Computer-Based Rock Description for Development and Exploration Geology
Published:January 01, 1991
Mark H. Scheihing, Gary L. White, 1991. "Computer-Based Rock Description for Development and Exploration Geology", 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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A computer-based rock description system has been developed for on-site description, graphical display and data transfer of cuttings, sidewall cores and conventional core. This system is designed to assist the geologist in efficiently collecting, graphically displaying and building databases of the geological and petrophysical attributes of cores and other types of rock samples. This system is used at the rig or core storage facility to collect rock description for later graphical display and processing in the office.
The rock description software is written as a HyperCardTM stack. This stack contains a wide variety of geological and petrophysical descriptors for siliciclastic, carbonate rocks and some non- sedimentary rocks. The geologist describes the core or sample by selecting description parameters by mouse- or keystroke for a given interval of core or discrete sample. For core description, there are no restrictions placed on the thickness of an interval of description. The description of an entire core or set of samples is stored as a set of "cards," each card containing the geological and petrophysical attributes of a given interval of the core or single sample.
Data entered into the rock description stack can be output as a graphical display, ASCII text file, or spreadsheet-style file. The graphical display uses a series of compiled programs to display the rock description data as lithology, sedimentary structure, and textural and comments columns. Extraction of data from the stack to make text and spreadsheet-style files permits easy transfer of rock description data to other computers and software.
Use of this system has dramatically reduced the time and laboriousness associated with the collection of rock description data. More importantly, this system has reduced the time involved in preparation of graphical displays and data entry for computer applications.
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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.