In any depleting reservoir in which the vertical stress is larger than the two horizontal stresses, there is the potential to induce normal faulting when the change of horizontal stress, ASh, exceeds a critical value of APp (the change in pressure due to depletion). Defining the stress path with depletion through time AS?/APp as ,4, we find the critical value of`4 to be 0.67 based on Coulomb faulting theory and a coefficient of friction of 0.6. In situ stress and pore pressure data from the Valhall and Ekofisk oil fields in the North Sea indicate that production-induced pore pressure reductions cause poroelastic reductions of the least principal stress. As a result, normal faulting appears to have spread out from the crests of the structures to the flanks. Pore pressure and stress changes accompanying production in the McAllen Ranch field in South Texas indicates that ,4 ranges from 0.3-0.4 which imply that production-induced normal faulting is not likely to occur.
While it is well known that fluid injection can induce faulting in oil and gas reservoirs, several studies have reported that both fluid withdrawal and fluid injection appear to have induced active faulting (see review by Grasso, 1992).
Active faulting in oil and gas reservoirs is of importance for a variety of reasons. Slip on active faults appears to be the cause of sheared casings of production wells in some fields (Maury et al., 1992) and, in others, shear slip on pre-existing faults and bedding planes appears to be a serious source of wellbore instability during drilling (e.g., Willson et al., 1998). Critically stressed faults in many low permeability reservoirs (i.e., faults that are active in the present stress field) contribute importantly to the overall reservoir permeability (Finkbeiner et al., 1998; Dohlakia et al., 1998). Meanwhile, reactivation of reservoir bounding faults can cause a loss of seal capacity and leakage to occur (e.g., Wiprut & Zoback, 1999). Slip on active faults may also control the vertical extent of hydrocarbon column that a fault-bounded reservoir can contain (e.g., Finkbeiner et al., 2000).
Mechanisms of Production-Induced Faulting
Increases in pore pressure can cause slip along preexisting faults by reduction of the effective normal stress on the fault plane as described in the Coulomb failure criterion:
[Equation available in full paper]
where x is the shear stress on the fault, Sa is the normal stress, Pp is the pore pressure and !a is the coefficient of friction (c.f., Jaeger & Cook, 1971). Thus, by raising Pp, the effective normal stress, Sa Pp on pre-existing faults is reduced. A fault can therefore slip at a lower shear stress than would normally be required.
Another mechanism of induced faulting in the vicinity of oil and gas reservoirs is poroelastic stress changes in the medium surrounding a compacting reservoir (Segall, 1985, 1989, 1992). In this case, faulting is induced by the superposition of preexisting stresses with the stresses caused by pore pressure decreases in the reservoir.
We discuss a third mechanism in this paper by which normal faulting is related to changes in poroelastic stress within the reservoir themselves. We demonstrate that the stress path associated with reservoir production is such that in some reservoirs the poroelastic decrease in the least horizontal stress can lead to normal faulting in the reservoirs. These processes may be active in several oil and gas fields where normal faulting within reservoirs appears to have been induced by hydrocarbon production (c.f., Doser et al., 1991).