Article pubs.acs.org/EF
An Experimental Study on Characterizing Coal Bed Methane (CBM) Fines Production and Migration of Mineral Matter in Coal Beds Paul Massarotto,* R. S. Iyer, Muthia Elma, and Timothy Nicholson School of Chemical Engineering The University of Queensland, Brisbane, Queensland 4072, Australia 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 nearwellbore 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.
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phosphates, chlorides, vanadates, and tungstates.5 The ash yield from proximate analysis of coal samples does not capture all the mineral matter such as salts and sulphides, which are volatized during that test. Qualitative and quantitative X-ray diffraction (XRD) is a commonly used technique for identifying and characterizing the mineral matter by low temperature plasma ashing of coal. The application of XRD and quantitative XRD techniques to coal is extensively described in the literature.6−9 The collection of field fines in sufficient quantity for both low temperature ashing and quantitative XRD was not possible. Therefore in this work, we have introduced a novel “comparative quantitative XRD” technique (CQ-XRD) to measure mineral content on a relative basis, leading to estimated quantitative migration of minerals from the formation coal matrix when in contact with water. Whole rock XRD analyses of coal samples under investigation were compared to each other, with the assumption that the error is consistent between the different analyses, for identical analytical conditions; useful conclusions can be drawn from the differences between XRD patterns. The research program ultimate aim was to use the University of Queensland designed true triaxial stress coal permeameter (TTSCP)10 to investigate the permeability of coal samples using typical water and fines concentrations and particle sizes. However, the quantity of fines required for such a series of experiments exceeds the amount that can be easily recovered from production sites. Therefore, in this work we also tested
INTRODUCTION Drilling of coal beds and high fluid production rates result in fragmentation of coal, generating coal fines.1 These fines are a mixture of released mineral matter, organic matter, and organic matter with bound mineral matter. In horizontal wells, mineral matter is also released when the drill bit traverses stone partings or wanders or branches into the roof or floor. Shear failure also occurs when the coal bed methane (CBM) well is put on production by applying a drawdown resulting in the lowering of bottom hole flowing pressure.2 This results in an increase in net stress; the associated shear failure occurs in a zone extending away from the fractured face, leading to microseismic events, dilatancy, initial permeability increase, fines creation, and fines movement resulting in eventual loss of permeability.2 These field fines have an average particle size of
