Utility of Mechanical Facies for Rock Classification, Characterization, and Correlation
Published:January 01, 1991
K. A. Alhilali, G. Shanmugam, 1991. "Utility of Mechanical Facies for Rock Classification, Characterization, and Correlation", 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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Mechanical facies, which represent the rocks' dynamic and mechanical properties, can be used to characterize and subdivide reservoir formations at the borehole and in areas between wells. For example, the mechanical fades of the Travis Peak-Cotton Valley sequence of east Texas and western Louisiana can be used to distinguish and accurately map poor reservoir calcite-cemented zones. Mechanical facies are derived from acoustic logs where both compressional and shear wave velocities are obtained.
The advantages of using mechanical facies for reservoir characterization, classification, and subdivision are:
Mechanical facies are a quantification of a rock's in-situ physical properties and can reflect the dynamic characteristics of the reservoir.
Mechanical facies may provide useful information away from the borehole by combining acoustic logging and tomographic survey data to classify and characterize reservoir zones laterally as well as vertically.
Mechanical facies can be used for well-to-well correlation.
Mechanical facies provide a continuous record and can be obtained in uncased or, under proper conditions, cased wells.
The versatility of mechanical facies for reservoir description and characterization is illustrated by several examples from diverse areas and for varying lithology.
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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.