Fracture-Conductivity Loss Caused by Geochemical Interactions Between Man-Made Proppants and Formations

Abstract

Summary The selection of proppant to provide highly conductive pathways in hydraulically generated fractures is typically based on the proppant crush strength, permeability, availability, and cost. Extensive libraries of laboratory-determined conductivity values, obtained using API standardized methods at a variety of simulated well conditions, are available for most proppants. However, post-fracture-stimulation well testing indicates that these values are often one to two orders of magnitude too high. In many fields, the productivity of fractures declines rapidly, requiring frequent restimulation treatments to remain economically viable. Proppant crushing and embedment, fracturing-fluid damage, and fines invasion are proppant-pack permeability-damage mechanisms that have been used to explain this loss of productivity. This paper reports on recent studies that have determined that alumina-based proppant materials may promote geochemical reactions that can occur at a surprisingly rapid rate, even at moderate temperatures, resulting in the loss of porosity and permeability and the creation of fines in the proppant pack. The compatibilities of several man-made proppants, ranging from lightweight ceramics to high-strength bauxites, with a variety of formations are presented. These findings indicate that the formation mineralogy plays a heretofore unrecognized role in determining proppant suitability. Most proppants were found to lose 50–60% permeability before stabilizing, while others were shown to lose up to 90% in only a few days. This paper describes the specialized methods that were developed to study these geochemical reactions and reports quantitative changes in permeability, proppant composition, and fluid changes. Surface analysis using scanning electron microscopy (SEM) and energy-dispersive X-ray (EDX) analysis provides dramatic visual confirmation of this damage mechanism. Results of this study have a great potential economic significance, suggesting new, important fracture design parameters that need to be determined to enable choosing the appropriate proppant for maximized stimulation longevity.