Application of an Ammine-Complex Adsorption Technique as an Alternative to Dielectric Constant Measurements (DCM) for Offshore Shale Typing

Abstract

TX 75083-3836, U.S.A., fax 01-972-952-9435. AbstractDielectric Constant Measurements (DCM) are often a significant component of field wellbore stability studies. The DCM technique is a recognised method for characterizing clay material, providing good correlation between specific surface area measurements and rock strength. The most accurate methodology is to characterize intra-reservoir shale whilst drilling. A portable DCM instrument can be used offshore; unfortunately such units cannot always be made available.Previous work has suggested a correlation exists between DCM specific surface area measurements and methylene blue titration (MBT) cation exchange results. However, a number of concerns were raised over the relative accuracy of the technique, and more importantly, on the consistency of testing between engineering personnel. Hence, an alternative, more quantitative method for accurately and rapidly determining the cation exchange capacity (CEC) of shale was required.A method, first proposed by Bardon et al., 1 based upon the interaction of ammine-transition metal complexes with the clay exchange sites, was selected and adapted for use offshore. The reactivity of the clay material could then be determined colorimetrically based upon cobalt hexammine trichloride depletion. This method was applied in an actual field situation.The Nelson field is located on the Forties-Montrose High within the Central Graben, UK Central North Sea, and some 113 miles east of Aberdeen. The reservoir, an oil producer, consists of submarine fan sands of the Forties Formation at approximately 7,200 feet TVDSS (true vertical depth sub sea). Serious problems were experienced with intra-reservoir shale while drilling the 1 near-horizontal 8½-inch section of Nelson well 22/11-N30y. Intra-reservoir shales were prognosed on a subsequent well, 22/11-N32 of similar trajectory. In order to * Formerly with Enterprise Oil plc minimise drilling risks, a full wellbore stability study was conducted. The ammine-complex adsorption test was used offshore, in place of DCM, to determine shale reactivity as part of a "shale type risk assessment matrix" developed from this study.A good correlation between specific surface area measurements and cobalt hexammine complex adsorption was found, providing a viable alternative to offshore DCM measurements for shale characterisation. It is believed by the authors that this is the first time this particular technique has been used in the field for this application.