Abstract

During a large water flood study on a cluster of fields in South Oman it became apparent that relative permeability constituted one of the major uncertainties impacting reserves in the cluster. At the onset of the study, only two experimental measurements were available that had been acquired with the currently recommended approach of wettability restoration and a combination of steady state and centrifuge experiments. Therefore, the team proposed to core five wells and embarked on a large scale special core analysis (SCAL) program, covering all predominant rock types, in order to get a better handle on the relative permeability characteristics.

This paper presents a case study of using a properly measured set of relative permeability data to replace the previously used analogue database and hence reduce uncertainties of waterflood recovery predictions. The experimental programme followed a recommended procedure of wettability restoration and a combination of steady-state and centrifuge experiments. When the experimental data became available, they were reviewed and numerically interpreted using the state-of-the art simulation techniques.

This has led to several insights that were missed in earlier field studies which used a set of simplified correlation functions/parameters for the cluster of fields without adequate special core analysis data calibration. Based on the new results the field's relative permeability characteristics are divided into two categories linked to rock types thus significantly reducing the uncertainty range. In this paper we will also highlight the procedure that was used to generate the new SCAL experimental dataset and the analysis that has been done to arrive at this conclusion.

The simulation effort and the subsequent analysis have reduced the uncertainty in relative permeability by a factor of three resulting in a significant improvement in the robustness of the development plans.

Introduction

The subject cluster consists of 23 fields and is located in the south of Oman, Fig.1. The fields in the cluster are characterised as green and brown. By "brown" we refer to high gross liquid production, rapid water cut development, variable well performance and marked pressure depletion. The fields are small to medium salt withdrawal related structure often with stacked reservoirs. In general, the majority of the fields are continuous Gharif clastic reservoirs at a depth of 0.6 to 1.6 km with medium to high fault density. Some fields consist of three reservoir formations: Mahwis formation (the Haima group of combro-ordivician age), Al Khlata and Gharif formation (Huashi group of Carboniferous/Permian age). The oil is medium to heavy and characterised by very low gas content.

Production since 1980 has recovered only a few percents of the cluster volume from mainly vertical wells producing commingled on all reservoirs as well as water injectors in the brown fields. This marks the cluster with high development potential for large-scale waterflood.

Initial screening study established the development maturity of the fields per reservoir units in terms of primary, secondary and potential tertiary recovery mechanisms. This enabled the integrated team to embark on data gathering (appraisal, surveillance etc.) aiming at reducing uncertainties and firming up viable life-cycle development options. In this paper, we will highlight only the SCAL uncertainty and its impact on the ultimate recovery. More details of the development plan and cluster risk management can be found in ref [1].

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