Numerical simulation of departures from radial drawdown in a faulted sandstone reservoir with joints and deformation bands

This paper is included in the Special Publication entitled 'Faulting, fault sealing and fluid flow in hydrocarbon reservoirs', edited by G.Jones, Q.J. Fisher and R.J. Knipe. Field measurements constrain the fluid flow characteristics of an analogue hydrocarbon reservoir in the faulted Entrada sandstone, Arches National Park, Utah. These data comprise maps of the geometry, inhomogeneous permeability, and porosity of fault zones, joints, and deformation bands in a region where two discontinuous normal faults overlap. Two-dimensional computer simulations of drainage of this analogue reservoir identify normal faults with highly permeable slip planes as the most important reservoir inhomogeneities. These faults compartmentalize fluid pressure over timespans greater than years while fluid can be drained on the kilometre scale along their highly permeable slip planes. Joints induce the second most important distortions of radial drawdown, influencing the timespans over which fault signatures are observed in pressure decline curves. The joints often extend to the boundaries of the reservoir. This also reduces the time before the rate of pressure decline accelerates due to boundary interaction. Zones of deformation bands less than 25 cm wide with a spacing ≥30 m have little effect on radial drawdown in our single phase fluid flow simulations. When drawdown spreads with time over the deformation structures in the analogue reservoir, the different structures simultaneously influence the change of pressure at the wellbore (pressure derivative). This temporal overlap prohibits an analysis of the effects of individual structures. Drawdown does not 'recover' to radial flow after an inhomogeneity is encountered.