Reservoir Properties Inferred from Seismic Response in Areas with Minimal Log Control
J.D. Loren, J.T. Kulha, K.S. Renbarger, R.M. Sneider, 1991. "Reservoir Properties Inferred from Seismic Response in Areas with Minimal Log Control", 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 integration of regional petrophysical, geological, and geophysical data permits the detection of reservoir properties and the prediction of seismic response where log control is minimal. Because the necessary acoustic and density logs are frequently unavailable, incomplete or inaccurate, even in mature producing areas, forward modeling to create synthetic seismograms and amplitude versus offset (AVO) response is difficult. Synthetic acoustic and density logs can be created from commonly available resistivity logs and a uniquely derived sand fraction curve. Examination of drill cuttings, sidewall, and whole cores enhances the accuracy of the sand fraction curve, defines reservoir quality and delineates exploration opportunities. Rock, log, and calculated data are transformed into the time domain and integrated into the seismic data by means of the synthetic acoustic transit time.
The process algorithms are project-calibrated with a learning set of diversely representative high quality log data. After verifying algorithm calibration, the synthetic log generation process can then be applied regionally with confidence, even in older wells with “ancient” resistivity logs. The synthetic acoustic and density logs are the basis for models of variable lithology and fluid content, from which synthetic seismograms and AVO response are generated.
Comparison of modeled synthetic seismograms and AVO response to actual seismic data demonstrates a better agreement based upon synthetic acoustic and density logs as opposed to measured acoustic and density, constant density or Gardner’s general relationship. The models can be easily modified by varying pore fluids and bed thicknesses to find the range of amplitude changes and AVO effects that can be expected in the seismic data.
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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.