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.

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