An Experimental Investigation on the Impact of Capillary Pressure, Diffusion Osmosis, and Chemical Osmosis on the Stability and Reservoir Hydrocarbon Capacity of Shales.

Abstract

Abstract Wellbore instability in shales is the most challenging and costly issue in drilling operations. Wellbore instability in shales can be attributed to many factors some of which have been well studied and documented. However, the physicochemical and mechanical properties alterations in shales which eventually leads to wellbore failure have been largely ignored. Water and ions movement in and out of shales plays a major role in the alteration of the physicochemical and mechanical properties of shales thus leading to wellbore instability problems and possible hole collapse. Water and ions can move in and out of shales by many mechanisms including, but not limited to, diffusion osmosis, chemical osmosis convective flow and capillary suction. This work presents experimental data analyzing the impact of chemical osmosis, and diffusion osmosis on water and ion movement when shale interacts with drilling fluids. The adopted experimental work minimized the effect of convective flow and capillary suction. Results show that water movement is not only controlled by chemical osmosis (water activity) as previously thought, but is also influenced by diffusion osmosis. This insight provides information and guidelines to optimize drilling fluids to effectively control and mitigate wellbore instability when drilling through troublesome shale. Furthermore, pressure transmission tests were used to experimentally measure capillary entry pressures of various non-wetting fluids (oil-based mud, crude oil and Nitrogen gas) through shales. These capillary entry pressures are needed for the estimation of shales seal capacity (h). Introduction The unfavorable interaction between shale and drilling fluids is considered to be the primary cause of many wellbore instability problems (Chenevert et al, 1969). Such interactions are very complicated and include mechanical, chemical, physical, hydraulic, thermal, and electrical phenomena (Van Oort et al, 1995 and Van Oort, 2003). The overall effect of these interactions is mainly related to the movement of water and ions into or out of shales (Al-Bazali, 2005). The physico-chemical and mechanical properties of shale around the wellbore, such as permeability, strength, pore pressure, and elastic modulus can greatly be altered by such movement. It is well-accepted that the adsorption of water results in shale strength and elastic modulus decrease, swelling and pore pressure increase (Hale et al, 1992).These changes around the wellbore may cause wellbore instability problems during drilling and completion operations. The magnitude of water movement and the effect of absorbed water on shale properties are influenced by the presence of ions in the solution (Ballard T. J. et al, 1992).Ionic diffusion may also result in the movement of ions into the shale formation, resulting in chemical alteration (Gazaniol et al, 1994). Horsrud etal. (1998) found the adsorption of potassium ions caused shale shrinkage due to cation exchange. Others have found that swelling can occur due to other ions and not necessarily potassium ions (Ewy & Stankovich, 2002). Simpson and Dearing (2000) experimentally demonstrated that ion diffusion altered the fabric of the shale and caused shale failure. From the above review, it was found that the movement of water and ions into or out of shale is critical to wellbore instability. Water and ions can move in and out of shales by many complicated mechanisms. These mechanisms include, but not limited to, chemical osmosis, diffusion osmosis, capillary suction and convective flow. In this work, we focused on the diffusive flow of water and ions. Namely, we investigated the impact of both chemical osmosis and diffusion osmosis on shale alteration. Our experimental set up and procedure was designed to minimize the impact of convective and capillary flow.