Steady-State Permeability Measurements on Intact Shale Samples at Reservoir Conditions - Effect of Stress, Temperature, Pressure, and Type of Gas

Abstract

Abstract Gas shales have gained significant attention in recent years as a source of natural gas, resulting in significant focus on storage and flow mechanisms in these rocks. The permeability of shale reservoirs is a key control that determines the producibility and profitability of the resource. In contrast to conventional reservoir rocks, gas shales have significant organic content and, often, most of the pores are in the organic matter. Furthermore, most pores are in the 1–100 nanometer size range, and gas can be stored in these pores as sorbed gas as well as free gas, which has raised concerns such as deviation of the flow mechanism from Darcy flow, and the effect of sorption on permeability in shales. The common industry practice of measuring permeability using crushed rock samples and helium as test gas at room conditions has been found to have a number of shortcomings, including inconsistent results reported by different laboratories using such methods. In this paper we present the results of steady-state permeability measurements on shale samples conducted at reservoir conditions, and demonstrate the effect of stress, pressure, temperature, sorption, and type of gas utilized for measuring permeability of intact shale samples. The results presented in this paper show that net stress, pore pressure, and temperature have strong effects on permeability of shales. Additionally, we show that the type of gas utilized for measurements, and sorption phenomenon can have a significant influence on the measured permeability, particularly for organic rich samples such as gas shales. We also demonstrate through examination of Klinkenberg flow model, that currently, with the exception of the effect of pressure on permeability, no existing model accurately predicts the effects of other factors such as temperature and type of gas on permeability of shale samples in the absence of actual experimental measurements. This paper documents that there may be significant errors associated with measurements that are conducted at room conditions using non-reservoir or non-sorbing gas, and accurate permeability measurements require tests to be conducted at conditions that are close to the in-situ conditions. Accurate permeability measurements conducted at reservoir conditions enable superior characterization, supporting sound business decisions in the development phase of a project, and more accurate production forecasting in the production phase.