Parameterizing and Block-Averaging Electrical Characteristics of a Reservoir: an Essential part of Electrical/Electromagnetic Evaluation of Production Process
A.J. Mansure, 1991. "Parameterizing and Block-Averaging Electrical Characteristics of a Reservoir: an Essential part of Electrical/Electromagnetic Evaluation of Production Process", The Integration of Geology, Geophysics, Petrophysics and Petroleum Engineering in Reservoir Delineation, Description and Management, Robert Sneider, Wulf Massell, Rob Mathis, Dennis Loren, Paul Wichmann
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The geology of an oil reservoir is not homogeneous at the scale of resolution of an electrical/electromagnetic (E/EM) measurement or at the grid scale of an oil reservoir simulator. This paper discusses issues governing how the petrophysical laws governing resistivity of homogeneous rocks average to give bulk or grid scale resistivity. The bulk-averaged resistivity of a heterogeneous material measured by a given E/EM technique is a function of the direction of current flow relative to any structure of the material. The difference in bulk resistivity between current flowing across and parallel to geologic structure is frequently significant resulting in a material that is anisotropic. Failure to recognize the importance of the direction of current flow can lead to significant errors in interpretating E/EM measurements. The averaging of core-scale properties (porosity, saturation, etc.) to give representative block resistivity has been analyzed using field data to ensure that the statistics (average, variance, correlation, etc.) are geologically realistic. Merely plugging average saturation and porosity into Archie's law, while ignoring how the saturation and porosity are spatially distributed, can give block resistivities that are as much as 50% off. The analysis of field data was generalized from a bedded unit to an amorphous block by considering a bimodal distribution of pore sizes. For this example the resistivity calculated using average properties can be off by a factor of ten. This demonstrates that the question of how to average properties to determine bulk resistivity is a fundamental petrophysical question, not just an artifact of the field data used.
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The Integration of Geology, Geophysics, Petrophysics and Petroleum Engineering in Reservoir Delineation, Description and Management
Bima Field, offshore northwest Java, is a sizeable reservoir containing reserves of approximately 700 MM bbls OOIP with a 50 BCF gas cap. At present only the northern 1/3 of the field is developed, with 7 platforms and 54 producing wells, of which 20 are horizontal. The field has multiple drive mechanisms and high viscosity oil (21 cp), resulting in rapid GOR and water-cut increase after 3 years of production. The high stakes (both reserves and facility investments) and the reservoir's complexities, make an effective reservoir management scheme critical. For this reason an integrated geological, geophysical and engineering description was carried out to provide a 3-D Reservoir Simulation Model to evaluate development options. Geologically, the Oligo-Miocene age Batu Raja Limestone was deposited on the Seribu Platform, a basement-controlled, fault- bounded structure. The Upper Batu Raja carbonate build-up is thickest on the structurally highest parts of the platform where the rock comprises a series of "cleaning upwards" cycles (muddy deposits overlain by progressively more grain-rich sediments). A Lower Miocene drop in sea-level caused subaerial exposure of much of the platform and leaching by meteoric fluids. This diagenetic event resulted in contrasts in the reservoir quality (porosity, permeability, fluid saturations) at various intervals of the Upper Batu Raja. Based on these dissimilarities, the reservoir was zoned into 6 model layers. Once zonation was established, well logs could be calibrated to whole and sidewall core. A dense grid of seismic data were used to map the Batu Raja structure. From these data, color seismic inversion sections were produced and calibrated to the well logs. The calibrated seismic data were then used to map the top of structure, the carbonate build-up's edges, the total thickness of the Upper Batu Raja (needed to control aquifer size in the model) and the thickness of the main pay zone (layers 1-3). Engineering reservoir description began with a detailed compilation of capillary pressure, relative permeability, production and DST data. The 3-D simulation model required special treatments, including varying the GOC depths to honor separate gas cap closures; making permeability pressure dependent in poorly-consolidated zones; and setting up horizontal well completion treatments. Results suggest that water injection into the oil rim and gas cap is an effective approach toward maximizing recoveries and minimizing gas cap resaturation. However, waterflood reserves are sensitive to injection timing. The synergistic approach of geological, engineering and geophysical input into the Bima reservoir study has had impact by delivering a reservoir management tool that can evaluate future development expansion and possible gas sales. The simulation model can also track fluid migration during the field's producing life. The geological/geophysical model led to an enhanced understanding of Batu Raja depositional and diagenetic processes that has potential in regional exploration strategies.