Application of nuclear magnetic resonance measurements in the evaluation of two coal seam gas wells in the Pedirka Basin

Abstract

In 2008, Central Petroleum was involved in an extended exploration campaign in the Pedirka Basin. The main targets were conventional oil and coal seam gas (CSG). A comprehensive logging program including nuclear magnetic resonance (NMR) measurements was acquired, with the scope of evaluating both targets in two wells. NMR tools measure the magnetisation of hydrogen protons present in the flushed-zone of the formation pore space. By calibrating this measurement in a water tank, NMR tools provide formation porosity independent of lithology, while classical methods for deriving porosity (density, neutron, etc.) are lithology dependent. While in conventional plays (clastics, carbonates) porosity is needed for evaluating the reservoir storage capacity, in coal beds porosity is needed for evaluating the surface areas of the pores. As methane in coal is bound to the coal surface, total pore surface affects the coal bed methane producing capacity. NMR measurements also provide information about porosity/grain size distribution, permeability and hydrocarbon saturation in conventional formations. This information can be very useful for evaluating coal seam gas, provided the conventional models can be converted and applied in coal beds. Evaluation of coal seam gas prospects using nuclear magnetic resonance is an industry first. This presentation highlights the benefits and difficulties of nuclear magnetic resonance evaluation of CSG prospects in these two wells.