Abstract

Well stimulation by hydraulic fracturing is a common method for increasing the injectivity and productivity of wells. This method is beneficial for many applications including oil, gas, geothermal energy and CO2sequestration, however, hydraulic fracturing in shale and other similarly complex geologies remains poorly understood. A 300×300×300 mm3block specimen of Fort Hays Limestone was hydraulically fractured in the laboratory using a true-triaxial apparatus to study complex hydraulic fracturing. This material is a member of the Niobrara Shale formation, a major unconventional oil and gas play in the Denver-Julesburg Basin. Hydraulic fractures were stimulated by injection of plastic epoxy. Injected epoxy clearly marked the fluid penetrated zones of the stimulated fractures, partially preserved the in-situ fracture aperture and bonded the fracture faces to give improved visualization of complex fractures in cross-sections cut after the experiment. The experiment resulted with a complex fracture network including prominent tensile hydraulic fractures, shear activated discontinuities and bedding plane separations. Acoustic emissions, injection pressures and injection rates were analyzed with reference to the fracture geometry to develop relationships between these parameters and to develop means of identifying complex fracture growth, as applicable in field scenarios where the actual fracture geometry is not easily measured.

1. INTRODUCTION

Hydraulic fracturing is a well stimulation method where fluid is injected into rock to create new fractures. These fractures are intended to function as high-conductivity fluid pathways enabling increased well productivity or injectivity. The hydraulic fracturing method can be used to improve oil, gas, geothermal, carbon sequestration, and deep waste injection well performance. Design and optimization of a hydraulic fracture treatment is non-trivial due to complexity stemming from multi-physical interactions between the in-situ rock and injected fluids.

A comprehensive understanding of hydraulic fracture geometry is critical for understanding and predicting fluid flow behavior through stimulated wells. Fracture geometries in field treatments are expected to be complex, including combinations of tensile and shear fractures [1,2]. This expectation is supported by mine-back studies [3,4] and microseismic monitoring where acoustic emission (AE) sources are typically dispersed through a volume of rock and do not clearly indicate discrete fractures. Some stimulation treatments are thought to produce complex networks of inter-connected fractures, accessing large rock volumes.

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