Introduction
A coalbed methane reservoir has very distinct characteristics in terms of the dewatering process. A dewatering stage needs to be done to reduce reservoir pressure. As the reservoir pressure decreases at a, gas desorbs from the coal matrix. However, in a dry coal reservoir system (such as the dry coals of the Canadian Horseshoe Canyon), the dewatering phase is not necessary. The uncertainty of a dewatering phase plays an important role in the feasibility of a coalbed methane reservoir development plan.
Well Inference
The effect of well interference on coal bed methane reservoir is somewhat different from that of conventional gas reservoir. As we can see from the simulation results, the effect of additional well creates a positive effect on gas production rate.
Gas production rate for an individual well from 40 acre spacing system is higher than that of 80 acre system in the early stage of production. Decreasing well spacing (interference effect) tends to accelerate the production of gas from the reservoir.
Closer spacing (interference effect) accelerates the depressuring of the reservoir. It can be explained in such a way that the additional pressure reduction caused by the second well increases the desorption rate, thus providing the system with more gas. In other words, the contacts between pressure transient between wells in a drainage system will accelerate the pressure drawdown and consequently will increase the desorption.
During the early stage of production, the relative permeability to gas increases as water is being produced from the reservoir. Therefore, gas rate increase during the initial stage of production. Instead of continuously increase, gas production at a certain time starts to decline. At this point of time, gas production rate will behave similarly with conventional negative interference. Gas production start to declines when the rate of desorption in the matrix system is less than the production rate in the fracture to the wellbore. The rate of desorption starts to decline at a certain time because the supply of gas from the matrix system already declined.
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CSG Well Spacing & Completion Examples
References:
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Zulkarnain, Ismail, Simulation study of the effect of well spacing, effect of permeability anisotropy, and effect of Palmer and Mansoori model on coalbed methane production, 2006, Texas A&M University