Geomechanical Modeling

During the production phase of a reservoir, natural phenomenon, such as movement of fluids, changes in pressure and stress in and around reservoirs occur. In high-pressure reservoirs, pressure depletion during production is associated with compaction within the reservoir causing stretching or extensional stresses in the overburden and underlying formation. This causes variation in velocities and formation thickness, which can be observed as 4D time shifts between successive vintages. Tracking the movements of fluids due to production, for example, gives valuable information about the depletion of a field, and can indicate areas of bypassed oil or gas.

Geomechanical modeling can be used to explain and predict 4D time shifts, using relationships which relate velocity changes to stress-induced fractional thickness variations for a range of lithologies. These relationships are used in the inversion of the observed pre-stack, offset-dependent time shifts for geomechanical properties. In addition to the obvious compressional effects within the reservoir, extensional stress is experienced above and at the edges of producing reservoirs. This extensional stress above the reservoir generally makes the largest contri- bution to the observed time shift.

Compaction of the reservoir generates significant changes in radial stress around its edges, which create a 4D anisotropy signature. CGGVeritas has extended geomechanical inversion to include the estimation of 4D anisotropy using offset-dependent 4D time shifts. This provides a unique solution for the delineation of depleted and compacted zones in the reservoir. Some of the benefits CGG has found by extending the geomechanical inversion to include the estimation of 4D anisotropy using offset-dependent 4D time shifts include: Improved understanding of 4D response of compacting reservoirs, production monitoring in fields with small 4D signatures, indication of potential wellbore instability in HPHT fields and on top of that it complements 4D elastic inversion techniques to improve discrimination of production effects within the reservoir.

Source: CGG Veritas

4D changes in vertical stress derived from inversion of 4D time shifts in the region of the Elgin field complex.