Fast Pressure-Decay Core Permeability Measurement for Tight Rocks

Matrix permeability is one of the most important factors used to evaluate long-term production of hydrocarbon reservoirs. However, for shale reservoirs that have ultralow matrix permeability in the range of tens to hundreds of nanodarcies (nD), the laboratory measurement of the matrix permeability of intact (nonparticlized) samples has remained a challenge. The widely used measurement methods, such as pulse-decay and steady-state, have two primary limitations: (1) measurements take hours or even days; and (2) sample fractures that are frequently present affect the measured permeability value. In this study, a new pressure-decay permeability method is proposed for ultratight rocks to overcome these limitations. The principle of the pressure-decay experiment is that system gas pressure is higher than pore pressure of the core sample so that gas penetrates the core sample and permeability is derived from the decrease of system gas pressure. In order to validate the proposed pressure-decay technique, experiments were conducted on a set of different types of rocks including a Marcellus shale plug, a set of Eagle Ford shale plugs, and a Marcellus shale plug with open, connected fractures. A series of comparative studies of the permeability results from pressure-decay, pulse-decay, and steady-state experiments on these core samples confirmed that the proposed pressure-decay experiment can provide accurate matrix permeability of ultratight rocks. Tests can be completed within one hour and the measurement range is from 1.0 x 10(-1) to 1.0 x 10(-6) mD. Further, the proposed method has the following advantages: (1) measured matrix permeability by the new pressure-decay method isn't affected by the open, connected fractures, whereas these fractures can make the pulse-decay and steady-state permeability results increase by three orders of magnitude; (2) the new pressure-decay experiment is approximately 10 times faster than the pulse-decay experiment and 20 times faster than the steady-state experiment; and (3) the proposed pressure-decay experiment can provide the gas-filled pore and fracture volumes during the permeability measurement, whereas pulse-decay and steady-state experiments cannot.