Spe 84848 3 perceived steam injection in terms of a

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SPE 84848 3 perceived steam injection in terms of a displacement process in which higher injection rates equated to higher oil production rates. This intuitive belief in the benefits of high injection rates was reinforced by the belief that high injection rates would heat more oil faster – with a consequent increase in production rate. Also during the field expansion of the process, steam would migrate to adjacent unheated vertical and horizontal areas of the field causing oil production there to increase, thus reinforcing this belief. Thermal reservoir simulation became widely available in the mid-1970’s. These simulators included the required physics, and showed the effect of gravity override. However, they were relatively difficult and expensive to use. In addition, limited computer power required studies to use relatively small number of grid cells, which tends to “smear” fluid interfaces and results. Although the simulation results were generally directionally correct, many operating unit personnel had limited faith in the results. In the mid-1980’s, two new analytical methods (Vogel 7 and Neuman 8 ) became available which are based upon the gravity override effects. Some key conceptual differences are that: (1) the dominant production mechanism for viscous oil steamfloods is gravity drainage of the heated oil, (2) oil rates are largely unaffected by steam rates higher than the minimum required to maintain the steamchest, (3) injection rates should be proportional to project area (not project volume), and (4) decreasing injection rates with time minimizes heat losses to produced fluids and casing blow. Figure 7 shows an example of the difference in heat requirements to maintain a steam chest at year one versus year five of a typical California steamflood pattern. The heat requirement in the fifth year is less than half the requirement in year one. The publication of these two analytical methods (1984 and 1985) was timely, given the price collapse which would occur in 1986. Early Monitoring and Management Examples Monitoring the use of injected heat in steam projects is not a new concept. The Deerfield steamflood pilot conducted by Carter Oil Company (now part of Exxon) in the 1950’s included thermal observation wells. As cited in the SPE paper reporting on the pilot, “Temperature data obtained from the thermal observation wells installed in the center five-spot enabled us: (1) to write energy balances on the reservoir and to determine the energy distribution, (2) to correlate energy requirements with steam front advance and (3) to form a picture of fluid movement through the oil sand.” 9 Their analysis included energy lost to produced fluids and fluids which migrated outside the pattern. Similarly, the concept of varying steam injection rates during the life of a steamflood was independently shown by both reservoir simulation and field testing in the early 1970’s.
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