Wettability Alteration Caused by Oil-Based Muds and Mud Components

Abstract

Summary Wettability alteration caused by oil-based muds and mud filtrates is of great concern in coring operations. This paper shows that mud filtrates from whole muds can change the wettability of water-wet cores significantly. Individual components primarily responsible for this wettability alteration are identified. Introduction Over the years, oil-based drilling fluids have become popular because they can overcome certain undesirable characteristics of waterbased muds - e.g., the ability to dissolve salts, promote clay disintegration and dispersion, interfere with oil and gas flow through porous media, and corrode pipe. Oil-based muds may be used where water-based muds are unstable or to minimize fluid-filtrate loss to the formation. Because of their excellent fluid-loss characteristics, such muds sometimes are recommended for coring applications where the in-situ saturations are to be preserved and core flushing with mud filtrate is unacceptable. preserved and core flushing with mud filtrate is unacceptable. Oil-based muds generally have diesel or mineral oil as the continuous liquid phase. Recently, it has been shown that the dynamic fluid loss to oil-bearing rocks can be significantly higher for oil-based muds compared with the API fluid loss. Because oil-based-mud technology is relatively new and these muds are not used as extensively as water-based muds, studies on the formation damage these muds cause have been published only recently. We now recognize that the problems with oil-based muds during drilling operations. while less numerous than those of water-based muds, are much more severe when considering alteration of rock surface properties. The main drawbacks include the following. 1. Oil-based muds usually contain more solids than water-based muds, so particle invasion may be more pronounced. 2. Some strong oil-wetting surfactants used to disperse solids in oil-based muds can convert formation rocks to an oil-wet state. This reduces the relative permeability to oil significantly and the producing water/oil ratio increases correspondingly. Because two-phase flow in reservoirs strongly depends on wettability, correct in-situ wettability conditions are crucial for core recovery and analysis. Because native-state cores sometimes are difficult to obtain during drilling, restored-state cores often are used for core analysis. In such cases, it is important to understand how to recover and infer original wettability from the altered state of the core. Some investigations have focused on this problem. A study published earlier showed that most commonly used water-based mud additives, some of which have been considered bland, can alter original wettability. However, the literature provides little information on the evaluation of oil-based mud provides little information on the evaluation of oil-based mud components. To understand which components alter rock properties, a thorough laboratory examination of each component independently is necessary before examining the whole muds. We expect that such a study will be important for improving oil-based mud formulations for coring operations and for minimizing damage in the flushed zone. This paper focuses on the wettability alteration caused by each oil-based mud component and discusses the permeability reduction these components cause. In this study, we used these common additives to oil-based muds: surfactants, organophilic clay, base oils, and inorganic salts. We measured wettability and permeability changes caused by individual components to water-wet permeability changes caused by individual components to water-wet and fractional-wet cores. In addition, we evaluated the synergistic effect of whole drilling fluids on core wettability and permeability. Capillary-pressure tests and contact-angle measurements were used to determine wettability. Berea cores were used as water-wet samples; this included Berea cores saturated with an asphaltic crude oil as fractional-wet samples and Berea cores treated with Quilon C, a chrome complex manufactured by DuPont Chemicals. Contact angles were used to measure wettability alteration. These experiments were performed on quartz (representing sandstone reservoirs), calcite (carbonate reservoirs), and asphaltene- coated glass slides.