One of the most challenging tasks of a log analyst is to understand logresponses. In order to achieve this it is essential to establish an accurate Bulk Volume Model for the reservoir. This includes mineralogy, porosity and saturations. Unfortunately, in the case of Dulang field, log data alone cannotbe used to derive accurately the Bulk Volume Model due to insufficient log dataand the presence of special minerals. These minerals which affect log responses significantly are Potassium-Feldspar, Siderite and Fe-Dolomite. Dulang field is a complex shaly-sand reservoir situated 130 kilometres offshore Peninsula Malaysia. This paper illustrates how results of core analysis have helped usunderstand log responses in the Dulang field. A special core recovery programwas carried out in three wells, namely, Dulang-A17, B20 and B21, which penetrated all the major reservoirs that contributed the bulk volume of STOIIP in the Dulang field. A low toxicity oil-based mud was used in well Dulang-B21to establish accurately the connate water saturation for quantitative calibration of log derived parameters. A bland water-based mud was used in theother two wells to preserve the rock wettability. The results of the coreanalysis enabled us to better refine our petrophysical model of the reservoir which led to a more accurate computation of reserves. Components of core analysis used in this study are Fourier Transform Infra-Red spectroscopy, Core Layering and conventional core measured petrophysical parameters (porosity, permeability, grain density and saturations). Introduction One of a log analyst's nightmare is looking at a bare minimum number of logcurves in a complex shaly sand reservoir to compute meaningful Net Pay (h), Porosity () and Water Saturation (Sw) for volumetric purpose. In the case of Dulang field, wireline log interpretation approach alone is insufficient to describe completely the formation. P. 107
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^
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