Formation compaction, if present, can have an important influence on thermal recovery methods, as observed in Western Venezuela, and elsewhere. This paper discusses the effect of formation compaction on oil production by cyclic steam stimulation and steamflooding, using a fully implicit steam injection simulator. The simulator accounts for three-phase mass and heat transport occurring in steam injection processes, for a wide variety of operating conditions. It employs an implicit formulation together with a Newtonian, direct solution approach, and was shown to be stable for large time steps.
It was found that oil recovery in a compacting reservoir increases with an increase in the uniaxial compaction coefficient. However, whereas cyclic steam stimulation yielded a favorable response in a compacting reservoir, the opposite was true for a continuous steamflood. A delay in implementing a steamflood in a non-compacting reservoir can lead to a considerable loss of recovery, in the range of 10 to 40% of oil-in-place, depending on the value of the uniaxial compaction coefficient. This finding has far-reaching implications for steamflooding subsequent to intensive depletion by cyclic steaming, or primary production.
Although formation compaction can be beneficial from the standpoint of cyclic steam stimulation response, there is strong dependence on the compaction coefficient. Furthermore, it was found that if the oil in question exhibits non-Newtonian flow behavior - reported for some Venezuelan oils - it must be accounted for in numerical simulations, otherwise the oil production rates may be in error by as much as 100%.
In a number of oil-producing regions in the world, fluid withdrawal has resulted in formation compaction, giving rise to surface subsidence, and the accompanying environmental problems. This was first discussed by Geertsma and later by van der Knaap and van der Vlis. The latter also discussed the causes of compaction. The more notable regions where formation compaction (and land subsidence) has been observed over a considerable period include the Bolivar Coast, Western Venezuela, and the Long Beach area, near Los Angeles, California. Large heavy oil reservoirs occur in both of these areas. In particular, steam injection has been a commercial oil recovery method in Bolivar Coast, since the early sixties. Until recently (1978), the oil production method was exclusively cyclic steam simulation, except for a brief steamflood project. Currently, a major steamflood is underway, described recently by van der Knaap, the early performance of which is "remarkably similar" to that of the earlier, much smaller steamflood.
It has been noted by a number of authors that formation compaction provides an important oil expulsion mechanism. For example, Ref. 2 gives an approximately one-to-one relationship between gross liquid production and subsidence volumes, for conventional oil production. Compaction is also known to be beneficial from the standpoint of cyclic steam stimulation. For example, de Haan and van Lookeren attribute 40% of the oil production to compaction. The role of compaction is not so clear in the case of a steamflood, especially as regards the extent of prior cyclic stimulation. de Haan and Schenk note that upon the completion of the compaction process, conditions would still be favorable for a process, conditions would still be favorable for a steamflood, viz. the oil saturation would still be reasonably high, in view of low recovery by cyclic stimulation, and the reservoir pressure would be low, facilitating steam injection.