Summary
This paper deals with the reservoir performance of the M-6 Project Area in the south-east of the Tia Juana field. The field is one of the four heavy oil fields located along the eastern shore of Lake Maracaibo in Venezuela, collectively known as the Bolivar Coast fields. Since the early 1930's the area covered by these fields suhsides, an observation which one could not fail to make in this swampy coastal area. The subsidence prompted the construction of coastal dykes for protection against flooding and also the installation of a network of benchmarks which are surveyed biannually, to monitor ground level changes in detail. It soon became apparent that there was a direct correlation between subsidence and oil withdrawal, implying that the compaction phenomenon acts as an oil production mechanism. This established a cause/effect chain of oil production/pressure decline/formation compaction/surface subsidence which has since been a fertile field for investigation.
The application of thermal recovery methods in these fields has been geared to make full use of the compaction pheonmenon. First steam soak is applied to accelerate oil production, bringing down the reservoir pressure to the lowest possible level. This maximises the oil recovery contribution by compaction, which is as high as 15% STOIIP. The resulting low reservoir pressure level is favourable for steam drive as follow-up phase.
Because subsidence is measured on a routine basis in the fields and compaction is not, an important aspect of the study is the subsidence/compaction relationship. There are theoretical considerations which indicate that compaction approximately equals subsidence volumetrically for reservoirs which are large in relation to depth. This is supported for the M-6 area by the results of the reservoir performance analysis. The compaction of the area has been compared with theoretical values based on formation compressibilities measured on Tia Juana core samples, petrophysical data and the reservoir pressure performance. To obtain a match the pressure data had to be adjusted for vertical heterogeneity of the oil sands and time delay in the pressure decline of shales caused by their extremely low permeability. The calculation procedure developed to describe the reservoir behaviour of Bolivar Coast type heavy oil fields during depletion enhanced by steam soak, is considered satisfactory for practical considerations (oil production forecasts, dyke design).
During the steam drive phase of M-6 the relative importance of compaction has strongly decreased. Additional performance is required, however, to enable definitive conclusions on the effect of steam drive on compaction.
