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44334-AC8
Modeling Natural Fracture Networks in the Context of Flow Simulations: Teapot Dome, Wyoming

Thomas H. Wilson, West Virginia University

Objective: The main objective of this research effort is to acquire an understanding of the role fracture and fault networks play in controlling oil production from the Tensleep Formation, Teapot Dome, Wyoming. The research is conducted in collaboration with the Rocky Mountain Oil Testing Center and Schlumberger. Fracture orientation, intensity, and aperture measurements obtained from the FMI logs are used to simulate layered and composite 2D fracture networks. Analysis of post stack 3D seismic data from the producing area of the field is incorporated into fracture network characterization and, eventually, into flow simulations to gain insights into reservoir scale properties of the fracture network. In year 1, the effort concentrated on evaluating the characteristics of the subsurface fracture networks. This year’s efforts concentrated primarily on the development of a starting model for fractured reservoir flow simulation.

Reservoir Characterization: Production from producing Tensleep wells was examined and a series of maps showing cumulative production were compiled. High producing wells are distributed along a fault bounded structural culmination. The highest producing wells lie along the fault trend in the structurally higher areas of the reservoir. Our earlier work suggests that fracture density (or intensity) varies from approximately 0.03 to 0.69 for the open fractures observed in Tensleep sandstones. These results are consistent with field based measurements presented by Gilbertson. FMI log observations also provide basic information on the range of fracture apertures associated with the reservoir model. Kinematic aperture and length distributions are often characterized as having fractal distribution. The aperture distribution is positively skewed and the starting model assumes a fractal distribution of fracture aperture and length. Fracture network models are upscaled using edge detection processing of the Teapot Dome 3D seismic data set. Numerous additional reservoir parameters were obtained from the literature on the field including reservoir pressure, temperature, porosity and permeability, original gas, oil and water in place and production histories.

Flow Simulations: Fracture characterization and flow simulation tools used in our studies include automated structural and fault interpretation workflows in Schlumberger’s Petrel software integrated with fracture modeling tools to simulate the Tensleep reservoir fracture network for use in Schlumberger’s ECLIPSE dual porosity simulator. The initial model consists of 60970 cells 100 x 100 feet in size distributed in five zones corresponding to major stratigraphic subdivisions of the reservoir and the water drive. Each zone includes a uniquely defined open fracture network. The model is characterized by matrix porosity and permeability, relative permeability, oil-water contact depth, and initial reservoir pressure. Preliminary simulations were conducted in collaboration with Schlumberger. Efforts in the coming year will focus on adjusting the properties of the fracture network to achieve a good match between simulated and actual reservoir production histories (Smith, work in progress). This iterative modeling process will help us better understand the nature of flow in the naturally fractured Tensleep reservoir, help design tertiary CO₂ recovery operations and increase carbon storage potential.

Impact: This year’s efforts provided the PI excellent opportunities to expand our relationship with Schlumberger and the Rocky Mountain Oil Testing Center. Outgrowths of the research were incorporated into a multidisciplinary proposal to examine scaling issues from grain to reservoir scale associated with carbon sequestration activities. The student (Valerie Smith) had the opportunity to present two papers on her research at the 2008 Annual AAPG meeting in San Antonio, Texas. The project also supported a two week visit this past summer to Schlumberger’s Calgary office where she worked closely with Schlumberger’s flow simulation group. As a direct outgrowth of her efforts and dedication to the project she will begin work with Schlumberger as a reservoir geophysicist this fall in their Carbon Services group.

Conclusions: The Year 2 effort resulted in the development of a detailed reservoir model for flow simulation. The reservoir model incorporates a fracture network derived from FMI log analysis, field observations and relationships published in the literature. Initial flow simulations were undertaken in close collaboration with Schlumberger. Three initial tests were conducted including: 1) production from just the matrix and aquifer water drive; 2) production from a fracture network with aquifer water drive (no matrix); and 3) a dual permeability case that incorporates both matrix and fracture network with the aquifer. Initial runs resulted in underproduction of oil and excessive water production. The dual permeability run encountered run-time difficulties. In Year 3 we will continue our collaboration with Schlumberger to help resolve obstacles encountered in the flow simulations and undertake some history matching exercises.

Acknowledgements: Appreciation is extended to individuals with the Rocky Mountain Oil Testing Center and National Energy Technology Laboratory for providing data and helpful advice. We thank Alan Brown (Schlumberger) for his comments and perspective and for providing Petrel and ECLIPSE software used in the project. Special thanks to Isabelle Pelletier Tardy and Samer Mualla of Schlumberger for their help in the modeling process.

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