Long-Term Hydrothermal Proppant Performance

Abstract

Summary The hydrothermal degradation of alumina-based proppants, which can lead to significant loss of fracture conductivity, has been the topic of recent papers. Most of these studies were necessarily compromised either by being performed on too short of a time scale to produce geochemical effects or by using unrealistic temperature conditions to accelerate the geochemical reactions. Those studies, however, provided insight that geochemical reactions likely occur during production from hydraulically generated fractures and gave evidence of possible long-term proppant instability. This paper presents proppant-pack-permeability and proppant-crush-strength data collected from a selection of hydrothermal tests performed in sealed test cells packed with proppant and formation material, with no flow and no mechanical closure stress. Long-term proppant performance was determined by evaluating the proppant after 0, 15, 45, 90, and 180 days of hydrothermal exposure at 300 and 450°F, resulting in verification that proppants degrade continuously with time at all temperatures. A typical longterm proppant-testing result was 80% loss in proppant-pack permeability and 40% loss in proppant crush strength, with just 180-days exposure at 300°F at typical reservoir pH. These tests were performed both with and without formation material present to demonstrate the impact of reservoir/proppant compatibility. Scanning-electron-microscope (SEM) and energy-dispersive X-ray (EDX) spectroscopy analyses verified that dynamic molecular rearrangement occurs over the entire temperature range, though at an accelerated rate as temperature increased. Extensive effort was made to quantify both the accuracy and repeatability of testing procedures used in this study and will be presented. This involves measurement of pack permeability using water before and after hydrothermal exposure, and single-proppant-grain crush-strength determination using Weibull statistical analyses. This paper presents substantial data to support the importance of proppant compatibility when selecting the best proppant for long-term fracture conductivity. It also suggests that methods in addition to American Petroleum Institute (API) crush and conductivity procedures need to be developed and implemented to rank proppant performance properly.