Comparison of Porosity and Permeability from Drill Cuttings and Core Samples for Mudrocks: A Proof of Concept Study

Abstract

Abstract Understanding CO2 migration through the reservoir seal is one of the main challenges in assessing caprock integrity, thereby ensuring safe and long-term CO2 containment and storage. The determination of seal rock properties using core sample analysis or well data is important to predict matrix migration of CO2 to minimise the risk of capillary leakage. However, core samples and continuous well log data of caprock sections are expensive and limited. This issue can potentially be resolved by using cuttings, that are readily available from drilling, with no additional coring cost. However, before drill cuttings can be used as an alternative, rock properties (porosity and permeability) values from drill cuttings must be correlated with rock properties from the preserved core data, for proof-of-concept purposes. In CO2 storage sites, the caprock zone is usually comprised of low permeability mudrocks. Due to the low permeability of caprocks with values <1 microDarcy (10−18 m2) conventional core analysis is insufficient to determine porosity and permeability. We here use helium pycnometry for porosity determination on plugs and mercury porosimetry on cuttings. Meanwhile, the selected methods for permeability determination is an unsteady-state technique for plugs, and pressure decay / GRI method for cuttings, as both methods can measure permeability down to or even below nanoDarcy (10−21 m2). For the proof-of-concept study, we expect a correlation between plug and cuttings permeabilities of R2> 0.8 as indication of success criteria, based on strong effect size from statistics point of view. Using data from literature, permeability and porosity values from core samples match the permeability from cuttings with R2 >0.9. The R2 value for cutting versus plug porosity is 0.95, which is satisfies the success criteria as well. The correlations show that porosity and permeability values from cuttings are comparable to values from plugs and can be used as an approximation for the uncored zone. This proof-of-concept study marks the beginning of a full fledge research study to establish porosity and permeability relationships in caprocks, which can be used as inputs in predicting CO2 matrix migration for safe geological storage of CO2. In addition, this approach can be applied to unconventional oil and gas studies, with potential cost saving from coring operations.