Understanding the mechanical behavior (compression, shear, or tension) of rocks plays an important role in wellbore‐stability design and hydraulic‐fracturing optimization. Among rock mechanical properties, strain is a critical parameter describing rock deformation under stress with respect to its original condition, yet conventional methods for strain measurement have several deficiencies. In this paper, we analyze the application of the optical method digital‐image correlation (DIC) to provide detailed information regarding fracture patterns and dynamic strain development under Brazilian testing conditions. The effects of porosity, rock type, lamination, and saturation (freshwater and brine) on indirect tensile strength are also discussed.
To examine the effect of rock type, 60 samples of sandstone (Parker, Nugget, and Berea) and carbonate rocks (Winterset Limestone, Silurian Dolomite, Edwards Brown Carbonate, and Austin Chalk) were tested under dry and saturated conditions with regard to lamination angle in laminated samples. A photogrammetry system was used to monitor the samples in a noncontact manner while conducting the indirect tensile experiment. DIC depends on the photogrammetry system, which helps to visualize and examine rock‐fracture patterns from the recorded images of the rock before and after deformation by assessing the strain development in samples.
The experimental results show the following.
Average tensile strength declines with increasing porosity for homogeneous, laminated, and heterogeneous rock specimens. Lower tensile strengths are observed in carbonate‐rock samples compared with sandstones, except for Silurian Dolomite.
Saturation reduces rock strength; for homogeneous samples, the highest strength decline (28%) was observed in Berea Sandstone, whereas the largest decrease (65%) for heterogeneous samples was observed in fully heterogeneous Edwards Brown Carbonate samples.
Increase of lamination angle (from 0 to 90°) affects the tensile strength. Average tensile strength observed for the Parker and Nugget Sandstones was greater in the direction perpendicular to the lamination direction (θ = 90°) compared with that of the parallel direction (θ = 0°).
Fracture patterns examined for homogeneous rocks are nearly centrally propagated and relatively linear. Three different fracture patterns (central fracture, layer activation, and noncentral or mixed mode) were investigated for laminated and heterogeneous samples.
Finally, DIC results illustrated the fracture creation and propagation with consistent strain mapping. The homogeneous samples produced a uniform fracture strain until the diametrical split, where the laminated samples were influenced by planes of weakness and fully heterogeneous anisotropic rocks produced winding and erratic fractures.