Visualisation and investigation of water retention in tight oil reservoir via real sandstone micro-models

Abstract

Hydraulic fracturing is considered as an effective approach for the development of tight oil reservoirs. Unlike conventional reservoirs, after a large volume injection of the fracturing fluid, the flowback rate in tight oil reservoirs is considerably lower whereas the underlying mechanism explaining water retention remains unclear. Therefore, in this work, to investigate the water retention mechanism, fracturing fluid injection and particularly oil production process were emulated within real tight rock chips. Displacement and water retention events were directly visualised and captured using a microscope. As a result, the discrepancy between temporary and permanent residence of water inside the pore space was clarified. Additionally, nuclear magnetic resonance (NMR) scanning was incorporated to study the pore-scale fluid flow behaviours. Results show that a large portion of water was retained in smaller pores and water retention rate is positively correlated with oil recovery, implying invaded water replaced the oil and resided in the pore space therefore increasing the mobile oil in the formation as oil flowing back. This work proposed an interpretation of water retention mechanism in tight oil reservoir after hydraulic fracturing and provided an insight to optimise the development and production of tight oil reservoirs. [Received: January 20, 2020; Accepted: April 23, 2020]