In 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 into the created fracture a proppant that will prevent fracture closure and provide adequate flow capacity. It is desirable that the proppant maintain a high conductivity for an extended time period to permit the efficient recovery of hydrocarbons without the need for restimulation. A number of mechanisms have been identified which can degrade the fracture conductivity over time, 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 to 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 been directed at evaluation of 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 structure, chemistry, and nature of these precipitants indicate 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. Further, other conditions known to occur in oil and gas reservoirs would naturally prevent the formation of zeolites.
Analysis of proppant samples retrieved from wells after extended time in the fracture will be shown. While it is recognized that barium sulfate or calcium carbonate scale can significantly impair productivity in some reservoirs, there is not yet evidence that zeolite precipitation poses significant concern in actual propped fractures, or that chemical treatment of the proppant surface is justified or effective at mitigating zeolite precipitation.
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.