Improved Petrophysical Core Measurements on Tight Shale Reservoirs Using Retort and Crushed Samples

Abstract

Abstract Gas shales have become an important resource for the production of hydrocarbons in North America, and are being explored as a resource on other continents as well, based on their rapidly increased importance to the North American market and promise to boost domestic production elsewhere. There are numerous pricing and geopolitical reasons for this active exploration, but regardless of where in the world they are being explored, gas shales share some fundamental properties that make them virtually impossible to analyze with conventional core analysis methods or conventional petrophysical models based on log data. These properties are basically that gas shales are tight, with permeabilities in the 10s to 100s of nD, have low (effective) porosity, typically less than 10%, and have high kerogen and clay content. While there are some variations of these themes (e.g. shales with higher detrital input, making them siltier or silty-laminated), in general the tightness of the rock and abundance of clay minerals and kerogen pervades, and that causes a number of challenges to analysis. We have developed analytical methods for evaluating these reservoirs on core by using crushed material to enable better access to the pore space, retort analysis to measure separately free, bound and structural water saturations and also distinguish water from oil, and pressure transient analyses for the determination of permeability. Conventional core analyses (e.g. Dean Stark), applied to kerogen- and clay-rich rocks fails in separating free from bound waters and water from light oils, thereby missing critical inputs into calculating effective saturations, effective porosities and clay-bound water volume. In addition, the amount of oil recovered from retort, as an independent quantity, can be a useful proxy for kerogen maturity. From a permeability standpoint, unconventional reservoirs are usually too tight to allow for steady-state permeability measurements, and microfracturing is often too pervasive to allow for reliable permeability measurements on whole plug samples. As a result, we have developed a crushed-sample pressure decay system to measure the nD permeabilities typical of shales, and a stepped confinement pulse-decay method for measuring micro-Darcy (and higher) permeabilities in more texturally complex, siltier or sandier unconventional reservoirs that typically have these higher permeabilities.