Larry W. Lake, 1991. "Simulating Fluid Flow Through a Geologically Realistic Permeable Medium", 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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For the past five years, we have been studying the Page Sandstone outcrop in Northern Arizona as a prototype for eolian oil reservoirs and aquifers. The work has encompassed detailed geological studies and highly intensive descriptions of the distribution of hydraulic conductivity or permeability. We are now at the point of using this information to evaluate predictions about fluid flow through naturally-occurring sands. Work on this topic is the subject of this presentation.
The first step in the study was to establish a “truth case,” a standard against which to measure the success of alternative procedures for flow prediction. Our truth case consisted of a highly detailed (more than 11,000 finite element nodes) numerical simulation of a miscible displacement through a portion of the outcrop wherein each nodal property was assigned according to the actual value existing at that point. This intensely deterministic simulation also allows us to evaluate the extent of geologic detail necessary for a good prediction; mobility ratio and the distribution of average permeability are the most important quantities governing fluid flow. Permeability anisotropy, fourth order-bounding surfaces and dispersion are much less important.
With the truth case in hand, we redo the simulations based on the amount of data that would normally be available for subsurface conditions. In this case, of course, it is the generation of interwell properties which is being tested. The work investigates two basic procedures: pseudofunctions and conditional simulation. The pseudofunction approach generates effective relative permeabilities (a concept borrowed from immiscible flow) to
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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.