Experimental Investigation on Permeability and Porosity Hysteresis of Tight Formations

Abstract

Summary The decrease of permeability and porosity with increasing net stress in consolidated and unconsolidated porous media is a well-known phenomenon to petroleum and geomechanics engineers. Conversely, permeability and porosity are observed to increase when net stress decreases; however, they typically follow a different path; this discrepancy is known as hysteresis. The trend of permeability and porosity hysteresis is a signature of porous media that depends on several chemical, physical, and mechanical properties. Understanding permeability and porosity hysteresis plays a significant role in production strategies of hydrocarbon reservoirs. The hysteresis effect on production strategies can be even more important in very-low-permeability reservoirs such as tight sandstone, tight carbonate, and shale/mudstone formations. The reason is that the stress change associated with permeability and porosity hysteresis can affect adsorption/desorption and diffusion-transport mechanisms that are among the main driving mechanisms in low- or ultralow-permeability reservoirs. In this study, matrix permeability and porosity hysteresis of nano-, micro-, and millidarcy core samples are measured for a wide range of net stresses (500 to 4,500 psia). The matrix includes nano- and micrometer-sized cracks (fractures) that are open or mineral-filled (sealed) cracks. The nano- and microdarcy core samples are from the Niobrara, Bakken, Three Forks, and Eagle Ford formations. The millidarcy core samples are from Middle East carbonate, Indiana Limestone, and Torrey Buff Sandstone formations. Bakken, Three Forks, and Middle East carbonate core samples are from oil-producing reservoirs, whereas others are from outcrop. The major experimental observations of this study are that (a) the stress dependency and hysteresis of permeability and porosity were observed to be larger for nanodarcy cores compared with those of microdarcy and millidarcy core samples; (b) stress dependency and hysteresis of porosity are smaller than those of permeability; (c) pore shape, pore size and their distributions, and mineralogy affect the stress dependency and hysteresis of both permeability and porosity; and (d) increase in permeability with increasing temperature and permeability hysteresis through temperature loading and unloading were observed in organic-rich core sample from Eagle Ford.