Experimental Investigation of Hydraulic Fracturing on the Montney Formation

Abstract

Abstract To assist in understanding the role of rock fabric, stress state, and fracturing fluid viscosity on fracture geometry, four large, dry blocks (28" × 28" × 36") sourced from a Montney formation outcrop were hydraulically fractured in a series of laboratory experiments. These tests investigated the role of texture on hydraulic fracture (HF) propagation with different fluid viscosities, and strike-slip and normal stress regimes. Each block was hydraulically fractured in two stages to allow comparison, within the same overall rock fabric, of fractures created by the different viscosities and injection rates. Pre-existing discontinuities, including calcite-cemented fractures and calcite layers were mapped prior to the test by a goniometer. The static elastic properties, failure parameters, dynamic properties, and fracture toughness of the host rock were measured in detail. During each test, the HF propagation was monitored by recording the generated acoustic emissions. Subsequently, the blocks were carefully dissected, and HF geometries were mapped in detail by laser profilometry. The results showed that heterogeneity in the rock texture and its location had a primary effect on the fracture propagation. The direction of the maximum stress played, at best, a secondary role. The fracture dynamics determined with acoustic emissions indicated a relatively symmetric, pancake-like shape with high dimensionless viscosity and more asymmetry with low viscosity fluid. Visual inspection of the resulting fractures showed truncated elliptical shapes with both viscosities and with the higher viscosity having less containment. Acoustic emissions clearly indicated HF initiation, propagation, and interaction with the rock fabric, indicating arrest and spreading along pre-existing interfaces in cases of low dimensionless viscosity, and more uniform propagation in cases of high viscosity but with the rock fabric having a significantly reduced impact. The experiments demonstrated the likelihood of the substantial impediment to successful proppant delivery and hydrocarbon production in the field based on observed occurrences of HF arrest on pre- existing interfaces (indicating possible height containment) as well as the formation of branches and step- overs. Our results indicated that sufficient knowledge of the fabric, in conjunction with proper selection of injection fluid and rates, may provide additional height containment and hence enhance lateral fracture extension and improve the depletion of the pay zone. Ultimately, optimum production may more likely be achieved with collection of further rock samples, laboratory testing, and modeling of pre-existing interfaces to allow relative ranking of these barriers.