Reducing Field Development Planning and Reservoir Management Uncertainty Through a Comprehensive Core Analysis Program in the Dulang Field, Offshore Peninsular Malaysia
Authors: Egbogah Emmanuel O. 
Published: December 2007

There have been published concerns that the log interpretation programs in the world's freshwater (low-contrast, low-resistivity) environments have not been providing accurate water saturation estimates, and that estimation of residual oil saturations is erroneous, as previous coring and core analysis procedures were not designed to maintain reservoir rock wettability. One of the most serious problems (and uncertainty) in field development plan optimisation and reservoir management in these freshwater environments, have always been high initial water saturation (with wide range of saturation values), inconsistent residual oil saturation results and lack of quality reservoir dynamic, mechanical and petrophysical properties data. To address these data uncertainty issues, a special drilling, core recovery and core analysis program was conducted on three key wells in the Malay Basin's Dulang Field, applying the latest technology, field-proven low-invasion core heads, bland water-base mud (WBM) and a low-toxicity, bland oil-base mud (LT-OBM). A minimum-flushing LT-OBM coring facilitated a more accurate determination of initial water saturation (Swi) required for recalibration of downhole log calculations and a more accurate estimation of reserves. A maximum-flushing WBM sponge-coring permitted an accurate determination of residual oil saturation (Sor). A sophisticated tracer technology was implemented using hexachloroethane tracer that provided accurate invasion profiles. The 210 meters of fresh, preserved cores recovered from the three wells, covering over 70% of the stacked reservoir system, were subjected to a comprehensive laboratory special core analysis program.

The results obtained from the core analysis program indicate a considerable narrowing (reduction) of the Swi range, from 19–61% to 20–43%. Water saturation exponent n has been improved from 1.89 to 1.64, and cementation exponent, "m" from 1.72 to 1.77. The impact of these improved parameter definition on reserves has been quantified through the computation of hydrocarbon-pore-volume of the cored wells. The results have shown an increase of 28.9% in HCPV, which translates to higher reserves; and have shown better-defined capillary pressure and relative permeability curves. The Swi and Sor data have been used to calculate recovery factors and expected ultimate recovery, the results of which duplicate those of computer simulation studies. All these provide greater confidence in further development and reservoir management of the field. This paper will discuss only details of the saturation results, including the quality assurance and quality control programs which validate the core recovery, core analysis and core-log data integration.

Introduction

The success of defining an optimum field development plan and reservoir management strategy for any field is critically dependent upon our knowledge and understanding of the reservoir rock and fluid properties, heterogeneity and the overall internal geometry / architecture of the reservoir system. Thus, the only sound basis for optimum development planning is a comprehensive, relevant and cost-effective data acquisition program. However, there are special problems associated with formation evaluation of fresh-water environments such as the Malay Basin. The reservoir quality of the reservoir sands is quite variable and primarily controlled by texture (grain size, sorting, angularity, etc), mineralogy (clay types, abundance, morphology, and location, and cements), degree of bioturbation and diagenesis. The salinities of formation waters are generally low and vary from less than 5,000 to 30,000 ppm. P. 411^