Abstract:
Permeability decrease with increasing net stress in consolidated and unconsolidated porous media is a well-known phenomenon to petroleum and geomechanics engineers. Conversely, permeability is observed to increase when net stress decreases, however typically follow a different path, this discrepancy is known as hysteresis. The trend of permeability hysteresis is a signature of porous medium that depends on several physical and mechanical properties. Understanding permeability hysteresis plays a significant role in production strategies of hydrocarbon reservoirs. Hysteresis effect on production strategies can be even more important in very low permeability reservoirs such as tight sandstone, tight carbonate, and shale formations. The reason is that permeability hysteresis can affect adsorption/desorption and diffusion transport mechanisms which are among the main driving mechanisms in low or ultra-low permeability reservoirs.
In this study, permeability hysteresis of nano, micro, and milli-Darcy core samples are measured for a wide range of net stress. We also perform cyclic (repeated three times) permeability and porosity measurement during stress loading and stress unloading measurements on three milli-Darcy cores. The nano, micro, and milli-Darcy core samples are from Niobrara outcrop, and Bakken and Middle East oil bearing carbonate reservoir cores, respectively. Our results show that, (a) stress dependency and hysteresis of permeability was observed to be inversely related permeability measured at initial stress conditions; hence, these characteristics are more pronounced on nano-Darcy and micro-Darcy permeability cores; (b) generally, both permeability and porosity decrease with increase in net stress and their hysteresis values are observed to be significant in the first hysteresis cycle; (c) cyclic hysteresis experimental information can be utilized for reservoir management purposes, especially in low and ultra-low permeability reservoirs; and finally, (d) cyclic permeability hysteresis information could also be used in proper designing of multi-stage hydraulic fracture and re-fracturing design of low and ultra-low permeability reservoirs.
Introduction
In the United State, oil and gas production form tight formations are becoming increasingly significant since 2007. This is mainly due to the advancement of multi-stage hydraulic fracture stimulation in horizontal wells. Thorough study can improve the deliverability of such reservoirs. In this study, we will investigate stress dependent permeability and hysteresis of nano, micro, and milli-Darcy permeability cores from Niobrara outcrop, and Three Forks and Middle East oil bearing carbonate reservoir cores. We will also present a cyclic permeability hysteresis during stress loading and unloading cycles.
Understanding of permeability dependency on stress and their hysteresis information values could be used to optimize production of low-permeability formations. For example, the irreversible permeability loss (formation damage) seen as permeability hysteresis during stress unloading could be minimized by early fluid injection (pressure maintenance) to stimulated reservoir volume. Also, this permeability hysteresis information could be used in proper designing of multi-stage hydraulic fracture and re-fracturing design of low-permeability formations.
