Operators of steamflood projects seem to prefer low pressure steam zones in their operations. The difference between steam zone temperature and initial reservoir temperature drives all energy requirements, which are usually the largest single cost in a steamflood project. However, lower steam zone pressure implies lower drawdown available for production. And lower steam zone temperature implies higher oil viscosity and therefore lower oil production rate. The influence of steam zone pressure and temperature on oil production rate is large. Theoretically, the influence on ultimate recovery (residual oil saturation) is moderate to none. Since costs encourage lower steam zone pressure and productivity encourages higher steam zone pressure, there should be an economic optimum.
The rate of change of energy requirements and oil production rate with respect to changes in steam zone temperature and pressure are determined analytically. If the reservoir geology is conducive to gravity drainage, even by a very tortuous path, then low steam zone pressure is highly favored. Lowering steam zone pressure usually comes at the cost of increased withdrawals, so it is important to carefully consider the requirements, consequences, and benefits on a case-by-case basis. A recommended method for this analysis is discussed. Significant project improvement after a pressure reduction has been reported. These production improvements and steam zone pressure in major steamflood projects are discussed.
Operators prefer to keep steam zone pressure as low as possible since pressure and temperature have a one-to-one relationship, temperature level above initial temperature drives all energy requirements (energy for steam zone growth, energy for heat losses, and energy produced and lost from wellbores and pipelines), and energy required for a steamflood project is usually the largest single operating cost. Some documented projects have steam zone pressure lower than tire pressure. A sampling of industry experience is given in Table 1.
In addition to the cost side, there is an incentive to develop as large a steam zone as possible. Several papers show pre-steam and post-steam core analyses with noted oil saturation changes. In Kern River Field, for example, "oil saturation is significantly reduced wherever a steam zone develops and is only slightly reduced in an underlying hot water zone?? (reference 2).