Laboratory Characterization of An isotropic Rocks

Abstract

Abstract A number of layered rock samples with large permeability contrasts has been collected mostly from the permeability contrasts has been collected mostly from the western United States. The collection provides samples from a variety of depositional geological environments with bedding thicknesses ranging from millimeters to decimeters. The objective of this study was to investigate the origin of the large permeability anisotropy. permeability anisotropy. To characterize these rocks, conventional laboratory methods have been used such as macroscopic petrographic description, mineralogical analysis using Fourier transform infrared spectroscopy, as well as permeability, porosity, conductivity and acoustic velocity measurements on Hassler cores orthogonal and parallel to the bedding surfaces. Additionally, novel techniques consisting of permeability scans on the rock surfaces with an experimental minipermeameter setup, petrographic image analysis from thin sections, and density profiles obtained from a computerized tomographic (CT) scanner were used. These provided detailed maps of vertical density, porosity and permeability changes within these rocks. Results of CT scanning at two different energies allowed us to simultaneously determine densities and estimate photoelectric factors. All of these rocks are layered and exhibit large porosity changes over very short vertical distances of sometimes as little as one millimeter. Some sandstone samples as well as an interlayered sand/shale sample were found to exhibit permeability anisotropies of more than one-hundred and vertical permeability anisotropies of more than one-hundred and vertical porosity variations of up to a factor of three. Conductivity porosity variations of up to a factor of three. Conductivity anisotropies of these samples, however, were generally less than 1.6, and acoustic an isotrropies less than 1.2. The sedimentological origin of these anisotropic rocks is attributed to an interaction of two or more depositional processes during lateral acceration or vertical aggradation, producing vertical variations in mineralogy, grain size and other petrographic characteristics. petrographic characteristics. Sedimentary rocks exhibit layering from the kilometer scale down to the grain size. Petroleum engineers have often indicated the importance of small-scale stratification on the permeability, hence producibility, of a reservoir. An understanding of the influence of small scale stratifications on permeability is thus essential for the design and scale-up of permeability is thus essential for the design and scale-up of petroleum recovery processes. For this reason, eleven samples petroleum recovery processes. For this reason, eleven samples representative of reservoir rocks were collected from a variety of depositional geological environments. Most of these samples consist of well consolidated, fine to coarse-grained sandstones, having layers with thicknesses less than one inch. On Hassler cores taken from these rocks, we conducted routine laboratory measurements such as mineralogical analyses and permeability, porosity, conductivity, and acoustic velocity measurements with porosity, conductivity, and acoustic velocity measurements with the objective of understanding the origin of permeability anisotropy. It is commonly believed that permeability is often controlled by the presence of low levels of clay minerals in the pore space or by shale streaks. However, while sandstones pore space or by shale streaks. However, while sandstones with shale intercalations account for some of the most anisotropic rocks, this paper shows that, even in the absence of shale partings, permeability anisotropies of more than one-hundred, and perhaps as high as one-thousand, can occur solely because of grain size variations in successive laminations. Specifically, we compared petrographic image analysis (PIA) results to density profiles obtained from a CT scanner, and permeability scans on the surface of highly anisotropic, permeability scans on the surface of highly anisotropic, essentially clay-free rocks. We found large vertical porosity and permeability changes, and were able to establish that they are permeability changes, and were able to establish that they are ultimately caused by textural factors such as grain size and packing. This represents an extension of work by Murphy et packing. This represents an extension of work by Murphy et al., who compared acoustic measurements to x-ray images in order to characterize centimeter-scale layering and heterogeneities in sedimentary rocks. P. 751