Abstract

The Sabinas sub-basin in northern Mexico contains gassy coals in the Upper Cretaceous Los Olmos Formation, based on both historical evidence and current desorption testing. The "Double Seam" coal is present at shallow depth (<500 m), has high vitrinite content (>86 vol%), is well-cleated, shows high diffusivity [average tau (τ) value is 56 hours] and has high natural fracture permeability (>30 mD) in the minesites. The coal averages 2.2 m in thickness but has a high ash content (32 wt%). A tonstein band is present in the middle of the Double Seam, consisting of vitrinite and inertinite embedded in a matrix of fine clays and quartz. Average desorbed gas content of this medium-volatile bituminous coal (Ro_max = 1.30%) is highest in Mine V (Esmeralda Mine at >9.0 cm³/g). Maximum methane adsorption at an equivalent depth of 300 m is 15 cm³/g (as-received basis; arb). Coal bed methane is mainly methane (98%) with heating value of 38.21 MJ/m³ (1026 Btu/ft³). The coal is under-pressured and reported to be dry, with possibly free gas in the cleat/fracture system and absence of discrete mineralization. In-seam horizontal drilling prior to longwall mining has resulted in the significant reduction of in-situ gas contents and in an increase of mined coal production per shift. The Sabinas sub-basin coals are suitable for a full-scale coal bed methane (CBM) development using in-seam single horizontal and multi-lateral horizontal drilling. Similarities, but also differences, exist between the Sabinas coals in Mexico and the same coals in the Maverick Basin, Texas.