Uncertainty Quantification and Reduction in Capillary Pressure and In-Place Volume Measurements on Tight Rocks

Abstract

For conventional hydrocarbon reservoirs, primary drainage capillary pressure (Pc) curve is used to quantify saturation distribution at discovery and original in-place volumes. However, for lower quality rocks (k < 10 mD), conventional centrifuge-based technique does not provide sufficient driving force to desaturate rocks close to irreducible saturation (Swir), leading to incomplete characterization of the saturation distribution. Although, most tight reservoir might not be thick enough to have water saturations (Sw) approaching Swir, accurate determination of Swir is critical as it is the endpoint for relative permeability. Uncertainty in Swir leads to uncertainty in water mobility at low Sw, water breakthrough, and modeled performance of production wells. In this work, we combine a portfolio of methods to quantify Pc for tight rocks to decrease this uncertainty. Tight outcrop plugs (0.01 mD < k < 10 mD) were thoroughly cleaned, and dried before porosity and absolute permeability measurements. Centrifuge air-brine primary drainage Pc measurements were conducted up to (i) 125-130 psi in a conventional equipment (up to 3200 RPM) using stock brine, (ii) ~250 psi in a conventional centrifuge using a heavy brine containing Cesium Formate, and (iii) 500 psi by spinning up to 10,000 RPM under overburden pressure in a high-speed centrifuge. Pc measurements were also conducted using Vapor Desorption, Digital Rock Physics (DRP), and Mercury Injection Capillary Pressure (MICP) on companion plugs. Centrifuge data were processed using Hassler-Bruner and Forbes technique, and a set of acceptable solutions were generated to quantify uncertainty. For tight rocks, Pc measurements with conventional centrifuge led to significant (~20%) uncertainty in irreducible water saturation (Swir), and thus in-place volumes. This uncertainty was reduced substantially either by incorporating high-speed centrifuge data or vapor desorption data. DRP-based calculations performed by 4 vendor labs agreed with each other, and with the experimental Pc measurements at low Pc values, but diverged significantly at high Pc. Uncertainty estimation from DRP-based Pc calculations was non-trivial, and therefore DRP-based data could not either quantify or decrease uncertainty in the in-place volumes. MICP-based measurements (scaled for interfacial tension) significantly overestimated in-place volume, as those measurements asymptotically approach Swir = 0. Vapor desorption measurements provided an anchor for Pc data at values starting 400 psi for air-brine system, and complemented the centrifuge measurements to decrease uncertainty. We combine the strengths of experimental and imaging techniques to decrease uncertainty in in-place volumes, particularly for low quality (tight) resources. Novel use of fluids and synergistic use of experimental techniques presented here significantly expand our capabilities to analyze capillarity in tight rocks.