Fractured Reservoirs: Orgin, Description, Evaluation
1991. "Fractured Reservoirs: Orgin, Description, Evaluation", 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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Fractured reservoirs have been recognized for over 40 years. Generally they have been, and still are regarded as a special type of reservoir with only a limited contribution to the total oil/gas reserves. DRUMMOND (1964) estimated that only about 13% of all reserves are contained in such reservoirs. It is the purpose of this paper to challenge this view. In order to do this it is necessary to take a closer look at the different types of reservoirs that might be called fractured traps.
The fractures that are of particular interest are the joints observed in almost all sediments. SCHEIDEGGER (1979) states: “Joints are ubiquitous phenomena”. Thus their actual presence, with highly variable spacing, is not a problem as any field geologist can confirm. Generally the orientation of these joints is vertical, or high angle, to bedding.
In order to be effective fluid conduits such joints must be OPEN. For reservoir depletion this means that they must be open NOW. One may question this additional requirement and argue that a joint once formed will never again close perfectly. Just how do we justify the necessity for gaping joints? Surely there are laboratory experiments that point in that direction. However, the simplest proof can be found in the process of artificial fracing, a daily operation in the oil patch for over 40 years, and a most successful one at that. The artificial fracture is produced in a positive (compressive) virgin stress environment by an excess pressure (injection pressure) within the fracture.
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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.