Thermal Recovery (aka Thermal Injection) is a process of injecting the heat into the reservoir to reduce oil viscosity or thin it and significantly enhance its ability to flow through the reservoir. It also reduces tension between rocks and liquids, which in turn improves oil mobility and results in easier flow. In addition, heated oil vaporizes, thus forming thinner oil via condensing. Thermal Recovery is the most widely used method worldwide for heavy oil projects and accounts for more than 60% of the global EOR production.
There are several variations of thermal recoveries, such as:
- Cyclic steam injection is also known as the Huff & Puff method, steam is injected into a well and then left in the well for a period of time, to heat the oil and make it flow more easily. The recovery rate of this method is less efficient than others and can only extract around 20% of the oil in place. It is a common practice to produce cyclic steams for a few cycles and then use the steam flooding method. Each cycle, inject-soak-produce can last between a few weeks to months.
- Steamflood or Steam Drive is the process of constant injection of steam to the reservoir through an injection well, to heat heavy oil and improve its viscosity. In addition, as steam condenses into hot water, the additional water drive pushes the oil toward producing wells. This technique results in a recovery rate of 25%-70% of the oil in place. Normally, steam is generated by using natural gas to heat the water.
- Steam-assisted gravity drainage (SAGD) is a steam injection in a more advanced approach, whereby two stacked (one above the other) horizontal wells are drilled in the reservoir, approximately 5 meters apart from each other. Pressurized steam injected into the upper well to heat the oil and make it more viscous, thus forcing heavy oil to drain into the lower well, from where it is produced to the surface. This method is mostly used in very heavy oil fields or oil sands. This technique results in a recovery rate of around 50 % of the oil in place.
- Fireflood or in-situ combustion used primarily in oilfields with high saturation and porosity, this method of thermal recovery generates heat inside the reservoir. In shallow depth formations, gas burners are used to provide fire burns in the formation. In deeper reservoirs, fire burns are generated by injecting air or other gas mixture with a very high concentration of oxygen. Fire reduces the viscosity of oil by the hot gases produced by the fire flames, pushing the heated oil to producing wells. Hence, the fire flooding produces two effects: 1) thinning oil and 2) a force to move oil. Sometimes water can be injected as well, which then becomes steam inside the reservoir, creating an additional force to move the oil. Combustion can be achieved by three various methods: 1) Dry forward, 2) Wet and 3) Reverse Combustion. In Dry Forward, combustion fire moves in the same direction as the injected air - towards a producing well. During Wet Combustion, water is injected behind the fire burn and then transferred into steam by the hot rocks that were heated by the fire front. This method is also known as COFCAW a combination of forwarding combustion and waterflooding. In Reverse Combustion, fire front moves in the opposite direction to the air injected. Fireflooding or in-situ combustion technique results in a recovery rate of 30%-40% of the oil in place.
In all the methods, thermal energy is generated at the surface. The source of water and power is critical. Footprint, design, complexity, and costs of surface facilities and down-hole systems are dependent on the method utilized. Fields where steam flooding is used, produced water requires recovery and separation, so it can be used as feed water for boilers. Natural gas is required to heat the water and generate steam. However, today, solar energy could be used to heat water to serve the same purpose. It is has been in use successfully for many years now in Oman. The decision of power generation is driven by many factors such as availability of natural gas, weather patterns (number of sunny days) and costs.
Although steam injection and fire flooding are the most commonly used, selection of particular technique is driven by the depth of the reservoir, formation thermal properties and fluid (crude) properties. Hence, one of the key decisions to be made during any thermal recovery project planning is understanding how fast the additional drive can be transferred to the reservoir in the most cost-effective way.