Proppant Diagenesis—Integrated Analyses Provide New Insights Into Origin, Occurrence, and Implications for Proppant Performance

Abstract

SummaryIn the application of hydraulic fracturing of oil and gas reservoirs, the objective is to create a conductive pathway for hydrocarbons to flow into the wellbore. This is accomplished by placing a proppant into the created fracture that will prevent fracture closure and maintain a high conductivity for an extended time period. A number of mechanisms have been identified that can degrade the fracture conductivity, including mechanical failure of the proppant grains, liberation of formation fines, proppant embedment, formation spalling, damage from the fracturing fluid, stress cycling, asphaltene deposition, proppant dissolution, and others. These factors in combination can reduce the effective conductivity by orders of magnitude as compared with the typically published conductivity data measured under reference conditions.Another mechanism for degradation of proppant over time has recently been postulated. This mechanism has been labeled "diagenesis" and refers to a dissolution and reprecipitation process that may reduce the porosity, permeability, and strength of the proppant pack as precipitants are deposited. Factors that were believed to control the occurrence and degree of diagenesis include closure stress, reservoir temperature, proppant type, and mineralogy of the rock formation. While much work has gone into evaluating this phenomenon by the industry, there remains uncertainty as to the prevalence of this mechanism and the prediction of its occurrence.This paper will summarize results from high-temperature static tests, extended-duration flow tests at reservoir conditions, detailed analyses of precipitates, the effects of this environment on the mechanical properties of proppants, chemical and mineralogical analysis of various reservoir shales, and evaluation of actual proppant samples that have been retrieved from producing wells.The paper will conclusively demonstrate that crystalline precipitates can be formed on the surface of all proppant types, including ceramic, sand, resin-coated materials, and even inert steel balls or glass rods. The nature of these precipitants indicate that they may be classified as zeolites. The chemistry and environment leading to the formation of zeolite precipitates will be reviewed. Testing indicates that zeolites can be formed without the presence of alumina-bearing proppant and appear to be largely dependent on the characteristics of the formation material and fluid. Conductivity tests carefully simulating reservoir conditions with actual reservoir shale core samples indicate that if diagenetic precipitation does occur it does not appreciably affect conductivity performance in the flowing conditions evaluated. Furthermore, other conditions known to occur in oil and gas reservoirs would naturally prevent the formation of zeolites.The results of this work will aid the stimulation engineer in proppant selection and treatment design. While there are many mechanisms contributing to the degradation of proppant performance, these studies indicate that zeolite precipitation is unlikely to be a dominant concern in most wells.