Effect of Gas Adsorption on Pulse-Decay Permeability Measurements

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Abstract
Pulse-decay method is a technique used to estimate cores permeability using gas injection based on Darcy's flow model. Helium and nitrogen are generally used when the target formation is an oil formation or liquid permeability is sought. However, new approaches suggest using CH4 to avoid correcting for properties difference between the testing fluid and reservoir fluid. This study focused on the latter approach to quantify the correction to gas adsorption. The Cui et. al, model based on Langmuir adsorption isotherm is modified to Cui-Freundlich isotherm which has been proven adequate to fit the adsorption on intact reservoir rocks. The model is developed using gas and rock properties and the approach is generalized by using reduced pressure and temperature to account for the gas compressibility. The results of the modified model showed that the new model can capture effective porosity of adsorption Φα used to correct the pulse-decay storage capacity parameters a and b. This correction in the storage volume and rock porosity values will enhance the permeability estimation in the ultra-tight rock samples. 1. Introduction Tight rock sample permeability is usually measured in the laboratory using gas. Unsteady-state gas permeability was proposed to avoid the shortcomings of the steady state gas permeability determination. Steady state gas permeability measurements are time consuming when used in very tight rocks because the time required for each steady state measurement. Bruce et al. (1952) introduced the use of unsteady-state gas permeability method to determine the permeability of tight rocks. Wallick and Aronofsky (1954) introduced the use of pulse decay set-up to measure the tight rocks permeability using the unsteady state gas permeability measurements. The experimental set-up involves monitoring the differential pressure evolution as a function of time across the rock sample. Each face of the rock sample is connected to a tank, upstream and downstream tanks. This set-up was introduced by Aronofsky et al. (1959). The upstream tank pressure is increased gradually by a specific pressure increment. Several modifications were made to enhance the pulse decay measurements and to shorten the experimental time required to measure the tight rock permeability (Jones, 1997). The pressure in the upstream tank (reservoir) should be raised in incremental that is almost 1 to 3% the value of the downstream transducer pressure. The downstream pressure should be stabilized and this might take hours or minutes based on the sample permeability. Then the pressure pulse is applied, this may be different than the initial pressure drop because of the dead volume.

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