
TY  - JOUR
A1  - Hillis, R.R. 
A1  - Crosby, D.G. 
A1  - Khurana, A.K. 
T1  - A MODIFIED FRACTURE GRADIENT RELATION AND ITS APPLICATION TO EAST TEXAS AND THE TIMOR SEA
JF  - The APPEA Journal
JO  - The APPEA Journal
Y1  - 1997
VL  - 37
IS  - 1
SP  - 536
EP  - 545
KW  - 
UR  - https://doi.org/10.1071/AJ96032 
N2  - 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 (&#x3c3;hmin) is given by:  where V is Poisson's ratio, &#x3c3;v 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 &#x3c3;hmin 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 &#x3c3;hmin 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 /&#x3c3;hmin 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. 
ER  -

