Abstract

Experimental studies of cyclic injection hydraulic fracturing are shown to enhance stimulated reservoir area and reduce the fracture breakdown pressure (FBP). Two types of injection protocols are experimentally investigated: monotonic injection with continuous increase of injection pressure, and cyclic injection, where injection pressure is cycled and increased progressively. Three different cyclic injection treatments are carried out: (1) Pre-breakdown cyclic injection fracturing where fluid is injected in cyclic manner till breakdown, (2) Post-breakdown cyclic injection which involves injection of fluid in cyclic fashion after breakdown, and (3) Pre-post breakdown cyclic injection in which injection fluid is cycled both prior to and after the rock failure. Real-time acoustic emissions (AE) were monitored and post-failure, fracture permeability was measured. SEM statistical analysis of the fracture process zone was done on core plugs extracted adjacent to the main fracture. Fracture permeability and the number of pre-breakdown AE were used as main metric to compare performance of cyclic injection with monotonic injection. Our test results show that when the monotonic injection is replaced by pre-breakdown cyclic injection fracturing, the FBP is reduced by as much as 64% compared to the monotonic injection pressure and the fracture permeability is enhanced by 2-to-3-fold even at 5,000 psi stress. It is hypothesized that the repetitive change in stress and asperity failure will enhance the fracture damage zone, resulting in multitude of secondary fractures around the main hydraulic fracture, thereby increasing the total reservoir contact area. This argument is further bolstered by our SEM statistical analysis of the fracture network which shows that the width of fracture process zone around the primary fracture is enhanced by a factor of 2.25 in pre-post breakdown cyclic injection fracturing. Our results provide the laboratory evidence which suggests that cyclic injection hydraulic fracturing has the potential to make field completions more efficient.

Introduction

Hydraulic fracturing is a technique in which fluid is injected into a borehole at pressures sufficiently higher than the tensile strength and in situ minimum principal stresses in order to induce fractures.). Hydraulic fracturing is an indispensable stimulation technique adapted to tap valuable energy resources like tight oil and gas and geothermal energy by creating new pathways that connect the in-situ fluids to the wellbore. However, in tight wells, i.e., formations with matrix permeability of less than 1md, it suffers from major challenges– (i) very high surface injection pressures are required to fracture deep reservoir systems, (ii) the increase in production rate is temporary and often drops-off to near pre-fracture levels (Kiel, 1977), and (iii) the environmental risks of activating shear-slippage of nearby natural faults and or fractures by high pressure fluid injection which causes seismic activity (Raleigh et al. 1976; Ellsworth, 2013). Therefore, there is an increased interest in developing a well stimulation technique that not only reduces the fracture breakdown pressure but also creates an enhanced permeable fracture network and simultaneously reduces adverse environmental impacts, i.e. induced seismicity. Several new modified stimulation methods of cyclic soft stimulation have been published recently; Zimmermann et al. (2019), Zang et al. (2013), Zang et al. (2017a, 2017b), Zhuang et al. (2016), Patel et al. (2017), Zhuang et al. (2017), and Hofmann et al. (2018). The ultimate objective of modifying conventional hydraulic fracturing techniques is to enhance the reservoir contact area, and simultaneously reduce the occurrence of larger magnitude seismic events.

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