Cuttings vs cores: are cuttings a reliable predictor of caprock porosity?

Abstract

Repurposing depleted oil and gas fields for underground storage may play an important role in the energy transition. Existing sample materials collected during the exploration and development phases of oil and gas fields may prove useful for answering questions for the safe repurposing of depleted fields. In the case of questions for caprock integrity, drill cuttings are typically available whereas core material is often not. In this study, porosity is measured on both cutting and core samples from the caprock interval of the same well. Porosity was measured by three different methods, and in each method the cuttings show much higher porosity than the core samples. Furthermore, comparison of the data to published mudstone compaction trends (porosity-depth trends) from the basin also indicate excess porosity for the cuttings samples. Based on scanning electron microscopy (SEM) images, the cuttings have persistent dilated grain contacts (intergranular cracks) that are not observed in the core samples. This indicates the excess porosity is due to volumetric changes and damage in the cuttings samples. Based on the results of this study, cutting samples from seal rock intervals are likely to produce erroneously high porosity values, and core pieces or well-established basin-wide trends are a better predictor of seal rock porosity.