There are three distinguishing phases in the production life of a wet CBM well: a period of inclining gas rate due to dominance of relative permeability effects, a period of rate stabilization during which the gas rate peaks followed by the depletion phase during which gas production rate declines.
The declining gas production rate can be attributed to many factors such as:
Declining reservoir pressure
Loss of productivity due to compaction and permeability loss
Loss of productivity due to fine migration that creates a choke skin near wellbore
While the decline in reservoir pressure is inevitable, the loss of productivity due to the choke skin and permeability loss for a declining CBM well can be remedied or slowed down through a multi-stage hydraulic refracturing. The impact of such fracturing on improving well productivity and boosting the gas production rate is investigated.
Reworking the well through refracturing could help clean up the zone near wellbore, re-activate or improve the coal network of natural fractures; lead to more effective dewatering and even create new contacts between the reservoir and formation. The area near wellbore will benefit from re-invigoration of fracture network and creation of an area of enhanced permeability.
In this study the benefit of completing a new vertical and/or horizontal CBM well with a multi-stage hydraulic fracture is examined. The production of wells with such completions are compared to typical single-stage fracturing and benefits of creating an inner zone of enhanced permeability in short-term and long term are investigated.
A numerical dual-porosity reservoir model is used to generate the CBM gas production forecasts.