Total gas-in-place, gas composition and reservoir properties of coal of the Mannville coal measures, Central Alberta

Abstract

The Lower Cretaceous Mannville coal measures in south central Alberta host one of the most successful horizontal coal methane developments, yet the contribution of thin coal seams and other organic rich strata to total gas-in-place and producible gas remains unaccounted. In this study, well log, core, fluid, and gas analyses in the Mannville coal measures are evaluated in order to quantify and characterise the total gas-in-place resource that may be accessed by a horizontal well completed in the main coal seam which is the usual practise. Regionally, the estimated gas capacity and content of the coals increases from northeast to southwest in parallel with the depth of burial and the level of organic maturation (rank), although local variations exist. The isotopic composition of the methane of coals currently at depths greater than 1500m have a strong thermogenic signature, shallower coals have a mixed biogenic–thermogenic signature, and the shallowest coals have a strong biogenic signature. The trend in gas composition is less well defined with the highest carbon dioxide contents occurring in the area of the lowest and highest ranks. Generally, the percentage of heavier gases (C2–C5) increases with maturity/depth of burial, but some low rank coals (Ro%≈0.30) in eastern Alberta and Saskatchewan contain significant C2–C5 hydrocarbons. The origin of the heavier gases in the low rank coals is unclear; migration from a deeper source is the most likely explanation.The gas adsorption capacity of the coals varies regionally with depth of burial (pressure), coal rank, and ash content. The highest adsorption capacity at reservoir pressure and temperature, approaches 400111cm3/g (SI unit)≈34scf/t (oil field units).scf/t, but most coals have values between 260 and 320scf/t. The gas content of the coals, as measured by desorption, ranges from 230 to 350scf/t and averages 310scf/t. Most of the coals are saturated with gas within the accuracy of the analyses. Notable exceptions occur adjacent to the Saskatchewan border where the lower rank coals may be markedly under saturated. The amount of methane in solution (calculated) at a reservoir pressure of ≈1000psig (6.9MPa) is calculated to be between 7 and 10 scf/t (≈3% of total gas).Currently, Mannville coal gas production is limited to an area in central Alberta of about 2200km2 (850miles2). Outside the producing fairway, sustained commercial production has not been achieved due to low permeability. In the producing and prospective fairway, the net thickness of the coal within ±20m of the main coal seam, varies from 0 to 10.8m and averages 5.1m. Here the thickest coal seam ranges up to 4.7m thick and averages 1.9m.Due to limited gas content data from core for all seams and wells, a protocol was developed to extrapolate existing core data to non-cored seams and wells through petrophysical logs. The protocol takes into consideration the correlation between gas and ash content and the maturity of the coals. The total coal gas resource density in the current area of production and prospective areas determined by applying this protocol for the average well has a low estimate of 5.4BCF/mi2(221BCF/mi2 (=billions of cubic feet/miles2) ~1.1E7m3/km2) (m3/km2), a median estimate of 5.9BCF/mi2, and a high estimate of 6.1BCF/mi2.