The heterogeneity of the rock fabric is a significant factor influencing the initiation and propagation of a hydraulic fracture (HF). This paper presents a laboratory study of HF created in six shale-like core samples provided by RITEK LLC collected from the same well, but at different depths. For each tested sample, we determined the breakdown pressure, the HF growth rate, and the expansion of the sample at the moment when the HF reaches the sample surface. Correlations were established between the HF parameters and the geomechanical characteristics of the studied samples, and deviations from the general relationships were explained by the influence of the rock matrix. The analysis of the moment tensor inversion of radiated acoustic emission (AE) signals allows us to separate AE signals with a dominant shear component from the signals with a significant tensile component. The direction of microcrack opening was determined, which is in good agreement with the results of the post-test X-ray CT analysis of the created HF. Thus, it has been shown that a combination of several independent laboratory techniques allows one to reliably determine the parameters that can be used for verification of hydraulic fracturing models.

In the last decade, the negative impact of hydraulic fracturing fluids on the reservoir properties has been noted, which has led to the new trend of improving characteristics and developing new hydraulic fracturing fluids. As an alternative option to the traditionally used cross-linked fluids based on guar solution, in our laboratory we have tested fluids having a branched spatial structure, which allowed them to hold and transport proppants, despite the low viscosity of this kind of fluids (100–200 mPa·s measured at 100 s−1). Existing theoretical models of hydraulic fracture (HF) propagation have some limitations in predicting the influence of fracturing fluids on reservoir properties. Unfortunately, in situ experiments in the target reservoir are difficult and expensive. Thus, laboratory experiments can be considered as a reasonable alternative for testing new fluids, since they can provide comprehensive information about the properties of the created HF before the application of a new hydraulic fracturing technique in the field conditions. This paper presents the results of an experimental study of hydraulic fracturing of granite samples in laboratory conditions. The injection of water- and oil-based unconventional fracturing fluids was performed to study the influence of fluid rheology on the dynamics of the hydraulic fracture propagation process and parameters of the created HF. We have found that the fracturing fluid viscosity affects the parameters of the created HF, such as aperture, propagation velocity, breakdown pressure, and HF surface tortuosity. The obtained relationships can be taken into account for Hydraulic Fracture modelling, which may increase the efficiency of the hydraulic fracturing in the field conditions.

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<p>Nowadays Hydraulic Fracturing (HF) is one of the most effective stimulation technique for hydrocarbon extraction from unconventional reservoirs, as well as enhanced geothermal applications. Practical applications of HF can have different aims. In one case, we need to stop cracks inside the host rock to avoid some HF breakthroughs into other formations and possible groundwater pollutions. The second situation is when we need to fracture several bedding planes in a reservoir which has a complex structure, especially in case of the presence of multiple natural fractures in unconventional reservoir. It is important to study hydraulic fracturing, its propagation and conditions of interaction with interfaces in laboratory conditions before expensive field application.</p><p>The present work demonstrates the results of a laboratory study designed to understand fracture interaction with artificial interfaces. For the first series of experiments, we used some natural materials such as shales, sandstones, dolomites and limestones with different porosity, permeability and mechanical properties. During these experiments we initiated hydraulic fracturing in homogeneous specimens with and without artificial surfaces, modelling natural fractures or bedding planes in unconventional reservoirs. For the second series of experiments, we used a combination of different materials to understand HF propagation in heterogeneous media, to study conditions of HF crossing or arrest at the boundaries between different types of rock. These laboratory experiments were done to create HF simulating natural processes in fractured and heterogeneous rocks or reservoirs.</p><p>Series of hydraulic fracturing experiments under uniaxial load conditions were conducted using the multifunctional system MTS 815.04. Before testing, samples were scanned by 3D CT System to characterize the rock fabric, and after testing, CT scanning was repeated to characterize 3D shape of created HF. The dynamics of HF initiation and propagation was monitored by Acoustic Emission (AE) technique, using piezoelectric sensors glued to the surface of the rock to record elastic waves radiated during the process of HF propagation. The experiments were made with different injection rates and fluid viscosities. Changes in radial strain, injection pressure and microseismic data over time were recorded.</p><p>As the result, these experiments indicate significant factors (rock heterogeneity, porosity, permeability, fluid viscosity and injection rate), influencing cracks initiation, propagation or arrest on the artificial interface. The fracture propagation and opening are characterized by measured radial deformation, fluid pressure and geometrical orientation in the sample volume. The experiments demonstrated, that fracture easily crossed artificial surface in the homogeneous limestone samples. And cracks initiated in limestone were arrested on the border with shale. In all cases combination of the AE and deformation monitoring allows to indicate fracture initiation, propagation and arrest.</p>

<p>Microseismicity and acoustic emission (AE) studies are a part of earthquake science. Compared to ordinary earthquakes, microseismic events are characterized by higher frequencies, lower magnitudes, shorter duration, and more complex source mechanisms. The researchers associate the induced seismicity with different processes: borehole breakouts, tunnel excavations, hydraulic fracturing, wastewater injection, and stimulation of geothermal reservoirs.</p><p>Acoustic emission represents elastic waves generated spontaneously due to the formation of microfractures when the rock is undergoing a sufficiently high load. AE can be used to obtain continuous data at various stages of the deformation process: from distributed plastic failure to localized macroscopic failure. The spatial distribution of AE events indicates the location of fractures, and the source mechanism provides information about the failure mode: a tensile fracture, a shear fracture, or a combination of both.</p><p>This work shows the results of an experimental study of borehole breakouts in sandstones. We measured AE during the deformation experiments and applied the moment tensor analysis to microseismic waveforms. We used a continuum mechanics model of Minakov and Yarushina [2021] to relate the laboratory AE data to the deformation processes. The comparison of the failure patterns and corresponding seismic responses obtained in laboratory and simulations, allows to classify the deformation regimes in real rocks based on seismic observables.</p><p>EG, MB, SS, and VS gratefully acknowledge support from the Ministry of Science and Higher Education of the Russian Federation under agreement No. 075-15-2020-119 within the framework of the development program for a world-class Research Center.</p><p> </p><p>References:</p><ul><li>Minakov, A., Yarushina, V., Elastoplastic source model for microseismicity and acoustic emission, <em>Geophysical Journal International</em>, Volume 227, Issue 1, October 2021, Pages 33–53, https://doi.org/10.1093/gji/ggab207</li> </ul>