Simulation of Thermally Induced Waterflood Fracturing in Prudhoe Bay

Abstract

ABSTRACT Waterflooding operations in the Prudhoe Bay field result in temperature reductions of up to 130 °F within the flooded region. As a result, the fracture gradient is significantly reduced, and the injection pressures may have to be reduced to avoid formation parting. The phenomena of fracturing resulting from thermally induced stress reduction was investigated to provide guidance for decisions on waterflood operating strategy. Thermal reservoir simulation was performed to determine the in-si tu temperature and pressure distributions following various periods of water injection. A finite-element stress-analysis model was developed to calculate the changes in in-si tu stress field. A three-dimensional fracture simulator was used to predict fracture growth and shape based on the calculated stress distribution. A thermal reservoir simulator was modified to include a simplified fracture mechanism and calibrated against step-rate test data. Fracture areas were then predicted with the modified reservoir simulator and correlated with the fracture shapes obtained with the fracture model. The fracture predictions confirmed field data which indicated that fractures created during well testing were short and confined to within the perforation interval. In addition, fractures created with clean water injection at pressures above the reduced fracture gradient were predicted to reach equilibrium lengths of less than about 200 ft.