CO2 and H2S Production During Steam Injection Into Carbonate Heavy Oil Reservoirs: Laboratory Results, Field Pilot Experiences and Recommendations On Mitigation

Globally about 1.6 trillion barrels of heavy oil are trapped in carbonate reservoirs. Based on public and non-proprietary data sources, a lot of in-place carbonate heavy oils are located in Canada (536 billion barrels) and the middle-east (130 billion barrels). Recovery of heavy oil from carbonate reservoirs with steam injection technologies is very challenging and notoriously difficult. This constitutes one of the toughest technical domains in unconventional oil recovery. The main reasons for these are the complex pore structures; varied reservoir quality and fracture distributions; and high heterogeneity of carbonate reservoirs. In the past 3 to 4 decades, a number of steam pilots have been conducted in many heavy oil carbonate reservoirs around the world. A few steam-based commercial heavy oil carbonate projects, mostly in the middle-east, are either approaching final investment decision (FID) or in post-FID construction or production phases.

Results from the steam pilots conducted in the past and currently ongoing are mixed but they provide a foundation from which to learn about complex nature of carbonate heavy oil reservoirs. This is especially so in cases where results of CO₂ and H₂S generation and production during steam injection pilots and/or related laboratory experimental tests are known. CO₂ and H₂S are both potentially harmful to the environment and human health. High CO₂ and H₂S emissions into the atmospheres can trigger regulatory penalties due to harmful effects of H₂S to human health and increasingly global climate change implication of CO₂ emissions. H₂S also has the additional disadvantage of promoting corrosion of equipment, facilities and pipeline connections. Capturing and disposal of CO₂ and H₂S are also expensive.

 A carbon constraint economy and a low-carbon future are environmental necessities and demand actions. As a result, finding ways or approaches to reduce volume of CO₂ and H₂S generated and emitted in major industrial or oilfield operational activities (such as steam injection in the recovery of heavy oil from carbonate reservoirs) require better understandings of the process of generation and production of CO₂ and H₂S in order to develop mitigation strategies.

Fortunately, "some" of the available published papers or conference presentations on laboratory experiments and field pilots contain information, data and operational experiences on the generation and production of CO₂ and H₂S during steam injection into carbonates, carbonate reservoirs or carbonate containing reservoirs or rocks with heavy oil. These information, data and operational experiences are "few" and have some limitations, but they can still provide foundation knowledge that are potentially useful in the “Assess” and “Select” Phases of EOR projects (pilots or commercials) when data and information are scarce. Such data can be used to develop preliminary mitigation strategies as well as HSE (health, safety and environmental) design and implementation considerations. Hence the objective of this paper is to review publicly available laboratory experimental data as well as ongoing and executed steam injection pilots in carbonate heavy oil reservoirs or rocks around the world with a view to:

 1)      Extract data and information on, or become familiar with the aspects of CO₂ and H₂S generation and production.

2)      Learn about pilot operational experiences that are vital to mitigation of CO₂ and H₂S generation and production.

3)      Use extracted data and information to recommend mitigation strategies that could potentially reduce or eliminate CO₂ and H₂S generation and production.

Reviewed experimental results or pilots might not be direct apple-to-apple analog to a heavy oil reservoir under consideration in terms of viscosity, API gravity, reservoir depth, aquifer absence or presence and strength, degree of hydrocarbon richness, relative mineral compositions and fracture intensity but significant insights can be gained from the operations, risk mitigations approach and execution strategies of these pilots that would help to minimize or eliminate CO₂ and H₂S generation and production.