Multiphase Simulations of Fractured Basement Reservoir Composed with Dfn Characterized Seismic and Subseismic Scale Features
Crystalline basement is not a usual target for exploration and production; however, a combination geologic conditions and processes can produce an economically significant reservoir. These conditions include fracture zones with sufficient porosity and permeability, nearby source rocks, a reservoir seal, and a favorable temperature and pressure history. The successful production of a basement reservoir requires approaches that recognize the complexity of the fracture and fault networks and their impact on production, especially water-oil displacement. A discrete fracture static-model (DFN) simulator coupled with a fully-implicit, three-phase, CVFE simulator developed at the University of Utah provides new insight regarding oil/water displacement mechanisms in these geometrically complex systems.
In this study, we built a DFN-based model of a portion of basement reservoir using two vertical seismic-scale features connected by 117 subseismic-scale features with different orientations. The seismic features were square with 1500-ft size. The subseismic network consisted of 300-ft sided fractures. The combined network was well connected. The distance between the two seismic features was set as 1000 ft. The system was set up with an aquifer support at the bottom and production from the top. Simulations over a range of permeability contrasts (base permeability contrasts was set as 1:100 between seismic and subseismic features) and pumping rates show the effect of fracture network properties on the water breakthrough. At low rates of production, water displaces oil uniformly in the seismic and subseismic networks. When breakthrough occurs, the water cut goes rapidly to 100%. At high rates of production, the water floods the seismic features produding a high, but less than 100% water cut after breakthrough, after which water cut gradually increases as the subseismic network floods at slower rates. The model can be used to optimize production rate recovery and economy.
AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009