A modified fracture gradient relation and its application to East Texas and the Timor Sea

Abstract

<jats:p>Theoretical fracture gradient relations are generally based on the assumption that the sedimentary sequence behaves elastically under conditions of lateral constraint. Hence the minimum horizontal stress (σhmin) is given by: where V is Poisson's ratio, σv is overburden stress, pp is pore pressure, and at is far -field tectonic stress. In driling practice, fracture initiation, or leak -off pressures, which are related to σhmin are most commonly predicted by the application of empirical stress /depth relations such as that proposed for offshore Western Australia by Vuckovic (1989): Leak -off pressure (psi) = 0.197D1145, where D is depth in feet. A modified form of the uniaxial elastic relation for the prediction of σhmin is proposed, such that: where the constants c and d are straight line regression constants derived from cross -plotting effective minimum horizontal stress and effective vertical stress. This relation, as opposed to previous empirical approaches to fracture gradient /σhmin determination, yields regression coefficients of physical significance: c represents the average Poisson's ratio term, v /(1 -v), and d represents an estimate of the tectonic (and inelastic) component of the minimum horizontal stress. This application of the modified fracture gradient relation, termed the effective stress cross -plot method, is tested successfully against published data from experimental wells in the East Texas Basin where independent estimates of Poisson's ratio are available. Leak -off pressures have been compiled from 61 wells in the Timor Sea. Leak -off pressures in the Timor Sea are somewhat lower than predicted by Vuckovic's (1989) stress /depth relation for offshore Western Australia, and a new, empirical stress /depth relation, which better fits the Timor Sea data is proposed: The effective stress cross -plot method is also applied to the Timor Sea data, yielding: Detailed pore pressure data were not available for the Timor Sea data -set and the effective stress cross -plot method does not fit the observed data any better than the new empirical stress /depth relation. However, the regression constants suggest an average Poisson's ratio of 0.26 and a relatively insignificant tectonic stress of 1 MPa for the Timor Sea. </jats:p>