In 2008, Central Petroleum was involved in a two-well drilling campaign in the Pedirka Basin. The main targets were conventional oil in the Algebuckina Sandstone and Murta Siltstone formations and coal seam gas (CSG) in the Purni formation. A comprehensive logging program which included nuclear magnetic resonance (NMR) was acquired with the scope of evaluating the objectives in both wells. NMR measurements provide information about porosity and pore size distribution, permeability and hydrocarbon saturation in conventional formations. This information can be very useful in evaluating coal seam gas, provided the conventional models can be converted and applied in coal seams. Evaluation of coal seam gas using nuclear magnetic resonance logging is an industry first. This paper highlights the benefits and difficulties of CSG evaluation using nuclear magnetic resonance technology.
Petrophysical properties that need to be assessed in the evaluation of potential coal seam gas productivity are adsorbed gas content and saturation, porosity, permeability and diffusivity of the coal bed. Total gas content and saturation can be measured and projected from a core sample or estimated from empirical correlations. Empirical correlations are based on sorption analysis of coal beds of different depth and rank (Kim 1977, Eddy et al. 1982, Ryan 1992). These empirical correlations show that coal seam gas content increases with increasing depth, pressure and rank, and decreases with increase in temperature. Increasing depth, pressure and temperature result in an increase in overburden pressure, which reduces the average size of the pores and the permeability, hence hindering the extraction of the gas from the coal bed. Research and laboratory work (Gan et al. 1972, Mahajan et al. 1978, Walker et al. 1981, Radlinski et al. 2004, Mares et al. 2009) completed on coal samples has established that rank methane content correlation is a pore size correlation.