| dc.description.abstract | Due to reduction of high quality oil resources and consequently increase of oil price around the world, new sources of energy should be found to relief the high demand of energy. Hence, countries like Venezuela, United States and particularly Canada came up with their unconventional reservoirs which contain bitumen, extra heavy oil and heavy oil as remarkable sources of energy. Exploitation of such kind of reservoirs was not beneficial in the past but in recent years due to the increase of oil price tendency to production from heavy oil reservoirs has incredibly increased. The known amounts of heavy oil reserves are 3,396 billion barrels of initial oil in place, including 30 billion barrels as futuristic additional oil. The whole discovered bitumen reserves are about 5,505 billion barrels of initial oil in place, including 993 billion barrels as futuristic additional oil. Cold Lake, Athabasca and Peace River which possess more than 60% of total natural bitumen resources are located in Alberta State of Canada. In this work Athabasca bitumen was used to saturate the porous media and initialize the experimental model.
There are different methods to produce from unconventional heavy oil reservoirs based on the principle of lowering bitumen and heavy oil intrinsic viscosity. This reduction may be performed using injection of steam inside the reservoir (increasing the temperature of reservoir contents) or by dissolution of special solvents into the bitumen (solvent injection) or by use of benefits of both of them. Hot water injection, in-situ combustion, steam-assisted gravity drainage (SAGD) and cyclic steam stimulation (CSS) are examples of thermal processes and vapor extraction (VAPEX) is an example of solvent injection method. Among the thermal processes, SAGD is the most efficient and promising method which was pioneered and developed by Butler et al. (1981b). This method benefits from advantage of horizontal well technology which has a greater contact area to the reservoir. With the remarkable advance in the drilling and reservoir engineering of horizontal well technology, and more successful reports on operations of horizontal wells around the world, the SAGD process, became one of the most important means of producing bitumen. Gravity drainage is the principal mechanism of recovery process that helps to recover melted, mobile bitumen together with the amounts of steam which are condensed due to contact with cold oil at the boundary of steam chamber via horizontal production well.
However, SAGD process has some deficiencies in practical implementation. To overcome these problems, several researchers conducted many experiments and tried different methods and theories. Among these methods, hybrid processes seems to be the best solution and alternative to SAGD. Hybrid processes benefit from advantages of SAGD and VAPEX as the main concept is based on gravity drainage but here, light hydrocarbons are co-injected with steam to improve bitumen viscosity reduction. There is a possibility to build a model physically or numerically and use it for many times to evaluate the special process and find out which parameters are most effective in the production process while the reservoir can only be produced once and then it is not possible to return its conditions to the original conditions. Hence, any fault and mistake is irrecoverable in a real case.
The first-hand target of this research was to investigate and evaluate the performance of SAGD and hybrid processes by laboratory test and simulation studies. Expanding-solvent SAGD (ESSAGD) and steam alternating solvent (SAS) are examples of hybrid processes which were investigated by laboratory experiments and simulation studies in this thesis in addition to the SAGD process. Before performing the laboratory tests and simulation studies, first of all it is necessary to perceive the fluid properties (PVT data), fluid-fluid and rock-fluid interaction parameters. The composition of Athabasca bitumen, density and viscosity as fluid properties and interfacial tension between steam and bitumen as fluid-fluid interaction parameter were measured and obtained experimentally. These properties were utilized in simulation studies. Furthermore, a cylindrical model packed with 2 mm sized glass beads were used to conduct the experiments of SAGD, ES-SAGD and SAS processes. Also, the effect of steam injection rate on SAGD performance, impact of solvent injection rate and type on ES-SAGD performance were investigated and eventually all of these processes were compared based on several efficiency indicators.
Results divulged that SAS and ES-SAGD represent better recovery factor and lower amount of cumulative steam-oil ratio (CSOR) which are the two main economic factors for evaluation of these processes. However, higher steam injection rate showed higher recovery factor at the expense of higher CSOR in SAGD process. To investigate the solvent type effect on ES-SAGD performance three light hydrocarbons namely, n-pentane, n-hexane and n-heptane were used. Among them pentane yields the best performance at the experimental conditions. Moreover, higher injection rate of solvent displayed promising performance in ES-SAGD.
Nowadays, by development of computer technology and thanks to the commercial numerical simulators, the petroleum engineers are able to simulate the real processes and history match the real case data in order to find the future economical plateau. The numerical models have more flexibility in changing parameters which are important for researchers.
However, there are some disadvantages in numerical simulations as they can only be used to predict the approximate gross performance of a reservoir. Nevertheless, the simulation model is useful to history match the experimental and field results in order to give us the better understanding of the mechanisms which are involved in the special process. In addition, some researchers were benefited from numerical results to adjust their physical models. However, several field scale simulation studies were carried out to compare the aforementioned processes. Results of simulation studies confirmed that ES-SAGD and SAS are more efficient and prosperous than SAGD process. Also, impacts of various geological and operational parameters were investigated for all processes. | nb_NO |
