Research paperEstimating pore fluid pressure-stress coupling

- A new method for quantification of fluid pressure-stress coupling.
- Coupling values range from 0.24 to 0.43, based on data from 11 case studies.
- Values are approximately half historical values determined using pressure gradients.
- Modelling shows how coupling need not involve poroelasticity.
- Case studies do indicate possible contribution from poroelasticity.

Fracture Pressure (FP) prediction is a vital component in well planning for safe drilling in high pressure areas, in the estimation of seal capacity and seal breach risk analysis, for fluid injection and in fracture stimulation for oil/gas recovery from tight reservoirs and mature source rocks.Coupling of pore fluid pressure (PP) and FP at basin-scale has been recognised for several decades, with coupling ratio values mostly between 0.46 and 0.87, i.e. for every 10 MPa of PP increase at constant depth, FP increases by 4.6-8.7 MPa. These values are similar to estimates for reduction of FP with reservoir depletion of an oil/gas field during production. Poroelasticity has been suggested as the principal reason for coupling. Most previous coupling values were generated by examination of pressure gradients. A new method to estimate pore fluid pressure-stress coupling is proposed which plots the measured value of FP from a Leak Off Test (LOT) minus the expected FP value on the trend for normal pressures (termed the Fracture Pressure Residual, FPr) against overpressure (OP), which is the magnitude of PP above normal at the same depth. The coupling value is now the slope of FPr against OP. FPr:OP coupling values from 11 global basin datasets range from 0.24 to 0.43, approximately half the previous quoted basin-scale coupling values.A pressure-depth model for rapid deposition on a continental margin, with OP exclusively generated by disequilibrium compaction and no change in effective stress in the overpressured section, represents a base case for compaction OP without poroelasticity. This model helps to reveal (1) that coupling values vary with the ratio of FP to lithostatic pressure (Sv); (2) since the 11 case study coupling values exceed the base case, one or more other (minor) coupling mechanisms are involved, potentially including poroelasticity; (3) values may be independent of tectonic environment; (4) one case study involving carbonate (chalk) is within the same range as those from siliciclastic sediments, and (5) the high coupling pore pressure-stress coupling values in the literature result from using fluid and fracture gradients including the shallow data where PP is normal, which distort the analysis.