Determining the potential of shale gas reservoirs involves an exhaustive process of calculating the volume of total gas, or Original Gas In Place (OGIP). The calculation of total gas relies on calibrating wireline logs to core data; which are considered to be an empirical validation or ‘ground truth’. However, inconsistency in sample preparation and analytical techniques within, and between laboratories creates significant uncertainty in calculating the free and adsorbed gas components which constitute total gas. Here we present an analytical program performed on samples of core to elucidate the causes of uncertainty in calculation of total gas. The findings of this program are used to propose improved methods of calculating total gas from core.
Free gas calculated from properties such as porosity and water saturation measured on core was found to be highly dependent on laboratory analytical protocols. Differences in sample preparation and water extraction method led to relative differences of 20% in water saturation and 10% in porosity observed between laboratories, leading to differences of 35% in calculations of free gas in place.
Adsorbed gas was evaluated using methane adsorption testing to study the changes in Langmuir parameters in samples with a wide variety of water saturations, clay content, and total organic content over a range of temperatures. It was found that the storage capacity of adsorbed gas artificially increased by a factor of two to three when experimental temperature exceeded the boiling point of water. This increase is related to the expulsion of clay bound water and subsequent availability of clay surfaces for methane adsorption.
Total gas in place is the sum of free and adsorbed gas volume estimates. The interaction and overlap of pore space between these two volume components are also important to consider. It is proposed to use a simplistic mono layer-based correction of volume of adsorbed gas from the free gas volume based on a composite pore size distribution from scanning electron microscopy (SEM) point counting and nitrogen adsorption data.
Pressurized sidewall core samples were acquired at reservoir conditions to measure free and adsorbed gas volumes during controlled depressurization under laboratory conditions. This provided a baseline measurement for comparison with calculations from traditional measurements to understand which laboratory protocol and sample preparation technique provided the most robust results
This study has elucidated methods to reduce the uncertainty in gas in place calculation and better understand resource distribution in dry gas source rocks.
