Porosity Measurements

Measurement of coal porosity makes use of coal cores or fresh samples derived from coal mines. Measurements require the preparation of samples suitable for the technique to be use. Part of the coal sample preparation in early times requires the drying of the samples for 24hours in the oven at 80oC (Dabbous et al., 1974). This process relieves the sample of all it moisture content. Later works indicated that the process of drying alters the coals structure and should not be used during measurements (Berkowitz, 1979). Due to coals dual porosity nature, most measurement techniques used are modelled from conventional methods but adapted to accommodate coals highly anisotropic nature and to measure different and specific porosity (micro or macro porosity).

The matrix porosity is usually higher than cleat porosity but most of the gas stored in coal is in the sorbed form in the matrix rather than as free form in the cleat. Therefore matrix porosity measurements are rarely carried out for coal sample (API, 1998). Should the matrix porosity be required, the Helium Boyle’s Law technique can be used. In this process only helium can be used because the gas is not adsorbed by coal. In case any of the gas is adsorbed, erroneous porosity values may be derived.

The Miscible Drive technique, commonly used for conventional reservoirs is routinely used to measure cleat porosity. Another technique used to measure cleat porosity is the mobile water porosity technique determined during measurement of unsteady state relative permeability (Gash, 1991).

3.4.1.1.1. Miscible Drive Technique

The miscible Drive Technique utilises non dried coal cores. In this technique the fluid saturating the core is displaced with a different fluid with different physical properties. Measurements are usually done under uniform confining pressure since cleat porosity is a function of confining pressure (API, 1989). Both the saturating fluid and the displacement fluid should be miscible (Meyer, 1982). If the tracer element such as sodium iodide is used, the technique is called “the miscible tracer technique”. Supposed the tracer element is adsorbed onto the coal, erroneously high cleat porosity measurements may be recorded (Gash, 1991; Dabbous et al., 1974)

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The coal cores are initially saturated with water. Before saturation the core with water Helium saturated with water vapour is injected into the core to remove mobile water in the core. The core is then evacuated to remove any free gas and the core is saturated at atmospheric pressure. Water is then injected at constant pressure with a constant back pressure from the bottom of the vertical hydrostatic core holder. A tracer element is then injected into the core until the concentration of the tracer in the effluent is same as that in the injector. Effluent volumes are monitored using electronic balance while tracer element concentration is monitored using electrical conductance and refractive index.

The tracer element is continually injected for long hours. The solution in the core is then displaced with distilled water in the same manner and the while monitoring the tracer element concentration and the effluent volume. The tracer element concentration and effluent volume is then used to calculate total displaced volume from which the system dead volume (core holder inlet and exit lines) are subtracted to calculate the cleat pore volume (Gash, 1989). The cleat pore volume is divided by the measured bulk volume of the core to calculate the cleat porosity.

This technique infers direct measurement of the reservoir rock. However, the technique is time consuming and requires pre-drilling, coring, core handling and core preservation. All these processes if not properly done will lead to erroneous results.

3.4.1.1.2. Multi Scale Imaging

This technique is new and mainly used by Digital core labs. Analysis involves characterising the heterogeneity and connectivity of pores and fractures, and distribution of minerals at the millimetre to micron scale as well as imaging and analysing the pore connectivity and mineral occurrences at the nano scale (Digital Core, 2012). With this analysis both micro and macro porosity of coal can be inferred. The cores are not simulated in this case.

This approach integrates 3D micro –CT (Computer Tomography) imaging with Scan Electron Microscopy (SEM) to characterised pores. Connected porosity regions are highlighted by supplementing imaging with functional fluids (figure 3.7 a and b). The porosity, mineral distribution and fracture network can be characterised and quantify at different scale from micro down to 2µm (Digital Core, 2012).

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Figure 3.7a: Quantification of connected porosity in 3D, (left)Bituminous coal, (right) Connected porosity map(darker grey = higher porosity) (Digital Core, 2012)

Figure 3.7b: Investigation of open and occluded porosity 3D, (top left) μCT image,(top right) SEM image (Digital Core, 2012).

One great advantage of this technique is that it is a non-destructive technique i.e. rock samples are not altered during measure consequently no erroneous result may arise as a result of sample destruction. The techniques also characterise the rock at all scales. However this technique is very expensive.

3.4.1.1.3. Other porosity Measurement Technique

Other methods used to evaluate coal porosity include; gas adsorption Techniques, Mercury intrusion porosimetry (MIP), imaging (Scan electron microscopy and Transmission election microscopy) and Nuclear magnetic Resonance (NMR). Procedures like MIP require injection

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of mercury at high pressures which may change the porous structure of the sample (suuberg et al., 1995). Other high pressure effects include shielding of larger pores by smaller pores thereby reducing the amount of larger pores (Gane et al., 2004). Most of the microscopic imaging techniques only provide local pictures of the samples (Sing, 2004) while gas adsorption techniques also destroy the sample and only provides information on the micro- and meso-pores (Yao et al., 2006). NMR is also a non-destructive technique which is less time consuming but makes use of measurements with portable equipment.