- Challenges
- Effiency
- Flexible Cement System
- Thermally Stable
- Mud Pockets
- Gas Migration

Gunnar DeBruijn, P.Eng., is technical manager for Schlumberger Well Cementing Services in Canada. Since graduating from the University of Alberta in 1990 with a degree in Mechanical Engineering, he has gained well cementing experience across Canada—from the foothills to the Arctic and offshore the east coast. This experience has involved shallow gas and heavy oil and has included world record extended-reach wells. Gunnar helps to adapt Schlumberger cementing technology to the Canadian market and also communicates Canadian needs to the company’s worldwide R&D community, driving the development of solutions to more effectively and efficiently achieve isolation and solve Canadian zonal isolation challenges.

Heavy oil cementing operations face several specific challenges, both technical and operational. Most heavy oil fields are in shallow unconsolidated sandstone, which can lead to loss of circulation during drilling and cementing. In addition, thermal operations—which can involve temperatures up to 340 degC (644 degF)—lead to stress in the casing, cement and formation.

Heavy oil extraction usually involves large numbers of wells, so operational efficiency is as important as meeting the technical challenges. Equipment must arrive on time, perform quickly and safely, and depart leaving behind minimal environmental footprint. 25,352 wells were drilled in Canada in 2006, decreasing to 22,973 wells in 2007. The decrease was all in gas wells—the number of heavy oil thermal wells increased. Rate of penetration (ROP) is generally high. One recent CHOPS project deployed 6–8 rigs, each of which took 1–2 days to drill the top hole and another 2–3 days to complete and case the well. This meant an average of more than two cement jobs per day and on some days considerably more. This high density of jobs requires excellent people, good equipment, and well planned logistics to meet peak work levels.

In some areas, such as Lloydminster, access to suitable water can be a challenge, both for cementing and subsequent SAGD operations. It may be necessary to consider using brackish and/or produced water in such cases.

As an industry, we do not yet fully understand the effects of heat transfer between the wellbore and reservoir, and are mostly learning from failures. Several operators working in Canada, California (Bakersfield), and other areas, have reported steam break-out at the surface due to cement and/or rock failure. Well heads have been observed to lift several feet when heated, clearly evidence that the casing and cement are moving. Leakage to the surface has forced shut-down of entire pads—perhaps 20–30 wells—until remedial action is taken.

One way to reduce thermal stress is to reduce the heat-up rate when steaming a SAGD or CSS injection well. Slow heating helps to spread heat and reduce differential stress. A well may fail if heated in 2 minutes, but not if heated over 24 hours or longer. Failure is most common at the first steam cycle—if well integrity is maintained after one cycle, it usually lasts for several more.

Soft formations offer little constraining pressure, and tensile pressures may lead to breakage. Cements with a low Young’s modulus, such as the flexible cement system using FlexSTONE technology, can deliver mechanical properties appropriate for these downhole stress environments. The set cement conforms to changes that occur during the thermal production process, offering better lifelong zonal isolation. FlexSTONE technology is currently qualified to 250 degC (482 degF), and work is in progress to address higher temperatures. The 2002 SPE paper 78950 “Implementation of Advanced Cementing Techniques to Improve Long Term Zonal Isolation in Steam Assisted Gravity Drainage Wells” describes the use of FlexSTONE technology in six SAGD wells in Christina Lake. To date, none have failed.

In some Canadian fields, Schlumberger is using RFC regulated fill-up cement, with 35–40% silica flour, to make it thermally stable. RFC cement is lower density than standard thermal cement, with a correspondingly lower Young’s modulus. A methodology for testing the strength of this and four other cement formulations is presented in the 2006 IADC/SPE paper 98896 “Effects of Long-Term Exposure to Ultra High Temperature on the Mechanical Parameters of Cement”. This test, simulating cyclic steam stimulation (CSS) over a 2-year period, heated samples to 340 degC (644 degF), then cooled them again. The study indicated that although other cement systems were more flexible, the RFC thermal was effective for the Cold Lake application.

While remedial squeeze cementing may be necessary to seal cracks, it is very important to get the initial cementing job right, with good mud removal. Mud pockets in the annulus can cause catastrophic failure when heated, including broken wellbores and collapsed casing. Schlumberger uses WELLCLEAN methodology to ensure that there are no channels or pockets of mud that can cause well failure. Where there is risk of losses, the LiteCRETE system gives good low density cement performance and is more likely to achieve cement back up to surface, clearing mud left in the annulus and formation. This makes the LiteCRETE system a good option for most of the wellbore, combined with RFC cement or FlexSTONE technology for use at the reservoir level.

When there is gas above heavy oil or bitumen, gas migration can be a challenge. Some heavy oil areas have reported surface casing vent flow (SCVF) failures as high as 25%. GasMigrationAnalyzer software can be used to assess the risk of SCVF. WELLCLEAN methodology, used together with cement slurries that incorporate performance additives such as GASBLOK, can overcome this problem and reduce SCVF to a minimum.

The results of tests such as described in the IADC/SPE paper, which heated samples in an oven, and geomechanics laboratory testing by specialists such as TerraTek, are inputs to industry simulators such as CemSTRESS software. It is important that simulators enable understanding of stress not only in the cement, but also in the casing and formation. For example, if the rock has a high Young’s modulus and can confine the stress in the cement sheath, then the tensile forces in the cement due to thermal expansion will be less of a problem.

Much work remains to be done in simulating downhole conditions and developing new cement technologies for thermal applications. Schlumberger will be focusing on these challenges, using experience from Canada and other heavy oil provinces and with support from the company’s global network of research and engineering groups.