Using horizontal wells to sequester CO ₂ and enhance coalbed methane recovery: A simulation study of operating procedures

A pilot project is being implemented in northwest West Virginia to study the technical feasibility of CO ₂ sequestration in unmineable coal seams. The project consists of four 3000 ft horizontal wellbores forming the exterior of a square with four horizontal wellbores at its center. Methane production occurs from all wellbores until the reservoir is sufficiently depleted, then the central wellbores are converted to CO ₂ injectors while production continues from the exterior wellbores. The methane production from an isolated pattern and the trends observed with regard to injection well length and pressure are not indicative of the performance to be expected when the pattern is repeated as part of a developed field. The amount of methane produced by a repeated pattern may be only a small fraction of that produced by an isolated pattern, especially if the isolated pattern were designed and operated so as to have a long project life. As a real sweep increases the region occupied by the CO ₂ increases and the region occupied by methane decreases. In the case of high permeability anisotropy, the plus-pattern with unequal length injectors provides good sweep. Long diffusion time constants can combine with high injection rates to produce poor displacement efficiency in the region behind the CO ₂ front. This can shift the optimal design from that for a reservoir that is otherwise similar, except with a short diffusion time constant. This is an abstract of a paper presented at the SPE Annual Technical Conference and Exhibition (Houston, TX 9/26-29/2004).