Fluid Flow in Cracks as Related to Low-Permeability Gas Sands

Abstract

Abstract A problem in formation evaluation of tight gas sands is that their permeabilities are sometimes surprisingly sensitive to variations in overburden pressure. Photomicrographs of pore casts show an interconnected system of Photomicrographs of pore casts show an interconnected system of sheet pores, which are somewhat like the surfaces of a randomized honeycomb. A mathematical relation for predicting the pressure dependence of flow rate in sheet predicting the pressure dependence of flow rate in sheet pores has been derived from the dimensions of the pores pores has been derived from the dimensions of the pores and the elastic constants of the matrix. The equation has been validated by measurements on artificial media containing cracks of known dimensions in glass and concrete. The observed pressure sensitivity of the gas sands used in this study requires the aspect ratio of the pores (in this case, the ratio of average large dimensions to sheet thickness) to be greater than 100. Aspect ratios have been determined by taking the large dimension from photomicrographs of pore casts or grain size and the thickness photomicrographs of pore casts or grain size and the thickness from mercury injection pressure or the slope of a plot of apparent permeability vs. the reciprocal of mean gas pressure. The latter gives the diffusive contribution to gas pressure. The latter gives the diffusive contribution to gas flow from which the pore size can be calculated. The two methods for measuring pore size give satisfactory agreement. The aspect ratio's for the sheet pores in tight gas sands are large enough to explain the dependence of permeability on overburden pressure. permeability on overburden pressure. Introduction Sensitivity of permeability to overburden pressure is often a key factor in formation evaluation of tight gas sands. Gas permeability reductions of more than an order of magnitude have been observed in dry cores when overburden pressure is increased to typical formation values. Although this sensitivity to pressure has been related to the presence of clays and shales, the current consensus is that the pressure behavior of crack-shaped pores-i.e., pores characterized by two large dimensions and one small pores characterized by two large dimensions and one small one-is largely responsible. A similar conclusion was reached earlier by Fatt with respect to the less severe, but still significant, sensitivity exhibited by conventional sandstones. In this paper, the effect of pressure on pore structure and consequent changes in gas permeability pore structure and consequent changes in gas permeability are examined for a variety of natural and synthetic porous media. Pore Structure Pore Structure Pore Casts The pore structure of tight sands is revealed Pore Casts. The pore structure of tight sands is revealed in three dimensions by resin pore casts such as those shown in Fig. 1. The cast is prepared by injection of epoxy resin into the sample, followed by etching after the epoxy is set. Pore casts for over 20 samples, 12 of which were selected for the variety they provided in geologic character, typically showed sheet pores that are linked to give random polyhedra. Within this structure are distributed relatively large pore spaces commonly formed by solution of individual grains and cements. These spaces are often filled partially with matrix material. Inspection of pore casts before and after etching, and dun sections of samples containing injected resin, indicates that individual grains are largely bounded by sheet pores; individual polyhedra, seen in the pore cast, appear to be associated with individual grains or local regions bounded by grain surfaces. Thin sections generally show that, as a sediment is compacted over geologic time by pressure-solution and recrystallization processes, the grains fit pressure-solution and recrystallization processes, the grains fit together more and more snugly and may even fracture, but they maintain their individual identities with respect to neighboring grains. The polyhedral structure, therefore, is related strongly to grain size distribution. Contact Between Grains The ability of etchant, used in preparation of pore casts, to penetrate the sheet pores through many layers of particles may imply the existence of areas of cementation or direct contact between grains. However, from examination of sheet pores studied to date, the areas of actual contact between grains generally are difficult to identify from the pore casts and probably are small. For some sediments with permeability less than about 0.005 md, it was difficult to prepare satisfactory pore casts, possibly because the casts had very poor pore casts, possibly because the casts had very poor structural integrity or because the resin did not penetrate the space, if any, between grains. From our observations to date, it seems likely that most tight sands of potential commercial interest contain a network of polyhedral sheet pores that largely control permeability. pores that largely control permeability. Pore Thickness. Electron micrographs of the sheet pore Pore Thickness. Electron micrographs of the sheet pore edges, such as shown in Fig. Id, for sands of less than 1 md permeability, show their thicknesses to range typically from about 0.2 to 4 mu m. The lower limit may be related to difficulties of preparing casts of even finer cracks, since the presence of much smaller pores is indicated by mercury porosimetry and NMR measurements. Surface Area The two-dimensional network at the surface of the pore cast permits estimates of crack length per unit area to be made. These can be translated to approximations of crack surface area, Ac, per unit volume. SPEJ P. 191