Evolution of the Reservoir Stress Path During Depletion and Re-Inflation and its Implications

Several researchers have studied the impact of reservoir depletion on rock mechanical behavior and associated changes in in-situ stresses and reservoir depletion stress path. However, the evolution of the reservoir stress path (ratio of change in minimum horizontal stress with reservoir pressure change) due to re-inflation of the reservoir during injection operations such as waterflooding is less understood.

In this work, a more fundamental approach to the evolution of the reservoir stress path under both depletion and re-inflation is studied through core plug uniaxial-strain experiments and field-scale numerical modeling which includes the impact of plastic deformation. Both unconsolidated and consolidated sandstones are considered. Results from the uniaxial-strain compaction experiments agree with findings from 3-D field scale numerical modeling that the injection stress path coefficient γ_h,i = Δσ_h/ΔP_inj is either similar or greater than the depletion stress path coefficient γ_h,d = Δσ_h/ΔP_dep.

The findings in this study have significant implications for waterflooding operations, infill drilling window estimation for re-inflated reservoirs, and CO₂ capture and storage operations (CCS). The assumptions on γ_h,i directly impact the allowable injection pressure, injection rate, and storage capacity for CCS projects. Hence a more thorough understanding could prevent overly conservative designs and promote a more accurate screening of potential waterflooding, infill drilling, and CCS projects.

The reservoir minimum horizontal stress path is the ratio of change in total minimum horizontal stress, Δσ_h to change in reservoir pressure, ΔP:

(equation)

It is crucial to understand the anticipated minimum horizontal stress changes in a reservoir during long-term production and during injection operations such as waterflooding because σ_h is closely related to hydraulic fracturing in a normal stress environment. When describing the reduction in σ_h from depletion, the stress path coefficient is defined as γ_h,d. When describing the increase in σ_h from re-inflation, the stress path coefficient is defined as γ_h,i. The Δ's in these definitions are defined to always give positive values of γ_h,d or γ_h,i. The assumptions on γ_h,i (injection stress path coefficient) could impact the allowable injection pressure, injection rate, and storage capacity for waterflooding & CCS projects and a more thorough understanding could prevent overly conservative designs.