Abstract
The modelling results from numerical simulations of the Early Cretaceous, Mannville coal measures with anisotropic permeability provide insights into development strategies not readily visualized or otherwise intuitive. The complex relationships between water and gas production, the contribution from multiple coal seams as well as from organic rich shales, and the impact of well interference combined with anisotropic fracture permeability are investigated through a series of numerical simulations of four well-pads (on the corners of a square mile of land with decreasing well spacing from 1, 3, to 4 laterals per pad). After 25 years of production, the two pads with optimally-oriented laterals with respect to the fracture permeability anisotropy produce 61% of the recovered gas for the 1 lateral/pad model, 52% for the 3 laterals/pad model, and 50% for the 4 laterals/pad model. Downspacing has a greater impact on increasing the gas production from pads with the poorly-oriented main laterals than from the pads with the optimally-oriented main laterals. The cumulative gas production at the end of the 25 year history is 4.2% higher for an optimally-oriented pad (pad1) and 1.1× higher for a poorly-oriented pad (pad3) for a model with 4 laterals/pad than 3 laterals/pad and an optimally-oriented pad is 1.1% higher for an optimally-oriented pad and 1.5× higher for a poorly-oriented pad for a model with 3 laterals/pad than 1 lateral/pad. Although downspacing from 3 to 4 laterals/pad has a greater impact on increasing the cumulative gas production from optimally-oriented pads than downspacing from 1 to 3 laterals/pad, the lower impact on poorly-oriented pads results in a lower total increase the cumulative gas production from the four pads. At the end of the 25-year production history, 9.0% more gas is recovered for the 4 lateral/pad model than the 3 lateral/pad model, which predicts 1.2× more gas than the 1 lateral/pad model. The recovered shale gas exceeds the recovered coal gas after ~7 years of production. The higher contribution of produced coal gas predicted due to downspacing results from a higher contribution of recovered gas from the main coal seam, while the contribution from the minor coal seams is lower. Downspacing has a minimal impact on the cumulative water production; after 25 years of production a difference of 1.0% is predicted between models with 4 and 3 laterals/pad and 1.7% between models with 1 and 3 laterals/pad. While downspacing increases the cumulative water production for the poorly-oriented pads (1.1× for 3 to 4 laterals/pad and 1.3× for 3 to 1 lateral/pad after 25 years), the cumulative water production for the optimally-oriented pads is lower over the majority of the production history (after ~4 years and 3.2% lower after 25 years for 3 to 4 laterals/pad and after ~6 months and 1.1× lower after 25 years for 1 to 3 laterals/pad).
Highlights
Designing and implementing strategies for coalbed methane field development in complex coal measures require evaluation of the interplay between geology, reservoir properties, well design, completions, and economics
Optimising field development of the producing Upper Mannville Group coal measures in the Western Canadian Sedimentary Basin in south-central Alberta is challenging due to permeability anisotropy of the coals and contribution of multiple coals seams and organic rich shales to production
The multi-well pad model with a well spacing of 1 lateral/pad, consists of two 900 m laterals through the main Mannville coal seam optimally-oriented in the direction of maximum coal fracture permeability and two 900 m laterals poorly-oriented in the direction perpendicular to the maximum coal fracture permeability
Summary
Designing and implementing strategies for coalbed methane field development in complex coal measures require evaluation of the interplay between geology, reservoir properties, well design, completions, and economics. Optimising field development of the producing Upper Mannville Group coal measures in the Western Canadian Sedimentary Basin in south-central Alberta is challenging due to permeability anisotropy of the coals and contribution of multiple coals seams and organic rich shales to production. In quantifying reserves and field development in the Mannville producing fairway, reserves and production have generally been considered limited to the main coal seam (1.3 - 4.5 m thick) and in which the horizontal wells are drilled and completed (Figure 1). Through numerical modelling [7] predicted that, for a model with average parameter values, 1.4 times (×) more gas and 3.0× more water is produced when the minor coal seams are included, while including shale beds predicts 1.7× more gas and 2.5× more water after 25 years of production from a 1000 m long, horizontal well than when only the main coal seam is considered. Our study is based on the metrics of the Mannville coal measures, the results are instructive and relevant to other coalbed methane developments where multiple coal seams and gas bearing shales occur
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