An NMR-assisted laboratory investigation of coal fines migration in fracture proppants during single-phase water production

Abstract

Proppants are injected with the fracturing fluid to keep the hydraulic fractures open. However, deposition of coal fines in the void spaces between proppants significantly damage flow conductivity of the fractures. In this study, we experimentally model coal fines migration in a proppant pack. Standard quartz sand (16–20 mesh) is used to prepare the proppant pack. The proppant pack is installed within a low-field Nuclear Magnetic Resonance (NMR) facility to record NMR response before and after the experiment of fines suspension injection. The injected suspension consists of water with 500 ppm concentration of coal fines of radius from 2.8 to 142.9 µm. Coal fines are generated by crushing an anthracite coal sample. The permeability is damaged significantly by about 41% after 80 pore volume injection. NMR response suggests that volume of the large-size pores (radius rp > 60 um) decreases while that of the medium-size pores (0.9 um <rp < 60 um) increases. The pore structure change is caused by two possible mechanisms: (a) reducing pore size by fines occupying pore space and (b) developing inter-particulate pore space of fines gathering at the damaged pores. Although the fines size (radius from 2.8 to 142.9 um) is significantly smaller than the pore size (<1005.2 um), the large-size pores can still be damaged by the fines. The results of this study can help to understand mechanisms of fines migration and conductivity evolution in/of the hydraulic fractures during Coal Seam Gas production.