Production history matching to determine reservoir properties of important coal groups in the Upper Pottsville formation, Brookwood and Oak Grove fields, Black Warrior Basin, Alabama

Abstract

The Black Warrior Basin of Alabama is one of the most important coal mining and coalbed methane production areas in the United States. Methane control efforts through degasification that started almost 25 years ago for the sole purpose of ensuring mining safety resulted in more than 5000 coalbed methane wells distributed within various fields throughout the basin. The wells are completed mostly in the Pratt, Mary Lee, and Black Creek coal groups of the Upper Pottsville formation and present a unique opportunity to understand methane reservoir properties of these coals and to improve their degasification performances.The Brookwood and Oak Grove fields in the Black Warrior Basin are probably two of the most important fields in the basin due to current longwall coal mining activities. In this work, methane and water productions of 92 vertical wellbores drilled, some completed 20 years ago, over a current large coal mine district located in these two fields, were analyzed by history matching techniques. The boreholes were completed at the Mary Lee coal group, or at combinations of the Pratt, Mary Lee, and Black Creek groups. History matching models were prepared and performed according to properties of each coal group.Decline curve analyses showed that effective exponential decline rates of the wells were between 2% and 25% per year. Results of production history matching showed, although they varied by coal group, that pressure decreased as much as 80% to nearly 25 psi in some areas and resulted in corresponding decreases in methane content. Water saturation in coals decreased from 100% to between 20 and 80%, improving gas relative permeabilities to as much as 0.8. As a result of primary depletion, permeability of coal seams increased between 10 and 40% compared to their original permeability, which varied between 1 and 10 md depending on depth and coal seam. These results not only can be used for diagnostic and interpretation purposes, but can be used as parameter distributions in probabilistic simulations, as illustrated in the last section of this paper. They can also be used in conjunction with spatial modeling and geological considerations to calculate potential methane emissions in operating mines.

A comprehensive model to history match and predict gas/water production from coal seams

“…Table 5 presents the history matching results, and the formation properties ranges according to Karacan (2013). The estimated parameters are within the range for Upper Pottsville formation.…”

“…The estimated parameters are within the range for Upper Pottsville formation. The estimated initial gas content for these layers is slightly higher than those from Karacan (2013). This is a result of higher gas production (gas peak flowrate = 580 Mscf/day, cumulative production 1.5 Bscf) compared to the maximum flow rate according to Karacan (2013) (300 Mscf/day, cumulative production 1.1 Bscf).…”

“…The ability of coal to adsorb gas is highly variable in the Black Warrior coalbed methane fields, and much of this variability may be related to differences of coal quality (Carroll and Pashin, 2003). Karacan (2013) analyzed gas and water production data from 92 vertical wells in the Brookwood and Oak Grove fields of the Black Worrior basin by history matching techniques. Completion data for these wells shows completion at Mary Lee group, or at combination of Pratt, Mary Lee, and Black Creek groups.…”

“…The well is completed at Pratt, Mary Lee, and Black Creek groups. The tubing head pressure was taken 10 psi (Karacan, 2013). The actual gas and water production rates were used to estimate the bottom-hole pressures for each group, neglecting the friction losses in the wellbore between the layers.…”

Inference of strata separation and gas emission paths in longwall overburden using continuous wavelet transform of well logs and geostatistical simulation

“…Coalbed methane in this field has been produced since the 1980s with vertical wells having multiple completions at the Pratt, Mary Lee, and Black Creek coal groups (Karacan, 2013a). These wells have also been logged extensively using gamma, density, resistivity, and caliper tools for locating completion intervals and identifying strata.…”