To investigate and quantify the impact of frictional interfaces and mechanical anisotropy of rocks on hydraulic fracture propagation, we performed two series of laboratory experiments. These include hydraulic fracture propagation through discontinuities as well as anisotropic rocks at a range of external stress levels. We inject high viscosity fluid into stacks of cuboids to simulate a set of discontinuities as well as anisotropic cubic samples. In addition to monitoring pressures and deformation, we record and locate acoustic emissions, yielding insights on fracture growth processes and the interaction between discontinuities/rock fabric and propagating fractures.
The results of our experiments indicate that on laboratory scale, rock fabric and structure have a significant impact on fracture geometries. Depending on external stress levels and rock mechanical parameters, fractures either terminate at, cross, or open discontinuities and exhibit complex geometries in anisotropic rocks. At elevated external stresses, the impact of these features on fracture geometries becomes less pronounced. Simple fracture mechanical models predict the results to some extent.
The generation and propagation of tensile fractures in rock by hydraulic fracturing is a process of great importance in engineering applications such as hydraulic fracturing for hydrocarbon or geothermal energy production. This process is well understood in hypothetical isotropic, homogeneous and continuous rocks (Yew 1997, Economides and Nolte 2000). However, anisotropy, heterogeneity and discontinuities – such as joints, faults, foliation, bedding and mechanical contrast between layers - are ubiquitous in most geologic formations.
The impact of pre-existing discontinuities on fracture propagation has been subject of several laboratory experiments (Renshaw and Pollard 1995, Llanos et al. 2017, Kear et al. 2017, Bunger et al. 2015, Teufel and Clark 1984, Anderson 1981, Blanton 1982, Lamont and Jessen 1963, Gu et al. 2011) and in-situ observations (Warpinski and Teufel 1987, Jeffrey et al. 1992, Jeffrey et al. 2010). In general, four phenomena may occur, when a hydraulic fracture hits a discontinuity: arrest, crossing, diversion into the discontinuity – i.e. opening of the discontinuity – and step-over, where the reinitiated fracture on the other side of the discontinuity exhibits an offset (Thiercelin et al. 1987). Yet, the underlying processes remain complex and not fully understood as there are many independent parameters – such as friction on and hydraulic properties of the discontinuity plane, mechanical properties of the surrounding material as well as fracturing fluid properties – involved in the process.
