Surface Roughness and Fluid Viscosity Controls on Propagation of a Hydraulic Fracture Across a Frictional Interface

Enhanced reservoir connectivity generally requires maximizing the intersections between hydraulic fracture (HF) and preexisting natural fractures (NF). Many analytical HF-NF interaction models are based on quasi-static elastic interactions and neglect effects of fault activation by fluid infiltration. We studied this interaction in laboratory tests of Solnhofen limestone (matrix permeability is 4 nD) under triaxial stress conditions (Confining and differential stresses are 5 MPa and 0-30 MPa, respectively). In the experiments a hydraulic fracture was initiated from a pressurized borehole drilled down the sample axis to within 6 mm of an inclined saw-cut fault (used as a proxy for a natural fracture). Saw-cuts were inclined at 30, 45, and 60^o to the borehole. Acoustic emissions (AE), fault slip, stress drop and pore pressure were recorded at a 5 MHz sampling rate. We also varied the RMS roughness (1, 2 and 4 µm), and the fluid viscosity η by using water (η=1 cp), and a water-based silicone emulsion (η=350 cp). Prior to the HF stage, we briefly slid each fault to measure its coefficient of friction. We independently measured the in-plane fluid transmissivity (K); the equivalent permeability of the faulted samples were 58 nD (smooth fault), 305 nD (medium), and 10.2 µD (rough).

Water-driven HF was able to cross the smooth fault, but was arrested by medium and rough faults at all experimental conditions. Increasing fault roughness promotes HF arrest and inhibits crossing. The HF driven by viscous silicone fluid could cross the medium roughness fault, but was still arrested by the rough fault, indicating a threshold for fluid diffusivity ( K/η) that would allow HF to propagate across the fault. Also, using the viscous fluid increased slip magnitudes and AE activity. Thus, pore fluid properties can significantly affect the HF-NF interactions. Other conditions being equal, local effective normal stresses along the NF will likely be affected by fluid diffusivity ( K/η): Rough faults with high-diffusivity will promote fluid infiltration, result in low effective normal stresses, and promote HF arrest. Conversely, when a viscous fluid is used, the chance for HF crossing is increased. In addition, the slip on the intersected fault is also increased; all making the viscous fluid a superior candidate for HF operations.