The effects of mineral composition, TOC content and pore structure on spontaneous imbibition in Lower Jurassic Dongyuemiao shale reservoirs

Abstract

Spontaneous imbibition (SI) is a capillary-force controlled process, which has been identified as one of the most important mechanisms to affect hydrocarbon recovery from unconventional shale reservoirs. In this work, we focus on the mineral composition, total organic carbon (TOC) content, pore geometry characteristics and fluid imbibition behaviors of the terrestrial Lower Jurassic Dongyuemiao shale reservoirs. The pore morphology is directly imaged and observed via field emission-scanning electronic microscope (FE-SEM). Nitrogen (N2) physisorption and mercury intrusion porosimetry (MIP) are applied to investigate the pore geometry characteristics. The slopes of deionized (DI) water/n-decane imbibition from fluid imbibition experiments are used to evaluate the pore connectivity characteristics of hydrophilic and hydrophobic pore spaces, respectively. Finally, the relationships between shale dominant mineral composition, TOC content and pore structure parameters on fluid imbibition behavior are discussed. Experimental results show that clay and quartz are the two dominant minerals in the Dongyuemiao shales. In this study, four typical lithofacies are identified in the Dongyuemiao shales, including laminated or massive mudstone, silty mudstone, shell limestone and argillaceous siltstone. Organic matter (OM) pores and interP pores are the dominant pore types in the Dongyuemiao shales, and most of the pores with sizes smaller than 50 nm. The imbibition slopes of n-decane are generally higher than that of DI water, indicating better pore connectivity of hydrophobic pores. Due to protection by quartz grains, samples with higher abundances of quartz are beneficial for forming a connected pathway for DI water migration, resulting in a better-connected water-wet pathway. The hydrophobic and hydrophilic pore networks in samples with higher TOC content are better connected, which are speculated to be related to the high volume of connected organic matter in shale matrix. Positive correlations occur between porosity/pore volume and fluid (including DI water and n-decane) imbibition slopes, suggesting that the connectivity of pore networks become better as porosity and pore volume increase, regardless of whether the pores are hydrophobic or hydrophilic.