Evidence of Direct Fluid Connections Between Hydraulic Fractures and Pre-existing Faults at the Cape Station EGS Project

Abstract

We present unique observations and models of hydraulic fracture-fault interactions from the Cape Station EGS project operated by Fervo Energy. We focus on the multi-stage, plug-n-perf stimulation of three horizontal wells near a vertical monitoring well that hosts a downhole pressure gauge. Faults were identified by clustering the microseismicity catalog through unsupervised clustering algorithms. We focus on a fault that intersects multiple horizontal wells and passes closely to the pressure gauge. The initial shear to effective normal stress ratio on the fault is between 0.31 and 0.45; approximately 7 – 18 MPa pressure increase is required to reach criticality for a friction coefficient of 0.6. The pressure gauge recorded two increases of approximately 20 MPa each, interpreted as follows. The first increase occurred during stimulation of the first well and is consistent with fluid leak-off and pressure diffusion away from a nearby hydraulic fracture into the low permeability reservoir. This pressurization of the reservoir brought the fault to a critically stressed state. The leak-off model cannot explain the second, more rapid and larger pressure increase observed during stimulation of the two additional wells, which is more consistent with pressure transmission along a high-permeability fault zone. Seismicity initiated near, but some distance away from where hydraulic fractures intersect the fault. The lack of seismicity close to a minor casing deformation implies that slip occurred aseismically in these regions. We test the hypothesis of pressure transmission along a high permeability fault zone using 3D modeling of fault zone fluid transport. The strong correlations between fault slip, fluid pressure, and well deformation highlight the crucial impact that fault structure and adjacent, permeable damage zones can have on stimulation. Our study demonstrates the importance of pre-existing faults and the potential value of physics-based modeling, integrated with geophysical and geomechanical data, in managing EGS stimulations.