Some of the technical challenges in coalbed methane (CBM) exploration are to:
Find the balance between good gas capacity (which tends to increase with depth) and good permeability (which tends to decrease with depth)
Predict the distribution of coal, preferably thick coal, which is typically deposited in a complex setting (multiple, lateral sub-environments)
Predict gas saturation for low-rank coals which are immature for thermogenic gas
To address these concerns, a robust exploration program was designed to evaluate the Miocene subbituminous coals in the Warukin Formation, Barito Basin, South Kalimantan for CBM potential. Specifically, the objective was to establish gas capacity and permeability trends versus depth and to collect core to measure gas content, saturation, and composition.
In 2011, twin wells were drilled at three locations in the southern part of the basin. Coal core and canisters were captured for gas desorption, gas composition, and coal properties. Langmuir parameters were obtained for gas capacity, saturation and critical desorption pressure. Both cased-hole injection fall off tests (IFTs) and open-hole IFTs were run.
Multiple attractive coal seams were identified in the three locations, based on gas content, saturation, and coal thickness. However, many of the deeper coals are thinner (less than 10 feet) and have significantly lower gas saturation. Additionally, all IFTs indicated very low permeability, although there is some uncertainty about the accuracy of these results due to less than optimal drilling parameters and borehole conditions.
Based on these results, our near-term focus has shifted to establishing flow via single- and multiple-well dewatering tests in the upper Warukin coal seams. Although the early drillwells with the comprehensive analytical program came at a cost, the value of the three, deep penetrations should not be dismissed given the pre-drill uncertainties. Reacting to early information (operational flexibility) is essential in unconventional exploration programs and must be balanced with the need to keep costs low (limited data acquisition, fixed well locations, pre-planned well designs).