They have curious names like X-DRAIN, Trenching and Fishbone, but these alternative well configurations may be the key to improving the effectiveness of Steam Assisted Gravity Drainage (SAGD) in situ operations.
“We looked at classical SAGD and thought maybe there are better ways to skin this cat,” says John Stalder, chair of OSLI’s Alternative Well Configuration (AWC) Group.
Stalder, who is also Principle Technical Advisor, Technology and Optimization at ConocoPhillips, explains SAGD has been the most commonly used method to access the solid bitumen found in the McMurray Formation of the Athabasca oil sands since its invention by Dr. Roger Butler in the mid-1980s.
Classical SAGD wells are positioned as stacked, parallel, horizontal pairs. The upper well is used to inject steam into the reservoir to create a steam chamber, which heats the bitumen until it is mobile and drains to the bottom of the heated region along with the steam condensate. The lower well collects or “produces” the steam condensate and bitumen, moving it to the surface. Over time, the steam chambers of adjacent SAGD pairs “communicate” so that the entire reservoir is heated, allowing much of the bitumen to be drained.
“SAGD is inherently energy intensive and requires closely spaced wells, making the process expensive. So we wanted to look at other possibilities,” says Stalder.
Roughly 80 per cent of the Athabasca oil sands deposit is buried deep enough that it can only be recovered in situ, meaning in place, and SAGD is the most common method currently used. Any improvements to efficiency or cost will be important.
Stalder points out SAGD is very energy intensive because steam is used to heat most of the reservoir, which results in high fuel costs and the associated environmental issues of air emissions and water usage.
Throughout 2010, simulations were carried out on six alternative well configurations that “look promising” for overcoming some of the challenges of SAGD.
Stalder explains alternative well configurations are needed to improve SAGD’s Cumulative Steam-Oil Ratio, or CSOR. That’s the amount of steam required to produce the bitumen. Right now it takes 2.5 to 3.5 cubic metres of steam to produce one cubic metre of bitumen.
By changing the position of one or both of the classical SAGD wells or replacing them with a different type of well, the AWC Group hopes to produce more bitumen earlier in the steam recovery process. Alternative well configurations may accomplish this by:
- accelerating bitumen recovery,
- reducing the amount of steam
- required to produce a barrel of bitumen,
- accessing stranded bitumen, and
- reducing the number of wells required.
Most of the alternative well configurations are designed to augment classical SAGD. However, XSAGD, X-Drain and Multi-Azimuthal fractures would change the classical SAGD layout. Stalder says XSAGD is similar to SAGD except that the injection wells are perpendicular (or oblique) rather than parallel to the producing wells. In a field application, with numerous wells, it would look like a grid. Some form of flow control is required between the injector and producer at the points where the wells cross. Steam chambers form above the points where the wells cross and grow spherically.
X-Drain, a GeoSierra/Halliburton concept, involves single vertical-well SAGD technology.It uses a vertical well with vertical planes, similar to hydraulic fractures, which extend outward like spokes on a wheel. The planes are created using a highly permeable propping agent, such as sand or glass beads, that are placed into the bitumen reservoir using a high viscosity fluid. Steam is injected into the upper portion of the well through a tubing string and moves outward through the highly permeable propping agent to mobilize
Stalder explains the bitumen and steam condensate drain downward through these planes toward a sump at the bottom of the well and are lifted to the surface through an insulated tubing string that is located concentrically inside the injection string. Over time, the steam chamber grows outward from the faces of the planes to create a vertical cylinder that continues to grow laterally outward in a radial fashion. Several vertical X-Drain wells would be needed to replace one horizontal SAGD well pair.
Multi-Azimuth Fractures use high permeability “fractures” to complement or even replace horizontal wells. A fracture could be 1,000 metres long and up to 15 centimetres thick, covering the full vertical height of the reservoir. Coarse sand or glass beads are used to prop the fracture so that it is very permeable. A vertical steam injection well could be drilled anywhere along the fracture, with a vertical or horizontal producing well near the bottom. A series of parallel fractures could be tapped by a single horizontal producer and single horizontal injector, both perpendicular to the fractures. These fractures could
also be used to link steam injection and production wells so that a lateral steam drive could be conducted in the bitumen, which normally isn’t possible because the immobile bitumen blocks fluid flow between adjacent wells.
Stalder says that field-testing XSAGD will be quite expensive and has been set aside until an economic collaborative project can be developed. X-Drain is being advanced for field-testing outside OSLI by GeoSierra/Halliburton.
Other alternative well configurations that supplement classical SAGD include the following:
- High Cross Injector SAGD is designed to accelerate communication between SAGD steam chambers. The concept employs one or more extra injection wells running perpendicular and above
a number of adjacent SAGD well pairs. Once communication is established it may be possible to convert to a lateral steam displacement process which will greatly improve steam trap control and allow higher production rates. As a result, production can be accelerated and thermal efficiency increased.
- Fishbone (sometimes called Herringbone)/Fishhook SAGD wells are designed to overcome barriers and baffles that can’t be accessed using classical SAGD well pairs. Fishbone wells have many relatively short side laterals drilled open hole before a slotted liner is installed in the main well. The side laterals, called fishbones, can have vertical and/or lateral trajectories to penetrate barriers and baffles. Even without these obstructions, fishbones can shorten the distance between the bitumen and a path to the surface. Both SAGD injection and production wells could include fishbones but the greater benefit will likely be for the production wells.
- Trenching is also designed to overcome barriers and baffles using a slightly different technique than
Fishbone SAGD wells. With Trenching, a horizontal well is drilled with a number of closely spaced laterals inclining toward the top of the reservoir that form a vertical plane above the horizontal well. The result is similar to digging a trench above and along the horizontal well.
A field test of Trenching is being planned for Nexen’s Long Lake project, while the Energy Resources Conservation Board has approved two Fishhook wells for ConocoPhillips’ Surmont operation.
While field testing takes place, the Alternative Well Configuration Group also will be looking at ways to improve SAGD by reconfiguring the internal well bore with flow distribution controls and other