Summary

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 the stress change associated with 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 our recent study, we investigated –

  • stress dependent matrix permeability of nano, micro, and milli-Darcy core samples for a wide range of net stress;

  • hysteresis of matrix permeability of these cores during stress loading and unloading cases;

  • cyclic matrix permeability hysteresis of milli-Darcy cores; and

  • effect of temperature on permeability of organic rich mudrock.

In this study, we further investigated –

  • cyclic fractured (jointed) and matrix permeability hysteresis during stress loading and unloading of nano-Darcy permeability Niobrara and Eagle Ford cores, and

  • pore pressure effect on matrix permeability of organic rich Eagle Ford core at 250 °F.

The following are significant observations of this study:

  • fracture permeabilities are more stress dependent than matrix ones;

  • permeability hysteresis during stress loading and unloading of fractured (jointed) tight cores is larger, compared to that of matrix;

  • matrix permeability decrease with stress and its hysteresis during the second and third stress loading/unloading cycles were negligible;

  • improvements in fracture permeabilities were observed during the second and third stress loading/unloading cycles. The possible reason is that when a repeated stress loading and unloading happens, a new flow path that connect the main fracture to micro and nano cracks could be created; hence, improve the effective permeability. This indicates that re-fracturing of nano and micro-Darcy formations could lead to improvement in connectivity of stimulated reservoir volume (SRV) region; hence, improved hydrocarbon recovery;

  • at constant confining stress, pore pressure loading (adsorption) leads to decrease in permeability, and pore pressure unloading (desorption) increases effective matrix permeability back, but with a hysteresis. The inverse relation of molecular diffusion vs pressure is also playing role to the inverse relation between pore pressure and permeablity.

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