An Accelerated Method to Measure the Swelling Behaviour of Caprock (Shale) in Steam Stimulation Process

Abstract

Shale formations are used to serve as hydraulic and thermal barriers in steam stimulated processes such as steam-assisted gravity drainage (SAGD) and cyclic steam stimulation (CSS). They are potentially subjected to swelling and softening when they are exposed to fresh water. A conventional swell test on clay shale is very slow and it can take months or years. In this study, an accelerated swell test is proposed, which applies an electrical potential gradient through the specimen. The applied external electrical field accelerates the ionic flows and subsequently speeds up the swelling process. Experimental results of this study on reconstituted bentonite specimens saturated with sodium chloride solution have proved the proposed idea. Possible electrochemical reactions and their influence on the experimental set up have been discussed. A mathematical model based on coupling the flow equations in micro and macro levels has been developed to quantify this process. Introduction Multi-component mass transport under electrical, chemical and hydraulic gradients is gaining increasing attention and interest in different fields of engineering. Acar and Alshawabkeh(1–3) applied an electrical field through soils to extract contaminant ions from the soil. Roy and Cooper(4) and Cooper and Roy(5) proposed the application of an electrical field to prevent the bit balling during the drilling process. In this study, the electrical field is applied to accelerate the swell test in shale specimens. Shale formations are used to serve as an hydraulic and/or thermal barrier in oil production. The high tendency of shale to absorb fresh water and its high swelling potential may cause problems, such as drilling bit balling(6) or wellbore instability(7). Fresh water can access shale formations in different ways from different sources. Practical examples include: the use of steam and water in the oil recovery processes and the use of water-based drilling mud in borehole drilling. Control of swelling is critical in these practical applications, and thus, characterizing the swelling behaviour in shale formations is of great importance in these engineering activities. However, a conventional swell test on shale is very slow and it can take months or years(8). Thus, an accelerated swell test could be very helpful to overcome this problem in cost and time savings. Ion extraction from macro voids in shale and the subsequent induced concentration gradients along the macro voids and inter-particle micro channels are the main sources of swelling in shale. However, due to the low permeability of shale, the ion transport through the bulk fluid of this material is a time consuming process(8). Consequently, the fluid flow towards the inter-particle micro channels has a very low rate. If the ion extraction from the bulk fluid is accelerated, the fluid flow towards the micro channels will occur faster and the specimen swells more quickly. In this study, an electrical potential gradient is applied to accelerate the ion removal from shale which finally leads to a quick swell test. Experimental results on reconstituted pure bentonite specimens saturated with NaCl solution proved this idea.