Acid demineralization with critical point drying: A method for kerogen isolation that preserves microstructure

Abstract

The geometry of kerogen-hosted pores partially controls storage and transport of hydrocarbons in organic-rich mudstones (commonly referred to as shales). Although these pores are considerably smaller than those encountered in conventional reservoir rocks, several laboratory techniques are available that provide some description of kerogen’s microstructure. Many of those techniques perform optimally if the kerogen sample to be studied is first isolated from the mineral matter in shale. It has been observed previously that while typical procedures for kerogen isolation result in a kerogen sample that is chemically similar to native kerogen in intact shale, these procedures result in a kerogen sample whose microstructure is considerably different from native kerogen. Here we describe a novel procedure designed to produce an isolated kerogen with more representative microstructure. The procedure involves the same solvent extraction and acid demineralization as in traditional procedures, but the final step of water removal, which is performed by heating in the traditional procedure, instead is performed here by critical point drying. Critical point drying is characterized by a liquid–gas phase transition without crossing a phase boundary. As a result, drying is not accompanied by capillary stress being exerted on the pore walls, which is responsible for drying damage in the conventional procedure. We present SEM images and surface area measurements suggesting that kerogen isolated by acid demineralization with critical point drying is microstructurally similar to kerogen in shale. We therefore propose that acid demineralization with critical point drying is an effective technique for preparing representative kerogen samples suitable for microstructural characterization.