Capillary pressure curve and liquid permeability estimation in tight oil reservoirs using pressure decline versus time data

Estimating oil recovery from shale is a difficult process because organic mudrocks have very low permeability (10nD - 100nD), high kerogen and clay content, a complex pore structure and low porosity (typically less than 10%). In addition, most of the pore sizes in organic mudrocks are within the nanometer range, thus making the process of oil intrusion very difficult to achieve and even harder to measure with certainty. Several methods have been proposed to estimate saturation profiles and capillary pressure curves in conventional rocks, but these methods fail in tight rocks because of the complexities inherent in these types of reservoirs. This report presents a new method that makes use of the pressure decline versus time data to estimate the capillary pressure drainage curve in tight rocks. The method was used to estimate the capillary drainage curve for an Eagle Ford shale and it was also used to determine the shale's liquid permeability using the early time data of the pressure decline curve. The data obtained during the experiment could be used to estimate fluid saturations as a function of time which can be very useful in determining relative permeability curves in the shale. This new procedure is easy, fast and can be reversed to estimate imbibition curves as well. It can be applied to both conventional and unconventional rocks and it can be useful in EOR experiments to estimate oil recovery as a function of time. Furthermore, this report presents the use of NMR as a useful tool in examining fluid distributions and characterizing fluid types within tight rocks, via a combination of NMR T₁ and T₂ data