The procedure to evaluate the basic petrophysical properties of hydrocarbon bearing shales varies substantially in the oil and gas industry, making it difficult to compare experimental results across labs. In this paper, we present a methodology that we hope will become an industry standard for preserving and preparing shales for petrophysical measurements.
We present the following series of measurements and their results to demonstrate the petrophysical properties of both well preserved and poorly preserved hydrocarbon bearing shales: native shale water activity, original fluid content, density, mineralogy, sea water and nitrogen permeability, swelling, Brinell hardness, and P-wave and S-wave velocities.
An adsorption isotherm test was used to measure the native shale water activity. We used the GRI method to measure the original fluid content and density. Mineralogy was determined with X-ray diffraction. A pressure transient technique was employed to measure the permeability of fluid and gas passing through a shale sample. We also compared our permeability data with the experimental data using the conventional GRI method. Brinell hardness was measured before and after swelling with a multi-scale durometer. Using a pulse transmission set-up, we also measured wave velocities before and after swelling. Young's modulus and Poisson's ratio were calculated with wave velocity data.
The large differences in shale properties between preserved and un-preserved samples as reported herein, clearly indicate that shales should be preserved at the well site and tested with a standard procedure ensuring minimum alteration of fluids from the shale. Failure to follow a standard procedure leads to measurements that do not reflect the "true" or in-situ properties of the shale. Instead, the measurements can be a factor of 2 or 3 different from the "true" value. The shale handling, preservation and measurement techniques and procedures presented in this paper can be used as a standard protocol for studying organic rich shales.