Summary
The success of steam-assisted gravity drainage (SAGD) has been demonstrated
by both field and laboratory studies mostly on the basis of homogeneous
reservoirs and reservoir models. A comprehensive understanding of the effects
of reservoir heterogeneities on SAGD performance is required, however, for
wider and more-successful implementation. This work presents a numerical
investigation of the effects of reservoir heterogeneities on SAGD using a
stochastic model of shale distribution. Two flow regions, the near-well region
(NWR) and the above-well region (AWR), are identified to decouple the complex
effects of reservoir heterogeneities on the SAGD process. Numerical simulations
are conducted with various realizations of shale distribution to compare SAGD
performance in terms of the effects of NWR and AWR. Hydraulic fracturing is
proposed to enhance steam-chamber development for reservoirs with poor vertical
communication, and the feasibility of hydraulic fracturing is discussed in
terms of in-situ stress and well orientation. Fracturing both injectors and
producers is found to improve steam distribution, oil production rate, and the
oil-steam ratio.
Introduction
Vast quantities of heavy- and extraheavy-oil (bitumen) resources have been
discovered worldwide. For example, an estimated original heavy oil in place of
more than 1.8 trillion bbl is present in Venezuela, 1.7 trillion bbl in
Alberta, Canada, and 20 to 25 billion bbl on the North Slope of Alaska (Burton
et al. 2005). Because of the significant viscosity of these crudes at reservoir
temperature, the technical and economic recovery of these resources presents a
significant challenge. With recent advances in horizontal-well technology,
steam-based in-situ recovery methods, aimed at thermal viscosity reduction,
have emerged for the exploitation of these resources (Butler 2001). Of all the
thermal methods, SAGD appears to be quite promising, especially for
bitumen.
In a typical SAGD process, two horizontal wells are placed close to the
bottom of a formation, with one well a short vertical distance (4–10 m) away
from the other. Steam is injected continuously into the upper well and rises in
the formation, forming a steam chamber. Cold oil surrounding the steam chamber
is heated, becomes mobile as its temperature increases, and flows together with
condensate along the chamber boundaries toward the lower well that functions as
a producer (Butler 1998). The SAGD technique enjoys many advantages over other
thermal methods, especially the conventional steamflooding methods. SAGD
overcomes the shortcomings of steam override by using only gravity as the
driving mechanism, which leads to a stable displacement and a potentially high
oil recovery. Moreover, the heated oil remains hot and movable as it flows
toward the production well, whereas, in conventional steamflooding, the oil
displaced from the steam chamber cools, and consequently the oil-phase
viscosity increases, as the oil flows to the production well.
In order to design an effective SAGD process, an understanding of the
complex physics of SAGD and reliable predictions of its performance are
essential. A vast literature on the SAGD concept has developed since it was
first introduced by Butler and his colleagues in the late 1970s (Butler and
Stephens 1981; Butler et al. 1981).
Butler developed a gravity-drainage theory on the basis of several
assumptions and derived a semianalytical numerical solution to predict the
oil-drainage rate. He and his coworkers also reported experimental data
obtained with a scaled visual model. Reis (1992) proposed modifications to
Butler's gravity-drainage model by using an empirical dimensionless-temperature
coefficient and the maximum velocity, and Akin (2005) also modified the model
by incorporating asphaltene-content-dependent viscosity to match the
experimental data in the literature better. Nasr et al. (2000) studied
steam-liquid countercurrent and cocurrent flows for different permeabilities
and initial gas saturations with a nonsteady-state, laboratory steam-front
dynamic-tracking technique.
Numerical simulation has been used widely to investigate the physical
process and practical operation of SAGD. For example, Edmunds (1998) analyzed
SAGD steam-trap control with 2D and 3D simulation models. He found that
establishing a liquid-saturated leg above the producer was feasible by
controlling the temperature of the produced fluid. The producer is shut in when
the temperature of the produced fluid approaches the temperature of the
injected fluid.
These analytical and numerical studies, however, were generally performed
for homogeneous, isotropic reservoirs. In reality, no reservoir is homogeneous
because of natural geological features, such as shale, faults, and fractures.
One example is the oil-sand deposit in Peace River, Alberta, Canada. It
contains a good deal of marine shale and mudstone that form continuous and
discontinuous shale barriers throughout the formation (Webb et al. 2005). The
heterogeneity introduced by the shale barriers and other geological features
plays an important role in the propagation of steam (Richardson et al. 1978).
Therefore, without understanding the effects of reservoir heterogeneities, SAGD
results for homogeneous systems cannot be applied directly to provide accurate,
reliable predictions for field-type systems.
During the past decades, several researchers have investigated the role of
reservoir heterogeneities on steam-chamber development for a SAGD process.
Joshi and Threlkeld (1985) studied reservoirs with shale barriers and
experimentally compared the effects of various well-configuration schemes and
vertical fractures. Yang and Butler (1992) conducted SAGD experiments with
reservoirs of two different types: reservoirs with thin shale layers and
reservoirs with horizontal layers of different permeabilities. These studies
were subject to experimental limitations. Reservoir heterogeneities were
simulated by including a limited number of impermeable barriers at designated
locations. Given the complex geological nature of shale, it would be better
instead to use a stochastic model based on geostatistical methods
(Pooladi-Darvish and Mattar 2002) to represent the shale distribution.
This study investigates the effects of reservoir heterogeneity on SAGD from
a simulation perspective. We use a stochastic model of the shale and sand
distribution and a fully featured thermal reservoir simulator. To interpret the
complex effects of reservoir heterogeneity on the SAGD process, two flow
regions are identified according to the characteristics of flows associated
within the steam chamber. Numerical simulations are conducted with a number of
equal-probability realizations in 2D and 3D to compare SAGD performance. The
intent of these simulations is to instruct regarding the thermal
gravity-drainage-process physics rather than to match the oil production of a
particular reservoir. For reservoirs with poor vertical communication,
hydraulic fracturing is proposed to enhance steam-chamber development, and the
feasibility of hydraulic fracturing is discussed in terms of in-situ stresses
and well orientations.
© 2008. Society of Petroleum Engineers
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History
- Original manuscript received:
1 August 2007
- Meeting paper published:
11 November 2007
- Revised manuscript received:
25 February 2008
- Manuscript approved:
3 March 2008
- Version of record:
25 October 2008
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