Modeling 3D porosity and permeability from GPR data in the Ellenburger Dolomite, central Texas

Abstract

Three-dimensional porosity and permeability were modeled in an Ellenburger carbonate reservoir analog from 2D crosshole and 3D surface survey ground-penetrating radar (GPR) data. Two-dimensional GPR crosshole velocity tomography, 3D migration of the GPR surface data, and porosity and permeability calibration to GPR attributes results in 3D porosity and permeability predictions that provide a consistent model of the paleocave structures and facies distributions. Picking the maximum instantaneous amplitude of the direct arrival wavelet for velocity tomography reduces uncertainties caused by a low signal-to-noise ratio, uncorrelated noise, and the interference between reflections and critical refractions at the earth/air interface. The GPR velocity is anisotropic with an average vertical to horizontal velocity ratio of 0.93, which is attributed to the dominance of the relatively horizontal orientation of the maximum porosity and permeability. Porosity and permeability trends are influenced by regional northeast-southwest and northwest-southeast striking conjugate fractures associated with the Pennsylvanian Ouachita orogeny and breccia facies generated by three episodes of burial and the resulting paleocave collapses. At depths [Formula: see text] from the present surface, large brecciated dolomite and limestone blocks have low porosity and low permeability. Between a depth of [Formula: see text] and [Formula: see text] irregular fracture orientations and distributions associated with collapse breccias have a higher average porosity and permeability. A deeper zone ([Formula: see text] to [Formula: see text] in depth), has intermediate permeability and porosity. Porosity and permeability could not be calibrated in open voids. Thus, the predictions are applicable to the core-scale to which they were calibrated, but are lower bounds for the whole volume, which contains breccias, karst, and fractures that increase both porosity and permeability at larger scales.

Conductivity and scattering Q in GPR data: Example from the Ellenburger dolomite, central Texas

“…The study volume contains disturbed host rocks (facies 3) and coarse-and fine-clast chaotic breccia facies (facies 4 and 5) that developed through three periods of alternating surface exposure and burial, from Middle Ordovician to the present (Loucks and Anderson, 1985;Kerans, 1988;Loucks et al, 1999;Gale and Gomez, 2007). Orientation and distributions of the porosity and permeability in the Ellenburger reservoir are controlled by the density and orientation of the conjugate fracture system Harbi and McMechan, 2011). Two set of fractures are identified (Gale and Gomez, 2007) in well cores from west Texas, and through 2D and 3D GPR reservoir analog characterization in the reservoir outcrop in central Texas Harbi and McMechan, 2011).…”

“…In the current crosshole tomography data acquisition, calibration traces are recorded at a known distance in air for estimation of the transmitter excitation time (Harbi and McMechan, 2011). These traces also are used to estimate the source spectrum because amplitude spectra at receivers in the crosshole survey are modified from the source spectrum by the attenuation (Quan and Harris, 1997).…”

“…The estimated average source spectrum over all the receivers, at radial frequency ω, is hA es i, and v is the velocity; x, θ s , and θ r are calculated a priori by ray tracing in the velocity model obtained by interhole velocity tomography (Zhou and Fullagar, 2001;Harbi and McMechan, 2011).…”

“…High-resolution crosshole GPR velocity and attenuation tomography are extensively used in characterizing shallow subsurface aquifers (Tronicke et al, 2004;Bachrach and Mukerji, 2005), reservoir analogs (Hammon III et al, 2002;Harbi and McMechan, 2011), monitoring hydrologic processes (Chang et al, 2004;Schmalholz et al, 2004;Hanafy and Hagrey, 2006;Deiana et al, 2008), fracture imaging (Kowalsky et al, 2005;Zhou et al, 2005;Liu, 2006), and monitoring subsurface contamination (Cassidy, 2008).…”

“…Then, we perform 2D interhole tomography for the real conductivity for the three crosshole data sets. Results of conductivity tomography and scattering are integrated with those from GPR crosshole velocity and surface data from Harbi and McMechan (2011).…”