NEW RESERVES IN OLD FIELDS: DO NOT UNDERESTIMATE THE GEOLOGIC RISK

摘要

New hydrocarbons from old fields are recognised as a significant contributor to the replacement of reserves. Impressive improvements in ultimate recoveries can be achieved through careful and well-engineered implementation of techniques such as water flooding, work-overs, infill drilling, gas injection, and even air injection. Key to success however is a well-suited understanding of the dynamic behaviour of the reservoirs in order to be in a position to optimise the implementation of the various technological options and thereby, maximise production rates whilst minimising costs. Understanding the reservoirs is always a lengthy process, which requires a well-kept database tirelessly exploited by an integrated team of geotechnicians. And in any case, our predictive ability remains highly dependent upon the quality of the data available with respect to the degree of complexity of the geology, even for fields well advanced in their mature stage. The older the field, the more likely the data records will be at best more difficult and lengthy to decipher, and more often, of unsuitable quality. This factor is sometimes greatly underestimated which translates into an underestimation of the uncertainties attached to the production and cost estimates. And as project promoters err on the optimistic side more often than not economic return may become adversely affected. Three field examples taken from the TotalFinaElf portfolio are discussed to illustrate these points. The first two fields were discovered and developed by the Company itself in the 70's, giving ample time to gather and analyse well targeted data and thereby mitigate the risks attached to the various IOR / EOR projects that were implemented subsequently, during the '90s. Both present geologic complexities that translated into continuously improving performance predictions - from the point of view of their accuracy - as knowledge improved. Ultimate recoveries in excess of 60% and 50% respectively are now anticipated with very satisfactory economic returns. On the contrary, the third field was discovered and developed by another Company during the '50s. Total purchased it in an open auction in 1991, with an expectation to increase production and reserves through the implementation of an extensive water-flooding scheme. Achieving the targeted increases proved much more difficult and costlier than anticipated. Modem data like 3D seismic, special logs, and numerous and precise pressure measurements, did help, but the geology proved much too complex to achieve the targeted range of production costs. The last example definitely draws our attention to the fact that whenever the data record or the time allocated to the evaluation are not sufficient for a precise description of the reservoir, that must include a satisfactory match with reliable and suitably detailed production data, the approach to the assessment of additional reserves should take into account the geologic risk to a much greater degree than most commonly done.