Outcrops and Modern Environments as Analogs for Characterizing Reservoirs
Published:December 01, 2006
2006. "Outcrops and Modern Environments as Analogs for Characterizing Reservoirs", Reservoir Characterization: Integrating Technology and Business Practices, Roger M. Slatt, Norman c. Rosen, Michael Bowman, John Castagna, Timothy Good, Robert Loucks, Rebecca Latimer, Mark Scheihing, Hu Smith
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A 3D geological model was constructed from a 3D quarry outcrop near Hollywood, Arkansas, for ‘reservoir’ flow simulation that can address the effects of small-scale (‘subseismic’) interwell heterogeneities on potential production problems in analog deepwater (and other) oil and gas reservoirs.
Dimensions of Hollywood Quarry are 380 × 250 × 25 m (1247 × 821 × 83 ft). The quarry exposes in 3D the upper Jackfork Group turbidites, which are often used as an outcrop analog for deep-water reservoirs in the Gulf of Mexico and elsewhere. The quarry is unique in that within its walls, numerous features can be examined which are typically found in subsurface reservoirs at larger scales, and which could dramatically affect reservoir performance.
Systematic characterization of this quarry was conducted in order to (A) simulate the effects of these features upon reservoir performance and (B) characterize them with instrumentation routinely used by reservoir geologists, geophysicists, and engineers. Techniques used to characterize the quarry include: photomosaic mapping, behind-outcrop coring and borehole image logging (FMITM), outcrop gamma-ray (GR) logging, measured stratigraphic sections, sequential photography of quarry walls, Digital Orthophoto-Quadrangle mapping (DOQ), Global Positioning System (GPS), including laser gun positioning of geologic features in 3D space, shallow high resolution (hammer) seismic reflection (SHRS), and Laser Imaging Detection and Ranging (LiDAR).
These combined techniques provided the basis for reconstructing the 3D geology within the quarry prior to its excavation. The resultant 3D geological model, constructed in GoCad™, includes spatially-oriented stratigraphic and structural features, and various up-scaling combinations. Results of fracture analysis were not included in the model, however, this analysis provided information on the orientation and origin of the fractures in relation to regional and local deformation. The model was imported into EclipseTM and 101 combinations of geological features, drilling scenarios, and drive mechanisms were simulated for a 10 year ‘production’ period.
Results indicated that presence and absence of faults is a major factor when producing this analog “reservoir.” Simulated production is reduced by 0.1 - 15% when faults are incorporated into the simplest “Tank” model. Partially sealing faults result in 1-10% less production than when faults are not sealing. A horizontal well across faults results in 4-21% higher production than with vertical wells.
The “Tank” model, provides 17% more oil in place (OOIP) and 12% more oil production than does more geologically realistic “reservoir” models. Adding geological information to the model, such as shale boundaries and faults, increases the accuracy of the initial volumetric calculations.
Stratigraphic and structural features within the 24 acres of geology reconstructed at Hollywood Quarry reveal horizontal and vertical complexities which could aid or hinder fluid flow in an analog reservoir at this, or larger scales. Systematic characterization of this quarry, and subsequent fluid flow simulation, has proven to be very useful in providing a better understanding of the geologic causes of various reservoir performance issues.