Murphy has commenced horizontal CSS at Cadotte (Seal Lake, Peace River Oilsands). Cadotte is an unconventional reservoir due to compositional gradient, faulting, water pockets, and variations in oil viscosity, rock dilation/fracturing and pay-zone thickness. Gravity and viscosity are marked by declining quality with depth, biodegradation and compartmentalization.
High oil viscosity and low water mobility at Cadotte cause low initial injectivity. High injectivity during CSS is achieved by high pressure injection to fail the formation mechanically and trigger fracturing and deformation (dilation-recompaction triggers relative permeability hysteresis). Evidence of dilation at Cadotte includes greater steam injectivity than is expected and surface uplifts (larger than can be attributed to thermal expansion/tensile fracturing).
History-matching high injectivities is challenging when reasonable fracture lengths/rock compressibilities are used. To match injectivities, most simulations (Cold Lake/California CSS) have either used larger compressibility (‘spongy-rock’ approach) or long fractures. Spongy-rock approach predicts a steady increase in injection pressure, whereas in early cycles pressures increase and then level off for most of the cycle.
We describe enhancements made to a commercially-available simulator to incorporate modeling of deformation and relative permeability hysteresis to match injection/production pressures at Cadotte that are otherwise difficult to reproduce. The geomechanical model explains surface heave and high injectivity caused by dilation due to shear failure, increase in pore pressure/formation compressibility, and decrease in effective stress: A dilation pressure is specified, below which behavior is elastic (low compressibility) and above which, higher compressibility is used. Above a maximum porosity, further dilation is not permitted (low compressibility). The hysteresis model calculates grid-block relative permeabilities that lie on/between imbibition-drainage curves, making it possible to use laboratory-derived relative permeabilities and still match field WOR.
Impacts of dilation-recompaction factors (fracturing pressure, maximum pressure, dilation pressure, recompaction pressure, and formation compressibility) are quantified through field results, history-match/sensitivity analysis, performance optimization, and uncertainty assessment, utilizing reservoir simulation, Latin hypercube designs and Monte Carlo simulation.
Addressing geological/reservoir/operational variances, this modeling project helps determine optimum CSS development at Cadotte. It provides insight on how to screen the reservoir/operational parameters for successful CSS application in geologically-complex unconventional reservoirs with varying PVT behavior.