An Integrated Approach to Reservoir Characterization in the Illinois Basin
D. F. Oltz, H. E. Leetaru, B. Seyler, S. T. Whitaker, 1991. "An Integrated Approach to Reservoir Characterization in the Illinois Basin", 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 Illinois State Geological Survey (ISGS) possesses reservoir characterization capabilities that derive from the use of geological and engineering data and techniques including: porosity and permeability measurements, core-flow experimentation, petrography, scanning electron microscopy and energy dispersive x-ray, x-ray diffraction, workstations and computer modeling of geological, geochemical and engineering data. Additionally, a large core and sample library is maintained by the Survey. The ISGS research program to improve and enhance oil recovery in the Illinois Basin utilizes these capabilities and facilities in a multidisciplinary geological and engineering approach to statewide reservoir characterization.
Hydrocarbon production in Illinois has followed a pattern typical of mature basins (fig. 1). Early discoveries were based on surficial geology and resulted in a peak in hydrocarbon production in 1908-1910. The introduction of seismic exploration methods in the late 1930s resulted in significant new discoveries and a production peak in 1940. The application of water-flood technology and hydraulic fracturing during the 1950s led to a lesser production peak in 1956. Since that time, however, production has continued to decline, with minor fluctuations due to variations in the price of oil (fig. 1). The application of improved and enhanced recovery methods could reverse this trend.
The Department of Energy (DOE) has shown in their nationwide analysis that two classes of oil, mobile and immobile, remain in reservoirs after normal production. In I1linois, DOE estimates some 1.5 billion barrels of mobile and 4.5 billion barrels of immobile oil exist in known reservoirs. I1linois ranks ninth in the nation in unrecovered
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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.