total stress or overburden stress sV (normally the maximum principal in-situ stress) can be derived by integration of the formation density, compensated (FDC) log. In tectonically inactive areas, the intermediate principal stress, sHmax, can be assumed approximately equal to the minimum principal stress: sHminsHmax<sV. In the more general case, when sHmax>sHmin, the value of sHmax in principle can be derived from the formation breakdown pressure pb as measured at the start of a fracturing job. For a fully plastering fluid, for instance, pb 3sHmin-sHmax-Pc where Pc is the pore pressure. In practice, however, sHmax cannot be determined accurately because the breakdown value will be influenced by hole geometry, hole integrity, mud-cake properties, and the extent to which the fluid penetrates into and pressurizes the pore space around the borehole. Also, hydraulic fracturing data are often lacking in many areas of the world. The only remaining way, then, to obtain information relation to in-situ stresses is to analyze the results of formation integrity tests (leakoff tests and casing-seat tests). In these tests, the pressure at which the formation starts taking fluid may range from sHmin to 2sHmin-Pc. Therefore, it is the lower end of the range of values from formation-integrity tests that is of interest for in-situ stress determination. Review of Previous Work The majority of hydraulic fracturing/formation-integrity test data quoted in literature are from the U.S. gulf coast and Santa Barbara Channel (CA). From these data, a number of authors have derived fracture gradient correlations to be used in planning drilling programs.