Abstract

Relative permeability is a primary parameter for determining the reservoir behaviour of coal seams. We present numerous relative permeability curves measured in the laboratory as well as curves from numerically matching field performance. Viscous fingering provides an explanation for the high values of residual water saturation.

Introduction

Relative permeability is a primary parameter in determining the response of coal-seam reservoirs. Despite the importance of relative permeability, measurements are infrequent and very few recorded examples appear in the literature. The main reason for this is that the properties of coal make the measurements difficult.

In the published literature, discrepancies exist between relative permeability curves from the laboratory and the field. High values of residual water saturation, often in excess of 80%, are observed.

In this paper we present numerous relative permeability curves measured in the laboratory as well as curves from numerically matching field performance. Some of the discrepancies in the results are postulated to be a result of the unfavourable mobility ratios that exist when gas displaces water. This leads to viscous fingering. Evidence to support this conclusion is provided in X-ray CT images of core displacements.

Viscous fingering behaviour is very dependent on heterogeneity. The extremely heterogeneous nature of coal is interpreted to be primarily responsible for the variability in relative permeability curves. We provide examples of how this is manifested in practice. Viscous fingering provides an explanation for the high values of residual water saturation.

Conclusions from this paper are important for scaling results from laboratory tests to field applications. Applications occur in coalbed methane and gas drainage from coal seams for mine safety. This work is also relevant to multiphase flow in other types of fractured rock.

Reservoir Properties of Coal

Coal is generally treated as a dual porosity rock containing micropores and a network of natural fractures. The fractures are known from mining as cleats. Some basic reservoir engineering aspects of coal have been described by Gray.

Coal is arguably the most difficult reservoir rock to work with for the reasons listed below.

  1. Coal tends to be friable and very heterogeneous, so sample selection is a major issue. Procedures for sample selection have been recommended by Hyman et al.

  2. The connected fracture network of coal has low porosity. Laboratory testing therefore requires accurate measurements of very small amounts of fluid.

  3. Very pronounced stress-dependent permeability is observed in coal. Furthermore, as the stress load is cycled, permeability tends not to return to the original value. The reason for this has not been clearly identified, but may include fines migration.

  4. Many gases strongly adsorb on coal, including methane, carbon dioxide and nitrogen.

Previous Measurements

Recorded measurements of relative permeability in coal are sparse. It has only been in the last few years that direct measurements of water relative permeability during gas injection have been reported. We reproduce below some of the previously published curves. To assist visual comparison, they have been replotted with the same aspect ratio and with water saturation on the abscissa axis.

In the early 1970s Reznik et al. reported on measurements of relative permeabilities under steady-state conditions, with water saturation both increasing and decreasing. With decreasing water saturation they were unable to measure water relative permeabilities, and instead computed values from the corresponding gas permeabilities by Corey's method. They acknowledged that the applicability of this approach to coal is tenuous. Relative permeability curves for decreasing water saturation are shown in Fig. 1a. The relative permeability to water is shown as a broken line because it was not directly measured.

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