Water Sensitivity of Sandstones

Experimental and theoretical studies have been carried out to elucidate the mechanism of water sensitivity of Berea sandstone and to quantify a number of important parameters. Based on the results of a number of novel experiments, a physical model has been developed. In this model, clay particles are released only when the salt concentration falls below a critical salt concentration. These colloidal clay particles remain dispersed in fresh water and are carried with the flowing fluid until they are captured at a local pore constriction, thereby decreasing the permeability. A mathematical model based on this mechanism has been developed. This model contains two parameters stemming from the rate equations of the release and capture of clay particles. Correlations of these parameters with flow rate and temperature are presented. Introduction The water sensitivity of sandstone is a colloidal phenomenon whereby the permeability of the sandstone is decreased rapidly and significantly after the sandstone is contacted with fresh water. This phenomenon is demonstrated by a standard water shock experiment in which the flow through a sandstone core is changed abruptly from salt water to fresh water. The results of a standard water shock experiment are shown in Fig. 1. The normalized permeability (k/kl) drops from 1.0 to about 0.01 after only 2 or 3 PV of fresh water have been forced through the core. Permeability reduction resulting from water sensitivity is of serious concern to the oil- and gas-producing industries. Water sensitivity, first recognized during waterflooding of petroleum reservoirs, is now a concern in many other field operations that require aqueous solutions, such as drilling, solution mining, and stimulation. Even though water sensitivity in sandstone has been recognized for 35 years, the literature on this subject is limited. The works of Gray,1 Mungan,2,3 Jones,4 and Hewitt5 are among the most widely cited. These papers document the phenomenon and concur that the water sensitivity results from clay swelling, clay particle migration, or a combination of these effects, depending on the composition of the sandstone. Clay particle migration is the most important mechanism of permeability reduction since sandstones containing very little or no swelling clays and a considerable amount of migratory or dispersible clays such as kaolinite and illite are water-sensitive. Gray,1 Mungan,2,3 and Jones4 have reported results relating permeability reduction to clay particle migration. Previous studies investigated the effects of salt solutions, pH, and rate of decrease in salinity on the water sensitivity of sandstone. However, an in-depth analysis of the processes of dispersion and plugging of clay particles and how these processes are affected by flow rate, temperature, and salt concentration has not been reported in the literature. Some effects have been explained inadequately or incorrectly. These are discussed in detail elsewhere.6 In addition, a mathematical model describing quantitatively the permeability reduction with time and other parameters has not been developed. Such a model would aid in understanding the dynamics of this phenomenon as well as in designing preventive measures. The study of water sensitivity is also of general scientific interest since the phenomenon involves a number of colloidal and interfacial phenomena, such as flocculation, peptization, filtration, and adsorption. Practical considerations and scientific interests warrant a comprehensive study of this phenomenon. In this paper, a mathematical model is developed, and comparisons with experimental observations are made. These observations include permeability restoration with saltwater reversal, sequential permeability reduction, effect of flow rate, and core length.