Invaded water flowback capability of high volatile bituminous coal after nitrogen displacing: from a perspective of fracture and seepage pore structural complexities

Abstract

Water-based fracturing is applied for stimulating coal reservoir. However, invasion of water can result in low flowback efficiency and low coalbed methane (CBM) yield. Nitrogen injection is an effective method to enhance CBM production. In this study, nitrogen displacing water experiments using water saturated high volatile bituminous coal were conducted, during which magnetic resonance imaging (MRI) tests were applied to determine flowback capability. To research the relationship between flowback capability and complexities of fracture and seepage pore, characterising methods including micro-CT and mercury intrusion porosimetry (MIP) were used. The results show that flowback capabilities of complex structural seepage pores and fractures were low. Water is easily flooded into simple structural seepage pores and fractures, and water flowback capabilities of these simple spaces are relative higher, compared with those of the complex ones. Water phase trapping during nitrogen displacing water happens due to water imbibition and Jamin effect occurring in complex structural seepage pores and fractures. [Received: April 27, 2019; Accepted: August 7, 2020]