Heavy oil is defined as liquid petroleum of less than 20 API gravity or more than 200 cp viscosity at reservoir conditions. No explicit differentiation is made between heavy oil and oil sands (tar sands), although the criteria of less than 12 API gravity and greater than 10,000 cp are sometimes used to define oil sands. Unconsolidated sandstones (UCSS) are sandstones (or sands) that possess no true tensile strength arising from grain-to-grain mineral cementation.
In-situ combustion requires standard field equipment for oil production, but with particular attention to air compression, ignition, well design, completion, and production practices. Air-compression systems are critical to the success of any in-situ combustion field project. Past failures often can be traced to poor compressor design, faulty maintenance, or operating mistakes. See Compressors for a detailed discussion of compressors and sizing considerations. Other discussions are available in Sarathi.
Predicting the production response to in-situ combustion (ISC) has been the topic of various studies. Complete numerical simulation of in-situ combustion is difficult because of the complex reactions and the thin burning front that requires small gridblocks for representation. The easiest method is essentially a tank balance, adapted by Prats. The oil and water produced are given by If the volumes are in acre-ft and the production terms are in bbl, a multiplication factor of 7,758 must be used. The estimate of 40% of the oil produced coming from outside the burned volume is an empirical value based on experience. This is the 0.4 term in Eq. 1. Figure 1, presented by Gates and Ramey, combines laboratory results and field observations from the Belridge in-situ combustion projects.
Many useful and reasonably accurate calculations can be made on in-situ combustion to predict the behavior of a proposed project. This page discusses the calculation process involved with behavior prediction. In-situ combustion prediction calculations will be explained in the following diagrams and example calculations. They start with a very simple heat balance and are then extended to more closely represent what happens in the reservoir. Start by assuming that no combustion data are available to get an initial idea of the feasibility of a project.
In-situ combustion processes are largely a function of oil composition and rock mineralogy. The extent and nature of the chemical reactions between crude oil and injected air, as well as the heat generated, depend on the oil-matrix system. Laboratory studies, using crude and matrix from a prospective in-situ combustion project, should be performed before designing any field operation. The chemical reactions associated with in-situ combustion are complex and numerous. They occur over a broad temperature range.
There are three main types of engine combustion. All three types of engine combustion convert the chemical potential energy found in the fuel to mechanical kinetic energy. The combustion cycle and fuel type required to complete this task are what distinguish the three engine types from each other. Two-stroke cycle engines or two-cycle engines complete their combustion cycle in two piston strokes that are accomplished with one revolution of the crankshaft. The two strokes are the power and compression strokes. The two-stroke engine is unique because it does not control the release of exhaust or the admission of an air/fuel mixture into the cylinder with a traditional valve arrangement, one intake and one exhaust. As shown in Figure 1, the process of filling the cylinder with an air/fuel mixture and exhausting the burned gases occurs almost simultaneously near the end of the power stroke. As the piston moves downward during the power stroke, first the exhaust port is uncovered and then the intake port is uncovered.
Gas turbines range in size from microturbines at 50 hp (37.3 kW) to large industrial turbines of 250,000 hp (190 kW). This page focuses on the gas turbine engine, the differences between types of turbines, and items to consider when they are applied as the prime mover. As shown in Figure 1 and Figure 1, the "open" Brayton cycle is the thermodynamic cycle for all gas turbines. Air enters the compressor inlet at ambient conditions (Point 1), is compressed (Point 2), and passes through the combustion system, where it is combined with fuel and "fired" to the maximum cycle temperature (Point 3). The heated air is expanded through the gas producer turbine section (between Points 3 and 5), where the energy of the working fluid is extracted to generate power for driving the compressor, and expanded through the power turbine to drive the load (Point 7).
In-situ combustion is the oldest thermal recovery technique. It has been used for more than nine decades with many economically successful projects. In-situ combustion is regarded as a high-risk process by many, primarily because of the many failures of early field tests. Most of those failures came from the application of a good process to the wrong reservoirs or the poorest prospects. The objective of this page is to describe the potential of in-situ combustion as an economically viable oil recovery technique for a variety of reservoirs.
The availability and economics of the prime mover fuel source and horsepower requirements frequently dictate that reciprocating internal combustion engines be selected to drive energy industry equipment. This section focuses on the reciprocating internal combustion engine and explores the difference between engine speeds, types of engine aspiration, and typical expected exhaust emissions. The three main types of engine combustion--two stroke, four stroke, and diesel--are discussed in the following paragraphs. All three types of engine combustion convert the chemical potential energy found in the fuel to mechanical kinetic energy. The combustion cycle and fuel type required to complete this task are what distinguish the three engine types from each other.
ISC is basically injection of an oxidizing gas (air or oxygen-enriched air) to generate heat by burning a portion of resident oil. This process is also called fire flooding to describe the movement of a burning front inside the reservoir. Based on the respective directions of front propagation and air flow, the process can be forward (when the combustion front advances in the same direction as the air flow) or reverse (when the front moves against the air flow). This process has been studied extensively in laboratories and tried in the field. The idea is that it could be a useful way to produce very heavy oils with high viscosity.