A SIMULTANEOUS DETERMINATION OF PERMEABILITY AND KLINKENBERG COEFFICIENT FROM AN UNSTEADY-STATE PULSE-DECAY EXPERIMENT

The unsteady state gas flow experiment is an appealing technique to determine the intrinsic permeability k1 and Klinkenberg gas slippage factor b of tight rock samples. "Pulse-Decay" and "Draw-Down" techniques consist in recording the pressure difference ΔP(t) at the core plug edges when a pulse of higher pressure initially applied at the inlet of the sample, relaxes through the core. The interpretation of the pressure-decay signal represents a challenge for a reliable estimation of k1 and b. On the basis of a previous analysis where the optimal experimental conditions were defined, the objective of this paper is to present an inverse technique that makes use of a complete model of interpretation. To begin with, we recall the classical method of interpretation based on a simplified model proposed in the literature. To circumvent some of the difficulties inherent to this method, we show how an inverse technique, based on the minimization algorithm of Levenberg - Marquardt (1963), can be used along with a complete flow model to perform the estimation of kl and b. Uncertainties on experimental parameters such as the volume of the upstream reservoir, dimensional characteristics of the sample and porosity of the material are introduced, the impact of which on the precision of the estimated values of k1 and b is evaluated. For the sake of clarity and to carefully analyze each parameter independently, the method is illustrated on synthetic signals.