Accurate Initial / Residual Saturation Determination reduces Uncertainty in Further Development and Reservoir Management of the Dulang Field, Offshore Peninsular Malaysia
Authors: Egbogah Emmanuel O. , Bin Tajul Amar Zarool Hassan 
Published: December 2007

There have been published concerns that the log interpretation programs in the Malay Basin have not been providing accurate saturation estimates, and that estimation of residual oil saturations are erroneous, as previous coring procedures were not designed to maintain reservoir rock wettability. The most serious problems (and uncertainty) in the Dulang Field development planoptimization was, in the main, high apparent initial water saturation (with arange of 19 - 61%), inconsistent residual oil saturation results and the lack of quality reservoir mechanical and petrophysical properties data. To address these data uncertainty issues, a special coring, core recovery and core analysis program was conducted on three key wells in the Field's Unit Area, 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).Minimum-flushing LT-OBM coring facilitated a more accurate determination of initial water saturation (Swi) required for recalibration of downhole log calculations and more accurate estimation of reserves. Maximum-flushing WBM sponge-coring permitted accurate determination of residual oil saturation(Sor). Sophisticated tracer technology was implemented using a hexachloroethanetracer that provided accurate invasion profiles.

The results of the core analysis indicate considerable narrowing of the Swirange. Water saturation exponent"n" has been improved from 1.89 to 1.64and cementation exponent from 1.72 to 1.77. The impact of this study on reserves has been quantified through the computation of hydrocarbon-pore-volume of the cored wells. The results so far have shown an increase of 28.9% in HPV, which translates to higher reserves and greater confidence in further development and management of the Dulang Field. This paper will discuss details of these results, including the quality assurance and quality control programs which validate the core recovery, core analysis and core-log integration results.

Introduction

Reservoirs can be effectively described and efficiently managed only when suitable data are available at field, well, core and pore levels. The level and quality of data also determine the degree to which reserves can be correctly estimated. The success of defining an optimum field development plan and reservoir management strategy for the Dulang Field are crucially dependent on our knowledge and understanding of the reservoir rock and fluid properties, as well as the internal geometry / architecture of the reservoirs. Thus, the only sound basis for optimum development planning is thorough data acquisition program. Recognized inaccuracies in open log data and concerns over reliability of early core analysis data were compelling reasons for further investigation of petrophysical reservoir parameters. [For example, available data indicated a high degree of uncertainty of initial water saturation, Swi, spanning a range of 19 – 61%. Clearly, more accurate Swi data are required to calculate reserves and determine an optimum development strategy for the field].

Well log data are usually converted to usable quantitative data through algorithms based upon physical assumptions. Any one or more of the variables can be wrong. For example, choice of an incorrect formation water resistivity(Rw, an extremely difficult parameter to establish) can lead to severe over or underestimation of water saturation (Swi), the consequence of which is incorrect reserves estimation and a less than optimal development plan. Cockroft and Robinson have shown that nearly all formation waters in South-East Asia are impacted by the presence of meteoric waters. The meteoric water was hydrodynamically emplaced over geologic time. P. 79^