High-Resolution Variograms from Optical Borehole Imaging for Borehole-Scale Geostatistical Rock Simulation and Lattice Boltzmann Flow Simulation in High-Permeability Carbonates
Michael C. Sukop¹ and Kevin J. Cunningham²
¹Florida International University, Miami, FL
²United States Geological Survey, Miami, FL
Geometry of the pore space is a fundamental control on flow in carbonates over reservoir exploitation time scales. Fluid properties and pressure gradients are temporally variable and can be changed by use of effective reservoir production, workover, and stimulation procedures. The geometry of the porous medium largely remains constant except if significant pressure variations deform or fracture the reservoir or acid fracturing is applied. Formulating methods to accurately simulate subsurface carbonate pore systems is a difficult challenge. A technique is presented that uses spatial statistics applied to digital optical borehole images to create realizations of carbonate megaporosity. Megapores are “equant to equant-elongate pores whose average diameter is larger than 4 mm, and for tubular or platy pores whose average cross-sectional diameter or thickness, respectively, is larger than 4 mm” (Choquette and Pray, 1970). Flow within megapores at the borehole scale was simulated using Lattice Boltzmann methods. The borehole-scale Lattice Boltzmann computations are orders of magnitude larger than conventional pore-scale applications.
Traditional geostatistical models are often based on sparse data obtained from cores or down-hole tests. Conversely, image data are dense and provide highly constrained variograms that reflect the detailed structure of the rock megapore system and its vertical arrangement within high-frequency depositional cycles. The high-resolution variograms obtained from borehole image data justify the fitting of unusually complex variogram models containing relevant stratigraphic information. Moreover, with and without conditioning to the borehole image data, satisfying stochastic realizations of the rock can be generated.
Current research efforts focus on accurate quantification of permeability and non-Darcy flow parameter measurements in the karst Pleistocene carbonate rocks of the Biscayne aquifer in southeastern Florida, which can have exceptionally high porosity and permeability values. Standard laboratory core permeability measurement techniques (e.g., air permeameter) are not suited to accurately determine the permeability of these rocks. Furthermore, benchtop analyses of limestone core representative of extremely high megaporosity have never been accomplished due to breakage of relatively fragile vuggy-rock intervals during drilling. In the southeast Florida study area, extremely megaporous aquifer intervals are attributed to a megapore system formed by Ophiomorpha ichnofabric (Droser and Bottjer, 1989; Cunningham et al., 2009; Cunningham et al., 2010; Cunningham and Sukop, 2011).
AAPG Search and Discovery Article #120034©2012 AAPG Hedberg Conference Fundamental Controls on Flow in Carbonates, Saint-Cyr Sur Mer, Provence, France, July 8-13, 2012