Summary
Steam-assisted gravity drainage (SAGD) (Butler 1991) has become the
preferred in-situ recovery concept for Athabasca bitumen. However, to maintain
steam-trap control, rates are limited by the close vertical spacing between the
parallel horizontal wells. In Cross SAGD (XSAGD), the injection wells are
perpendicular to the producing wells. Sections of the wells near the crossing
points are either restricted from the start or later plugged to achieve a
significant rate and thermal efficiency advantage over SAGD. This
numerical-simulation study shows XSAGD especially advantageous in cases in
which low pressure is required.
Introduction
SAGD is very effective in mobilizing bitumen and achieving high recovery
from thick, high-permeability reservoirs. The key to achieving a high initial
production rate is to effectively heat the full length of the region between
the parallel, horizontal, injection and production wells and then maintain this
region at high temperature throughout the operation. This minimizes bitumen
viscosity, allowing the maximum rate of gravity drainage and production.
Typically the injector is placed approximately 5 m above the producer. This
close spacing of the wells has a distinct advantage during the early portion of
the process of establishing the steam chamber. However, this close spacing
poses a challenge to avoid short-circuiting of the steam from the injector
directly into the producer later on. This challenge can result from hot
channels caused by uneven spacing between the wells or pressure gradients along
the completions, or because of heterogeneity. Even in the ideal case, excessive
drawdown can draw live steam into the producer, risking sand-control failure as
well as inefficient heat management. Some degree of steam-trap control must be
used to minimize such problems, but this itself limits SAGD rates.
Once a steam chamber has been established, it would be beneficial to move
the injection and production wells farther apart, possibly both vertically and
laterally, to improve steam-trap control at higher production rates. XSAGD
essentially is an attempt to move the points of injection and production
farther apart at a strategic time to improve performance. The concept is to
drill the injection wells above the production wells with spacing similar to
that used in SAGD, but unlike SAGD, the injectors are placed perpendicular to
the producers. Portions of the wells near the crossing points are plugged after
a period of steam injection, or the completion design may restrict flow near
these crossing points from the start. The plugging operation or restricted
completion design effectively blocks or throttles the short circuit between
wells at the crossing points, with the effect of moving the points of injection
and production apart laterally. The increased lateral distance between the
injecting and producing segments of the wells improves the steam-trap control
because steam vapor tends to override the denser liquid phase as injected
fluids move laterally away from the injector. This allows rates to be increased
while avoiding live steam production. Of course, XSAGD is not conceived to be
used for a single well pair, whereas SAGD can be implemented as a standalone
well pair. XSAGD is better suited for several adjacent producers with several
perpendicular injectors to achieve a more or less rectangular (half-pad)
development with a “checkerboard” grid formed by the crossing of the wells.
There are at least two penalties with this XSAGD concept. First, only the
points near where the wells cross are effective in establishing the initial
steam chamber rather than the entire length of the wells. This restricts the
initial production and injection rates at the very time that present-value
economics strongly favor high rates. This leaves XSAGD behind SAGD at the
start. Second, the plugging operation requires additional cost and poses a
serious practical challenge to operations, namely how to selectively plug hot
wells operating within a steam chamber. Completions that are restricted at the
crossing points from the beginning may avoid the risks and costs of later
plugging, but such completions will allow limited short-circuiting of the
injected steam throughout the life of the process with some impact on thermal
efficiency.
© 2007. Society of Petroleum Engineers
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History
- Original manuscript received:
31 January 2006
- Meeting paper published:
1 November 2005
- Revised manuscript received:
9 August 2006
- Manuscript approved:
10 October 2006
- Version of record:
20 February 2007
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