In-situ Casing Consolidation by Electrokinetic and Electrochemical Methods

Abstract

Abstract Companies drilling in the unconsolidated sea-floor mud and shallow over-pressurized zones of the Gulf of Mexico are encountering cement wash-out problems while setting the initial structural casing (0-300 ft depth). Jet-drilled casings can fail due to insufficient frictional bearing capacity. This paper presents a method of increasing the bearing capacity of a jet-drilled (or pile-driven) casing in-situ by applying a potential difference such that the casing is anodic compared to a remote cathode. It has been shown experimentally that clayey formations will swell and stick to a simulated anodic casing by the combined electrokinetic processes of electroosmosis and electrophoresis. Any cavities around the "casing" are eliminated and the formation is flush against the metal surface, increasing bearing capacity. The formation around the "casing" dries out due to electroosmotic migration of water away from the anode and shear strength is further increased. Increases of 50% to 1000% in shear strength and bearing capacity were achieved in a matter of hours in the laboratory, depending on the type and water content of the surrounding soil formation. In addition, it was found that there exists an optimum level of electrical treatment for each formation type, beyond which the shear strength begins to degrade. The process has a modest power requirement and is not expected to cause safety or environmental problems. Introduction Offshore drilling operations usually involve the placing of an initial structural casing to a depth of 300 ft or 50 in order to penetrate through the silty mud of the sea-floor and down to the more solid formation below. This structural casing is usually sunk under its own weight as far as possible and then either jet-drilled or pile-driven the remainder of the way. Jet-drilling is less expensive and thus the preferred option, however, casing placed in this manner often fail under the combined load of its own weight and that of the subsequent conductor casing (Fig. 1) and in these cases pile-driving is the only option. The potential solution to this problem presented here is to increase the bearing capacity of a jet-drilled casing, such that it is able to withstand higher loads without slippage or failure, by means of applying a positive charge to the casing (i.e. making it the anode in an electrochemical cell) after it has been jetted into place. The casing is then consolidated in place by a combination of electrokinetic phenomena, as explained in the following section and as shown in Fig. 2. This technique of electrokinetic (or electroosmotic) consolidation has been applied to many other geotechnical and soil mechanics problems such as slope, dam and foundation stabilization, arresting settlements of bridge supports and increasing the bearing capacity of pre-driven piles. In each case the shear strength of the soil around the anode was found to have increased significantly and the published results show that field applications have been extremely successful. The soil consolidation and increase in shear strength is invariably accompanied by a decrease in water content around the anode. Published results by Bjerrum et al. show this technique to be successful even in extremely soft, silty clays. A survey of the literature also finds the enhancement in the shear strength to be, in the time scale of the applications involved, permanent and irreversible. While most previous field applications of electrokinetics have been on land, there exists published work demonstrating that the procedure also works well offshore, both in theory and in practice. Indeed, the highly conductive nature of marine sediment is helpful to the process as it decreases the level of voltage required to provide an adequate consolidation current. It is expected that this factor will be an advantage when implementing the procedure to solve this particular problem. The objective of this study was to characterize the electrokinetic consolidation process in terms of the current level and treatment time required to achieve the optimum bearing capacity. P. 215^