SHOULD THE PHYSICO-CHEMICAL PROPERTIES OF SHALE AND KEROGEN BE MODELLED USING KEROGEN “TYPE”?

During the last decade, advancements in horizontal drilling and hydraulic fracturing technology have led to impressive growth in shale gas production. More than 300,000 shale wells have been drilled in the US that contributes to more than 50% of U.S. natural gas production. However, despite the use of advanced hydraulic fracturing techniques, there is still a low recovery of gas in place (<20%) and a rapid decline in well productivity. The primary reason for low productivity is the lack of understanding of fundamental molecular scale properties of shales, especially involving kerogen. Although recent progress has been made on modeling kerogen structure and its volumetric properties, these studies are based on kerogen “type” which do not account for heterogeneities that exist within a particular kerogen “type”. In this study, by utilizing Marcellus Shale samples, we determine the heterogeneities in the molecular structure of kerogen with a similar kerogen “type” and similar maturity. These heterogeneities within a kerogen “type” show that the previously built structural models of kerogen have limited applicability as they could underestimate/overestimate several physico-chemical properties of kerogen and shale. This could lead to the inaccurate prediction of the type and amount of extractable hydrocarbons, the potential of shales for CO₂ sequestration and waste disposal, and interactions of hydraulic fracturing fluids in the subsurface environment.