Abstract

This paper presents a case example to illustrate how a consistent geomechanical approach and coherent data integration are used to provide the basis for the evaluation of formation mechanical properties, characterization of in-situ stresses, identification of wellbore failure mechanisms, and calibration of a wellbore stability model. The calibrated model is then used in conjunction with the proposed build-and-hold and catenary well profiles to generate the mud weight windows for an ERD well. To provide further insight into the wellbore stability with respect to wellbore trajectories and in-situ stresses, contour plots of mud weights are generated for a critical depth interval. These contour plots delineate potential instability areas and are critical for predrilled well trajectory designs. Additionally, an innovative approach to establish the lower limit of mud weight for wells that cannot be drilled without failure zones (breakouts) forming around the wellbore is presented. This approach enables difficult wells to be drilled, with the lower mud weight reducing the risk of formation damage and increasing the rate of penetration.

Introduction

Resak Field, located offshore of Peninsula Malaysia, is operated by PETRONAS Carigali Sdn. Bhd. (PCSB). The field is currently under development and 13 wells will be drilled radially from a production platform in a ‘bow-tie’ pattern. In order to cost effectively drain the proven reserves without having to add a lightweight structure and pipelines connecting to the processing platform, some of the development wells will be drilled extended-reach. Resak LOC-11 is an extended-reach well designed to drain the southeastern portion of the field reserves. Two wellbore trajectories, build-and hold and catenary, were proposed. Fig. 1 and Fig. 2 show the schematics for the two well plans.

Extended-reach drilling (ERD) can be extremely costly if not optimally designed. Occurrences of borehole instability-related problems, such as stuck pipes, fishing and sidetracking operations, have been reported to increase with increasing wellbore inclinations1 and extended-reach drillings. In addition to escalating costs, wellbore instability can also result in poor hole conditions, which ultimately would affect the quality of reservoir characterization and the effectiveness of primary cementing. Poor primary cementing in high angle gas wells is of particular concern because of possible gas migration during cement setting. Faced with the challenges of delivering the well within AFE, the Resak project team decided to conduct the wellbore stability assessment for the proposed ERD well. The results of the geomechanical analysis will be used to:

  • decide which wellbore trajectory, and

  • establish the mud weight program that will mitigate wellbore instability and improve operational performance.

An extensive in-house database is available through comprehensive data acquisition programs during the exploration and appraisal phases. Due to the data quality of older wells, only the most recent appraisal well data (Resak 6F-18.4) was selected to provide the basis for rock mechanical properties and in-situ stress characterizations as well as wellbore stability model calibrations. The calibrated model was then used in conjunction with LOC-11's well plans (hole deviations and azimuths) to generate the stable mud weight profiles. The safe operating mud weight window can then be implemented together with good oilfield drilling practices to mitigate potential borehole instability-related problems. The purpose of this paper is to present a consistent geomechanical approach for developing the optimal mud weight program for LOC-11 ERD.

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