Knowledge of initial-residual saturation values are required to estimate recovery efficiency and the economic viability of an oil recovery project. Accurate knowledge of the oil content of a reservoir is critical in the economic evaluation of any prospect for enhanced oil recovery after conventional recovery by primary means and any injection of water and gas. The accuracy of these data is very critical for implementing post-waterflood techniques because project economics are very sensitive to oil saturation in a partially depleted reservoir. Consequently, attempts have been made to determine oil saturation values from various techniques. However, the commonest and most economical method has remained the routine analysis from cores cut with water-base muds. But these render inaccurate results due to inherent flushing during coring as well as oil shrinkage and expulsion due to gas expansion during core lifting to the surface. Typically, such results are corrected for shrinkage and the resulting value is arbitrarily increased again by a bleeding factor of 10%–15%. There are no published industry correction data to correctly account for flushing and blowdown in conventional cores. This paper presents the core analysis results of a highly controlled, bland water-base sponge coring program with a PDC maximum-flushing corehead designed to simulate a waterflood The results from 29 full-diameter core analysis from 200 feet of recovered core yielded an average bleeding / expulsion factor (E) of 1.062 which we believe to the first such published data set to provide a measured E factor for correcting residual oil saturation values from conventional cores. The results are supported by a well designed and properly implemented quality control program. The results of the sponge coring program also compare very well with residual oil determined from other sources. Introduction Oil saturations determined from analysis of cores that are cut with water-base muds are always less than in-situ saturations because of drilling-fluid invasion and oil flushing during coring, and oil shrinkage and expulsion due to gas expansion as the core is lifted to the surface. The core is subjected to pressure and temperature reduction as it is brought to the surface. Thus, the gases released from solution with pressure decline cause shrinkage of the oil volume, and as the gases expand and escape from the core, they expel some of the core fluids. Use of pressure-retaining core barrel or sponge coring techniques can eliminate this second effect but do not eliminate flushing. Two well-known flow processes can occur to cause oil flushing during core cutting and retrieval:mud filtrate invasion caused by overbalance pressure at the drill bit during coring operations andfluid expansion caused by a drop in pore pressure during core retrieval. Jenks et al provided information indicating that overbalance pressure is the prime factor in oil stripping during coring operations. Consequently, accuracy in reserves determination requires that residual oil saturations be corrected for the effect of high pressure gradients around a wellbore which can arise during both drilling and production. Rathmell et al point out, however, that routine core analysis oil saturations that are adjusted for bleeding and shrinkage, can give reliable values for residual oil saturation after waterflooding in many sandstone reservoirs, particularly those containing low-viscosity oils. However, Rathmell and group have not provided a correction factor value for adjusting the oil saturations for bleeding and shrinkage. In consideration of this problem, Kazemi proposed the following equation: (1) P. 445^
Recent studies have suggested the possibility of spontaneous emulsification as a mechanism for enhanced oil recovery (EOR). The discussions have, however, remained essentially qualitative. A study was therefore undertaken to estimate quantitatively the contribution of spontaneous emulsification as an EOR mechanism. The tests were conducted on several bulk liquidlliquid systems as well as by displacement experiments in unconsolidated synthetic sand packs. Spontaneous emulsification was found to be a mechanism for EOR: the estimated extra contribution to EOR due to this mechanism was found to be significant in laboratory scale displacement experiments. Tertiary recovery was always greater when spontaneous emulsification was evident than otherwise. Results of tests on bulk liquidlliquid systems indicate that the occurrence or absence of spontaneous emulsification can be correlated with the values of 'partition parameter'. It may be concluded that higher oil recoveries may be achieved in chemical EOR processes where interface mass transfer (and the accompanying spontaneous emulsification) occurs. The evaluation of efficiency of residual oil mobilisation through the capillary number theory (with and without spontaneous emulsification) is also discussed. Displacement tests with spontaneously emulsifying systems showed that residual oil left behind a conventional waterflood was mobilised in a range of capillary numbers much less than that which applies to low-tension waterfloods.
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^
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
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