Crisman Annual Report 2009 - Harold Vance Department of ...

Well Spacing and Infill Drilling in Coalbed Methane Reservoirs
Objectives
Reservoir simulation has been used to describe
the mechanism of gas desorption and diffusion in
coal to reflect the response of the reservoir system
and the relationship among coalbed methane
reservoir properties, operation procedures, and
gas production. The objective of this work is to
investigate well spacing and completion design
practices under various development scenarios by
using reservoir simulation.
In a coal bed methane reservoir there is a natural
fracture system which conducts the fluid flow to the
wellbore and matrix system where essentially all
gas is stored. Instead of gas being compressed in
the pore space, most is adsorbed on the surface of
it. Considering the small pore size in this reservoir
system, the Klinkenberg effect or slippage factor
could effect the permeability change during
reservoir depletion. The amount of gas adsorbed is
quantified by an adsorption curve (isotherm curve)
of the Langmuir equation. As the reservoir pressure
declines during production from the fracture system,
gas desorbs from the coal surfaces. Flow gas from
the coal matrix to the fracture system is a molecular
diffusion, expressed by Fick’s law rather that Darcy’s
law. Because of the adsorption curve’s convex shape,
it becomes very important to attain low reservoir
pressure; this is a much more important factor than
in conventional reservoirs. After long dewatering,
water production will decrease and gas production
increase and peak after water production has
significantly declined from its original rate. Predicting
the time and magnitude of this peak is a large part
of the early evaluation of the wells. Eventually the
wells decline and have a more conventional rate
pattern. In the later stage of depletion (effective
fracture permeability increasing during matrix
desorption at lower pressure), this rock mechanic
can be described by the Palmer-Mansoori effect.
Approach
A reservoir simulator will be developed to determine
the effect of various spacing and completion
decisions on recovery for particular scenarios of
reservoir properties/description. The outcome of
the simulation and history matching will typify the
reservoir of interest and will be used to develop
further analysis, such as:
» Determine where the Palmer-Mansoori permeability
and the Klinkenberg effect are important in
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reservoir mechanics
» Demonstrate the importance of various parameters
on spacing
» Determine desirability and expected performance
of either vertical or horizontal wells
» Develop well spacing correlations to determine
optimum well spacing for new reservoir
development and guideline for several practical
circumstances
Accomplishments
A single well, 2D, single phase reservoir simulator
has been developed using Macros Visual Basic.
Reservoir simulation results for different sorption
pressure cases are presented in Fig. 1. The work is
still being continued to accommodate multiphase,
Klinkenberg effect, and Palmer-Mansoori effect.
Gas Rate (SCFD)
1.E+07
1.E+06
Project Information
1.4.4 Effects of Infill Drilling Coalbed-Methane Reservoirs
Contacts
Bob Wattenbarger
979.845.0173
bob.wattenbarger@pe.tamu.edu
Pahala D. Sinurat
Simulation Result for Constant Pressure Case
1.E+05
0.1 1 10 100 1000
Time (days)
Sorption Pressure = 1103.20 psi
Sorption Pressure = 882.56 psi
Sorption Pressure = 661.92 psi
Fig. 1. Reservoir simulation results for various sorption pressure.
Significance
Residual method can be applied in developing a
reservoir simulator for coalbed methane reservoirs
to provide a rapid screening approach looking at the
prospect of development or purchase or production
improvement.
CRISMAN INSTITUTE
Crisman Annual Report 2009