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

Rate Transient Analysis in Shale Gas Reservoirs with Transient Linear Behavior
Introduction
Many hydraulically fractured shale gas horizontal
wells in the Barnett shale have been observed to
exhibit transient linear behavior, characterized by
a one-half slope on a log-log plot of rate against
time. This transient linear flow regime is believed to
be caused by transient drainage of low permeability
matrix blocks into adjoining fractures, and is the only
flow regime available for analysis in many wells.
Objectives
A hydraulically fractured horizontal shale gas
well will be modeled as a horizontal well draining
a rectangular geometry containing a network of
fractures separated by matrix blocks (dual-porosity
system). The solutions presented by El-Banbi for
a linear dual porosity model will be extended and
applied to this system. The effects of desorption
and diffusion will be assumed negligible in this
paper since they will not be important at reservoir
pressures of interest in the Barnett shale.
The objectives of this research are:
» To develop mathematical models to analyze these
multi-stage hydraulically fractured horizontal wells
» To develop a rate transient analysis procedure for
analyzing these wells to enable the determination
of reservoir characteristics, drainage volume/
original gas-in-place (OGIP), fracture network
characteristics and assessment of the effectiveness
of different hydraulic fracture treatments.
Accomplishments
The hydraulically fractured shale gas reservoir system
was described by a linear dual porosity model which
consisted of a bounded rectangular reservoir with
slab matrix blocks draining into adjoining fractures
and subsequently to a horizontal well in the center.
The well fully penetrates the rectangular reservoir.
Convergence skin is incorporated into the linear
model to account for the presence of the horizontal
wellbore.
Five flow regions were identified with this model.
Region 1 is due to transient flow only in the
fractures. Region 2 is bilinear flow and occurs when
the matrix drainage begins simultaneously with
the transient flow in the fractures. Region 3 is the
response for a homogeneous reservoir. Region 4
is dominated by transient matrix drainage and is
Crisman Annual Report 2009
the transient flow regime of interest. Region 5 is
the boundary dominated transient response. New
working equations were developed and presented
for analysis of Regions 1 to 4. No equation was
presented for Region 5 as it requires a combination
of material balance and productivity index equations
beyond the scope of this work.
It is concluded that the transient linear region
observed in field data occurs in Region 4, drainage of
the matrix. A procedure was presented for analysis.
The only parameter that can be determined with
available data is the matrix drainage area, Acm.
It was demonstrated that the effect of skin under
constant rate and constant bottomhole pressure
conditions is not similar for a linear reservoir, as
the constant bottomhole pressure shows a gradual
diminishing effect of skin. A new analytical equation
was presented to describe this situation
It was also demonstrated that different shape
factor formulations (Warren and Root, Zimmerman
and Kazemi) result in similar Region 4 transient
linear response provided that the appropriate f(s)
modifications consistent with lAc calculations are
conducted. It was also demonstrated that different
matrix geometry exhibit the same Region 4 transient
linear response when the area-volume ratios are
similar.
Project Information
1.2.8 Modeling and Analysis of Linear Transient Flow
Regime in Shale Gas Reservoirs
Related Publications
El-Banbi, A.H.: 1998, Analysis of Tight Gas Wells. PHD
dissertation, Texas A&M
U., College Station, Texas.
Bello, R.O.: 2009, Rate Transient Analysis in Shale Gas
Reservoirs with Transient Linear Behavior. PHD dissertation,
Texas A&M U., College Station, Texas.
Contacts
Bob Wattenbarger
979.845.0173
bob.wattenbarger@pe.tamu.edu
Rasheed Bello
CRISMAN INSTITUTE
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