Can cuttings replace cores for porosity and pore size distribution analyses of coal?

Abstract

Core data play a significant role in analyses of fundamental petrophysical properties, such as porosity and pore size distribution (PSD), which can be used to estimate the storage capacity and recovery rate of hydrocarbons. However, coring is time-consuming, and it is sometimes difficult to retrieve an intact core, particularly from coal reservoirs that are characterized by low mechanical strength and development of fractures. In most cases, only drill cuttings are available for analysis. Therefore, it is important to determine whether drill cuttings can adequately represent formation properties such as porosity and PSD. In this study, coal samples were prepared for analysis, including three groups of artificial drill cuttings—group-A (particle size of 1.00–1.70 mm), group-B (2.36–3.35 mm), and group-C (4.75–6.70 mm)—as well as one core plug. Nuclear magnetic resonance (NMR) measurements and mercury intrusion porosimetry (MIP) were performed to determine the porosity and PSD. The results for the porosity and PSD from the three groups of cuttings agree well with those from the core plug, which suggests that the effects of mechanical-pulverization and size fractionation during the sample preparation can be neglected for cuttings with particle sizes of ≥1 mm in most cases. The exception is one sample that having high mineral contents, because many mineral blocked pores in the sample can be released during mechanical-pulverization. The recommended particle size of drill cuttings for determining the porosity and PSD is 1–1.7 mm, which can provide accurate analytical results and avoid interference from other factors such as wellbore collapse, particle gravity dispersion, and drilling mud contamination.