Abstract

Dielectric 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 the1 near-horizontal 8 1/2-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 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.

Introduction

During a phase of infill development drilling, serious problems were experienced unexpectedly with long sections of intra-reservoir shale's when drilling the near-horizontal 8 1/2-inch section of Nelson well 22/11-N30y (referred to as the "N30y").

A subsequent well, 22/11-N32 (referred to as the "N32"), was projected to have a similar trajectory in a similar part of the field, with the risk, once again, of encountering problematic intra-reservoir shales. In order to try and reduce the drilling risks, a full wellbore stability study was initiated of which Dielectric Constant Measurements (DCM) were a significant component. The ultimate aim of this study was to develop the ability to "type" the intra-reservoir shale's in real time, at the wellsite, during the drilling process. The aim being to reduce the time required to assess continued drilling risks and to take the necessary decisions to maximise hole length. For the purposes of maintaining continuity of drilling operations on the platform, the project had a relatively short deadline in which to produce results that could be applied to N32.

The DCM technique is a recognised and accepted method for typing clay material, giving good correlations between specific surface area measurements and rock strength.

During the wellbore stability study at least three intra-reservoir shale types were identified as having different strengths, requiring mud weights ranging from 10.6 to 11.5 lb/gal in order to maintain hole stability. Given that these shales are hard to define when encountered whilst drilling horizontally, a relatively sensitive method of classifying clays offshore was required. This data was then to be used in the decision tree for ascertaining the intra-reservoir shale type, thus better enabling appropriate action to be taken.

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