An Experimental Study on Characterizing Coal Bed Methane (CBM) Fines Production and Migration of Mineral Matter in Coal Beds

Abstract

Drilling through coal beds and high initial fluid production rates can lead to shear and compressive failure of coal, resulting in the creation of coal fines. In overbalanced drilling, these fines mix with the drilling fluid and are carried into the near-wellbore zone. During initial water production, the fines are transported to the producing wellbore through the coal beds. Both transport processes result in blocked cleats and pores in the near-wellbore, high-pressure drop zone, reducing permeability and lowering production rates of gas and water. Excessive fines production will also lead to excessive wear and plugging of equipment. The objective of this study was to characterize a set of coal fines from a coal bed methane (CBM) field, with the aim of understanding their origin and character toward their effective management in the field. This study focused on fines originating from horizontal and vertical wells drilled in the Moranbah CBM field in Queensland, Australia, where the horizontal drilling process was suspected of creating excessive fines. We divided this study into two parts; in the first, we prepared fines from four different coal cores sourced from a nearby vertical well and characterized them. In the second, field fines collected over time from vertical wells connected to horizontal wells were characterized, with a novel technique called comparative quantitative X-ray diffraction (CQ-XRD) to establish organic-to-mineral ratios. We also report on key differences in CQ-XRD-derived mineralogy between field fines and core-derived lab fines; changes to particle size and helium-derived density over time; petrography and proximate ash yield of the four cores; and comparisons in water properties of field formation water with lab leachate water.