--> Observations On Fluid/Fracture Pressure Coupling Ratios

AAPG Asia Pacific Region GTW, Pore Pressure & Geomechanics: From Exploration to Abandonment

Datapages, Inc.Print this page

Observations On Fluid/Fracture Pressure Coupling Ratios

Abstract

Early attempts at creating fracture pressure models incorporated the effect of variations in fluid pressure on fracture pressure by equating fracture pressure to fluid pressure and then adding to the fluid pressure a portion (x) of the difference between fluid and fracture pressure. The net coupling for fluid pressure and fracture pressure of this approach at constant Sv would be 1-x. This approach typically creates fluid/fracture pressure coupling ratio values between 0.1 and 0.5 and fracture pressures were limited to the pressure range between fluid pressure and Sv.

Observations on the variation of fracture pressure with fluid pressure reduction associated with hydrocarbon production yielded fluid/fracture pressure coupling ratio values as large as 0.8 and as small as 0.36. The cause of the coupling was interpreted as poroelastic response to the effective stress reduction during hydrocarbon production.

Sedimentary basin-scale studies of fluid and fracture pressures expressed as gradients yielded estimates of the fluid/fracture pressure coupling ratio values between 0.60 and 0.76 and also interpreted coupling as a poroelastic response. Reexamination of the basin-scale data without use of pressure gradients and without the direct use of fluid and Sv pressures as limits on the calculated fracture pressures yielded fluid/fracture pressure coupling ratio values between 0.28 and 0.43 for 11 basins examined. Poroelasticity can be excluded as an explanation for most basin-scale coupling.

Uniaxial strain models indicate that fluid/fracture coupling ratios can be related to Poisson’s ratio/stress ratio and a Biot coefficient (usually set equal to 1.0). With this model, low coupling values would be associated with high Poisson’s ratios (plastic). Coupling values obtained from LOT and fluid pressure from 11 basins are compared with coupling values derived from stress ratios derived from the same data set. The two approaches yield similar, but not identical coupling values. In all cases the coupling value derived directly from LOT and fluid pressure data was larger by values ranging from 0.004 to 0.10. The cause for the difference is not known. Poisson ratio values associated with the stress ratios ranged from 0.40 to 0.44. These values are consistent with expected plastic properties of rapidly sedimented, high porosity shales. Poisson’s ratio data can also be obtained by laboratory testing or by calculation from Vp and Vs data. Fluid/fracture pressure coupling values obtained from interpretation of fluid and fracture pressure data will be contrasted with coupling values obtained from laboratory tests and velocity studies.

Published laboratory-measured Poisson’s ratio values near 0.5 (plastic/very low coupling values) appear to be restricted to samples associated with very low effective stress and shallow burial. Reduction in porosity by compaction (increased effective stress) or cementation is associated with reduced Poisson’s ratios and higher coupling values. This model of reducing Poisson’s ratios with burial depth/compaction contradicts some existing models of Poisson’s ratio behavior which project Poisson’s ratio values increasing from near 0.20 at the seabed (brittle) and rising to 0.5 (plastic) at 20,000 feet below mudline.

A basin model with normal compaction to top of overpressure and a lithostatic-parallel fluid pressure gradient with depth below top of overpressure, (constant shale porosity/vertical effective stress) would be characterized by near-constant Poisson’s ratio and coupling values with depth below the top of overpressure. These types of fluid pressure profiles are typical of mud-dominated, stratigraphic successions found along continental margins. The onset of chemical diagenesis and cementation, most likely at temperatures between 100o and 120oC would be expected to decrease the Poisson’s ratio and increase the coupling coefficient.