How flow velocity affects the conductivity of propped coal fracture: New insights

Abstract The conductivity of propped fracture in coal exerts a profound effect on the transport of coalbed methane (CBM) from low-permeable coal seams with hydraulic fracturing stimulation. Understanding the conductivity dynamics caused by flow velocity is crucial to the accurate prediction of CBM productions and the deployment of proper development strategies. However, the changes in the conductivity and pore space of propped fracture in coal due to the movement of proppants at different flow velocities remain unclear. In this paper, online nuclear magnetic resonance (NMR) technology was first used to investigate the impact of flow velocity on the flow channels and conductivity of propped fracture in coal. Results show that the conductivity of propped fracture in coal exhibits an initial increasing and then a declining trend with increasing flow velocity velocities. It is also observed that at low flow velocities, the sensitivity of propped fracture conductivity increases with an increase in proppant concentration, size and effective stress. As a comparison, the sensitivity of propped fracture conductivity increases with increase in proppant concentration and size, but decreases as effective stress increases at high flow velocities. The T 2 spectra show that the conductivity is positively correlated with the pore space of the propped fracture. The experimental results and theorical analyses suggest that the dynamics of the conductivity is due to the movement sequence of the proppant particles with different sizes that are subject to varying dragging force caused by flow velocity.