Summary
The retention of fracturing fluid in a proppant pack reduces the
dimensionless fracture conductivity, Fcd, resulting in poor well productivity
regardless of fluid type (gelled oil, crosslinked polymers, viscoelastic
surfactants, or foams). Analytical expressions derived in this paper can
be used to calculate the extent of fracture cleanup under a set of production
conditions. Several dimensionless parameters describing the fluid, fracture,
and reservoir properties are introduced that affect the equilibrium cleanup. A
second, transient, fracture-cleanup model is also proposed and is used to
estimate the dimensionless-parameter values.
The equilibrium model predicts that cleanup increases greatly with
decreasing dimensionless yield stress of the fracturing fluid. The
magnitude of cleanup is also greater for cases in which the permeability ratio
of the clean portion of the fracture to the fouled portion is high. The cleanup
is expected to increase with an increase in the ratio of reservoir and fracture
mobility until an optimum is reached and then decrease with increasing the
mobility ratio. The critical process of parameter estimation is achieved
by making experimental measurements and by history matching published fracture
flowback data using the transient model.
Introduction
Injecting viscous proppant laden fluids into a hydrocarbon reservoir creates
hydraulic fractures. The fracture is held open by the proppant and
provides a low resistance pathway for the production of oil and gas into the
wellbore. Much of the fracturing fluid resides in and around the fracture
at the end of the treatment,1–4 and this residue impedes hydrocarbon
flow. Fracture cleanup is the process of transporting the residual fluid
(most commonly polymer-based) from the fracture back to the earth’s
surface. Increasing the removal of the residual fluid from the fracture
results in a higher proppant-pack permeability, which increases the hydrocarbon
productivity.
Fracture Impairment and Cleanup
Fracture-face skin refers to reservoir permeability impairment along the
fracture surface. This impairment is caused by saturation changes, clay
swelling and migration, wettability changes, relative permeability hysteresis,
and capillary pressure changes, as well as pore-throat blocking caused by
internal polymer filtercake in high-permeability formations.1,5,6 It has
been shown that fracture-face skin has little impact on well productivity in
low-permeability reservoirs unless the fracture-face permeability is reduced by
two to three orders of magnitude.6 Cinco-Ley and Samaniego7 provide a
general relationship for estimating the impact of fracture-face skin on well
productivity.
A choked fracture occurs when something impairs the proppant-pack
conductivity. Proppant-pack conductivity can become impaired by proppant
crushing, immobile fracturing fluids and fracturing fluid residue (such as
dehydrated polymer), and formation fines that infiltrate and plug the proppant
porosity. Fracture conductivity is reduced by non-Darcy (inertial) effects
due to high flow velocities.8,9 It is also reduced by saturation changes
and relative permeability hysteresis in the proppant caused by the production
of two or three phases simultaneously.10,11
Previous work has illustrated the impact of fracturing-fluid viscosity,2
inertial forces,12 viscous fingering,13 flowback practices,14–16 gel
residue,3,17 and breakers4,18,19 on fracture conductivity and fracture
cleanup.
In a departure from previous approaches to the fracture-cleanup problem,
this work focuses on the development of an analytical model that enables
identification of key parameter groups influencing fracture-fluid
cleanup. The purpose is to clarify the relationship between variables
influencing fracture-fluid cleanup even though this model will not simulate the
entire fracture-cleanup process as accurately as complex reservoir simulators
that account for more of the aforementioned damage mechanisms.
© 2005. Society of Petroleum Engineers
View full textPDF
(
1,597 KB
)
History
- Original manuscript received:
19 January 2003
- Revised manuscript received:
2 November 2004
- Manuscript approved:
20 January 2005
- Version of record:
15 March 2005
View Cart
