4D seismic, 4D geomechanics and hydraulic stimulation in the low permeability South Arne chalk field
Geomechanical models have many purposes in reservoir management. Applications include the prediction of reservoir compaction, subsidence and long-term well-bore integrity, optimization of drilling trajectories and mudweights, and the design of hydraulic stimulation and perforation campaigns. The basis for all geomechanical applications is an accurate knowledge of the subsurface stress state, pore pressure and the mechanical properties, which in combination form a geomechanical (or mechanical earth) model. To increase trust in their predictions, these models need to match data observations that are linked to the subsurface stress-state. In the initial phases of reservoir production, such observations typically include observations during drilling (e.g., loss of drilling mud allows estimation of an upper bound for pore pressure, and inflow events give a lower bound for pore pressure), observations of drilling-induced fractures and wellbore breakouts, as well as leak-off and formation integrity tests. During reservoir production wellbore failure in shear and compaction gives indications of production-induced localized strain. On a field-wide scale, time-lapse seismic time shifts in the overburden are now commonly ascribed to reservoir compaction and serve as a field-wide calibration method for geomechanical models. Reservoir compaction causes overburden elongation and an associated velocity slow-down (Hatchell and Bourne, 2005), and this causes an increase in two-way traveltime between base- and monitor survey, termed time-lapse time shift. This close link between observed time-lapse time shifts and the modelled reservoir compaction and overburden elongation is now being used as a field-wide calibration tool for geomechanical models (e.g., Staples et al., 2007; Herwanger and Koutsabeloulis, 2011).