Roberto Suarez-Rivera

Onset of Hydraulic Fracture Initiation Monitored by Acoustic Emission and Volumetric Deformation Measurements

In this paper, the results of laboratory studies of fracture initiation, early propagation and breakdown are reported. Three experiments were conducted on a low permeability sandstone block, loaded in a polyaxial test frame, to representative effective in situ stress conditions. The blocks were instrumented with acoustic emission (AE) and volumetric deformation sensors. In two experiments, fluids of different viscosity were injected into the wellbore, fluid injection was interrupted soon after the breakdown pressure had been reached. This allowed us to investigate hydraulic fracture initiation. In the third test, fracture initiation criteria were applied to stop hydraulic fracture propagation significantly earlier, prior to breakdown, and as it propagated a short distance from the wellbore. The analysis of AE results shows an increase in AE activity and a change in the AE spatial correlation, during the fracture initiation. This early stage of fracturing correlates strongly with the onset of rock volumetric deformation, and is confirmed by the analysis of ultrasonic transmission monitoring. The rock microstructure, after the test, was investigated by analysis of scanning electron microscope images. These indicated the development of leak-off zone near the wellbore and a dry hydraulic fracture at the farther distance from the wellbore.

Understanding the Effect of Rock Fabric on Fracture Complexity for Improving Completion Design and Well Performance

In the past, containment of hydraulic fracture height growth has been evaluated based on an assumption of rock formation layers with contrasting conditions of minimum horizontal stress, and to a lesser extent, Young’s modulus, leak off rates, and fracture toughness between adjacent rock layers. Most recently, large-block hydraulic fracturing experiments in the laboratory, and observations of fracture propagation (natural or induced) in core, have provided evidence that the rock fabric plays a significant role in arresting fracture height growth and also in promoting fracture complexity. In addition, unconventional reservoirs are often over-pressured. And, as the pore pressure increases, the stress contrast tends to be reduced, and the role of rock fabric becomes dominant. In this paper, we investigate the effect of weak interfaces on fracture geometry and height containment by conducting hydraulic fracturing tests on large blocks from tight shale outcrops, under simulated effective stress conditions. We define rock fabric as the presence, orientation and distribution of bed boundaries, lithologic contacts, mineralized fractures, and other type of weak interfaces. This rock fabric creates discontinuities in the stress and strain fields and affects the way the rock deforms and fails. Continuous monitoring of acoustic emissions and using acoustic transmission during fracturing, allows understanding the process of fracture initiation and fracture interaction with the weak interfaces. Post-test CT x-ray scanning and detailed dissection and photographic imaging provide a good record of the fractures. In addition, these post fracture measurements allow comparing the fractures created with results from acoustic emissions localization. The experimental results clearly demonstrate the importance of rock fabric to understand and predict fracture complexity and fracture height containment.

Hydraulic Fracturing of Tight Shale Monitored by Acoustic Emission and Ultrasonic Transmission

In this work we study the effect of fluid viscosity on hydraulic fracturing initiation and near-wellbore propagation on block samples of tight shales subjected to representative effective in-situ stress conditions. Combined analysis of acoustic emission, ultrasonic transmission and volumetric deformation indicates that the viscosity of the injected fluid had a strong influence on hydraulic fracturing initiation, fracture propagation and fracture geometry. Injection of high viscosity fluid into the stressed tight shale resulted in fracture initiation at a bore pressure higher than the overburden stress and occurred significantly earlier than the borehole breakdown pressure. After initiation, the hydraulic fracture propagated symmetrically from the borehole in the direction parallel to the maximal horizontal stress, causing significant volumetric deformation of the rock. In the case of injecting a low viscosity fluid into the stressed block, fracture initiation occurred at a borehole pressure significantly lower than it was required with the higher viscosity fluid, and occurred almost simultaneously with the bore pressure breakdown. AE measurements during hydraulic fracturing allowed us to estimate that the average speed of hydraulic fracture propagation was approximately thousand times faster for the low viscosity fluid than for the high viscosity fluid.