Laboratory study of hydraulic fracturing in heterogeneous materials

Real rocks are heterogeneous and contain cracks of various scales. The influence of these cracks on hydraulic fracturing was first studied in the 1980s, when experimental studies showed that the presence of natural fractures can affect the trajectory of the hydraulic fracture. Another type of inhomogeneous material is a homogeneous matrix with fragments of a different material with significantly different mechanical and filtration properties. By varying the composition and size of these fragments, it is possible to create samples with different characteristics. The interface between the matrix and the fragments acts as a weakened area, similar to cracks in a nonuniform fractured material.

Here we present the results of experiments conducted to study the propagation of hydraulic fracture in a heterogeneous material composed of a mixture of gypsum, cement, and fillers. Marbles chips and fine gravel were used as additives in the mixture. The porosity of the original samples without additives was 0.44 and the permeability was 4.15 mD. The porosity and permeability of samples with marble chips were 0.32 and 6.9 mD, respectively, while those with gravel were 0.30 and 2.74 mD. The height of the samples was 60 mm with an outer diameter of 104 mm. Brass inserts with an internal diameter of 10 mm and a length of 18 mm were placed in the center of both sides of each sample during manufacturing. The length of the uncased part of the well was approximately 24 mm.  The prepared sample was placed between two circular aluminum bases, with piezoelectric transducers mounted in the recessed working surfaces. Silicone liquid PMS-5, with a kinematic viscosity of 5 centistokes, was used to saturate the sample. During hydraulic fracturing, silicone liquid PMS-200, with a viscosity of 200 centistokes, was applied. The samples were subjected to radial stresses of 0.5 MPa, along the lateral surface, and axial stresses of 3 MPa were applied along the axis. The main focus of the experiments was on studying the acoustic emission accompanying the propagation of the hydraulic fracturing crack.

The experiments have shown that the presence of inclusions significantly affects the shape and development of hydraulic fractures: under conditions of radially symmetric lateral compression, fractures with three branches form in heterogeneous materials, while fractures with two wings form in homogenous samples. The fracturing pressure in samples with fillers is higher than in homogenous ones, and the highest fracturing pressure values are achieved when there is a more rigid and durable filler present. It has been established that acoustic emission pulses in an inhomogenous material have a wider frequency range than in a homogeneous one. In samples containing fillers, more acoustic emission pulses are recorded, especially when there are rigid fillers present. It is also worth noting that acoustic emission occurs earlier in experiments with homogenous samples relative to the time at which the maximum pressure is reached during hydraulic fracturing. The results of the acoustic emission source location are consistent with the observed patterns of cracks on the surface of the samples.