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Scaling and Plugging Issues in Husky Energy’s Lloydminster Thermal Projects

Article by Franklin Foster, Ph.D., with the assistance of Allan Koch, Husky Energy
[note: this article made possible by support and funding from the Petroleum Society of CIM, Lloydminster Section]

 Producing Heavy Oil is a challenge. This website describes many of the ways and means that this challenge has been met over the years. One approach is to heat the heavy crude to make it flow more easily. The general term for applying heat is "thermal recovery". One of the oldest and most successful thermal projects is one currently operated by Husky Energy east of Lloydminster and referred to as the Pikes Peak location.

 Pikes Peak has seen an evolution of thermal techniques which in turn have spun off to broader applications. The site began production in 1981 using Cyclic Steam Stimulation (CSS). In this approach, high temperature, high pressure steam is injected into a well for a period of time and then the well is put into production until produced volumes fall off noticeably. Then steam is injected again following which production is resumed, and so on. The length of time for steam injection and production may vary from well to well depending on a number of variables.

 A somewhat later technique, again applied at Pikes Peak, was Steam Drive. In this approach, steam is injected continuously down a separate well from the production well, once interwell communication has been established (normally after 3-4 CSS cycles). The effect of the steam drive, where injectors and producers are arranged in regular "patterns" (inverted 7-spot, or 1 injector for every 6 producers, in the case of Pikes Peak) greatly increases "sweep" of the oil and improved recovery over CSS.

 The third era of thermal techniques focused on Steam Assisted Gravity Drainage (SAGD), mainly over thicker bottom-water areas on the edges of Pikes Peak. In this approach, thanks to directional drilling, pairs of horizontal well bores are drilled. Steam is injected into the upper well bore causing oil to precipitate downward into the lower well bore from where it is produced.

 Currently (2007) the Pikes Peak site is home to 239 directional wells, 5 SAGD pairs and 2 horizontal producers. The site produces approximately 8,000 barrels per day (bbls/d) but even more impressive is that this now mature field has seen recovery factors as high as 85 percent. In fact, in 25 years of production, Pikes Peak has produced an estimated 55 million barrels of crude oil. This has made it one of the most successful Thermal Recovery projects in the world. Other Thermal Projects in the Lloydminster area at Bolney Celtic and Lashburn bring Husky's current Thermal Production up to 20,000 barrels per day.

 The oil produced at Lloydminster comes from the Lower Cretaceous Mannville Group, approximately 500 meters below the surface. The pay zones are generally 15 to 25 meters thick and some areas have bottom water (i.e. the oil is floating on top of pools of salt water). The oil is measured at 12° API in terms of viscosity or 22,000 centipoise at 15° C [i.e. about 20,000 times as viscous as water]. The steam injected is at 230° C and typically 100% steam quality in the SAGD Areas.

 Most horizontal well bores are not completed with pipe casing but rather with a permeable liner or wire-wrapped screen. It is these that are subject to plugging, thus restricting the flow of oil and decreasing the well's production. There are many causes of the plugging ranging from fine particulate matter such as quartz, feldspar, clays, or pyrrhotites; typically "glued" or "baked" together by asphaltenes and waxes in the crude oil; to carbonate and other chemically based scaling. Each of these plugging mechanisms require a different response so it is important to try and identify the exact cause or causes.

 One case study might illustrate both the challenge and the method. Laboratory analysis of a recovered well liner from a Bolney well indicated that the plugging resulted mainly from silicate particles which had effectively been glued to the liner by asphaltenes and wax, whereas experience with screens recovered at Pikes Peak indicated a Calcium Carbonate based scale, likely with a heavy oil coating. The response was to flush the horizontal well bore with various acid treatments such as a complex staged hydroflouric acid treatment (with a solvent pre-flush), which evolved over time to simply bull-heading 1% HCl acid down the annulus (termed "poor-boy" acid jobs) as a much cheaper and operationally easier alternative of boosting oil production. Eventually, coiled tubing with a special rotating jetting tool (which sprays the acid at high pressure) was used to improve placement of the 1% HCl acid solution along the entire horizontal well bore length.

 Plugging mechanisms continue to be variable from well-to-well and poorly understood. Sample analysis reveals complex combinations of particulates, waxes, asphaltenes, and carbonate scales. Different remediation techniques (and stimulation fluids) have yielded mixed results over time but efforts continue to learn more about plugging and scaling mechanisms and adapt best practices to the solution. It has been observed that there is a generally declining response to repeated acid treatments performed on the same well.

 Husky Energy's horizontal well optimizations continue to evolve with new, more cost-effective technologies (such as running calipers inside the liner and under-reaming or "drilling-out" any buildups inside the liner) and ongoing experimentation with solvents/surfactants and modifications to the stimulation fluids. It is indeed vital to continue to meet these challenges as plugging and scaling in horizontal wells has a major negative impact on well productivity in Thermal Projects.