Several researchers have suggested that perturbed stress fields associated with active faults significantly affect flow properties in the subsurface. To examine this insight quantitatively, we combine an elastic dislocation model of active faults and a stress-compliant function for fracture apertures to evaluate flow properties in a fractured basement reservoir in the Yufutsu field, Japan.
The most productive well in the reservoir shows about 90° of rotation of the axes of horizontal stresses along the well trajectory, whereas the directions of horizontal stresses at nonproductive wells are commonly oriented consistently with those of the regional stresses. Additionally, results of step-rate hydraulic injection tests at the nonproductive well indicate that fracture apertures fluctuate significantly in response to the magnitude of stresses normal to the fracture surfaces. These observations support the important effect of perturbed stress fields on flow properties.
The observed stress orientation profiles are correlated to those from a geomechanical model. Nonproductive wells were drilled in the area where the regional stress state prevails, and the most productive well penetrates the area near the tips of active faults, where the stress concentration leads to enhancements in fracture permeability by opening or shearing. The modeled perturbed stress fields combined with a stress-compliant function for opening of fracture apertures can explain the observed difference of well performances to the first order. The model demonstrates that areas near the tips of active faults have high potentials for hydrocarbon production in fractured basement reservoirs.