Roy D. Nurmi, 1991. "Integration of Borehole Data for Improved Reservoir Characterization", 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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Most reservoirs are undersampled, thus making integration of all available data necessary for reservoir characterization. Although there is a wealth of data in individual wells, it's effective integration generally requires a good understanding of measurements strengths and limitations, including vertical resolution and volume measured. The importance of understanding the details of measurements has been demonstrated by recent reservoir studies from Egypt to India. The heterogeneous variation of porosity in carbonate reservoirs (at every scale of examination from microns to kilometers) requires careful integration to adequately characterize such reservoirs. The routine evaluation of the standard measurements (core, well log, and testing) in such systems (Cretaceous and Tertiary) are often found to be erroneously interpreted because there are many types of heterogeneities. Moreover, fractured carbonate reservoirs require even more careful integration of core, well log data, and well testing data, especially when the analysis includes thin permeable streaks as are present in some giant Khuff gas reservoirs of The Gulf. These and other giant reservoirs need detail geological reservoir studies and high resolution measurements. Core petrophysical data and well log data are often both needed and should be routinely integrated rather than only being compared for quality control.
The geological and geometrical analysis of borehole data sometimes allows the projection of structural and/or sedimentological information away from the borehole. ‘Similar fold’ modelling can be used to construct structural cross-sections around individual wells using dip data on a geological workstation. The value of this approach has been demonstrated in the analysis of faulting
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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.