Reservoir Compartmentalisation in the Roma Area

Abstract

Abstract Application of the reservoir compartmentalisation (RC) theory has increased reserves and opened new development and exploration opportunities in the Roma area. Reservoirs with pessimistic reserves assessments have been restudied using RC concepts, this resulted in reserve increases of 10% to 40%. These results encouraged workovers and tie-ins of old wells that were suspended for more than 10 years; some of these projects resulted with production and reserves better than expected. Another benefit of the RC theory is the sound explanation of unstable pressure trends. Pools assessed as small, because of rapid pressure decline during short testing, have been shut in for more than 10 years. During this time the pressure has slowly increased up to the initial reservoir pressure. The pressure support from an aquifer was believed to explain this behaviour. However, the RC model postulates that the depletion is the result of a small compartment intersected by the wells; and the pressure increase up to the initial reservoir pressure the result of gas coming from the adjacent compartments. In other words, the reservoir behaves as a small reservoir in the short term but as a large one in the long term. Finally, the understanding of the pressure behaviour of compartmentalised reservoirs have also increased the accuracy of production forecasts since it is possible to model a pressure decline for a particular arrangement of compartments. Case studies of two fields in the Roma area presented in this paper illustrate the importance of understanding compartments and being able to identify and prove significant additional reserves. In summary the RC theory has added value to the Roma area, not only by increased reserves but also reducing the risk of development and exploration prospects. Introduction The Roma Shelf is a hydrocarbon province located 500 km west-northwest of Brisbane, in the state of Queensland, Australia (Fig 1). The Roma area contains 42 fields and 80 pools (Fig 2). The first discovery happened by accident in 1900 when the Roma Council was drilling a water well and it blew out with a gas flow. Since then 402 wells have been drilled and 138 of these were successful. Currently, the Roma area is producing an average of 18 mmscfd and 20 BPD of condensate from 60 wells. The main producing horizon is the Precipice sand (Early Jurassic). The secondary horizons in order of importance are the Showgrounds sand (Middle Triassic), the Rewan (Late Permian), and the Tinowon sand (Early Permian). The total OGIP of the Roma fields amounts to 300 BCF and the total cumulative production, as of March 1996, is 198 BCF; that is 68% of the OGIP. The fields expected to recover another 26 BCF during the next 6 years which will give a total recoverable reserves of 224 BCF, or 75% of the OGIP. The growth of recoverable reserves during the last 5 years is illustrated in Fig 3. Field studies and recompletions were the main sources of reserves growth during this period. The field studies included the revaluation of reservoirs that produced better than expected, thus yielding an increase of recovery factor and OGIP. On the other hand, the recompletions included the perforations of zones that performed poorly during open hole DSTs, some of these zones resulted in improved flow rates and significant reserves. The contribution to reserves growth by development and exploration wells drilled during the same period were lower than the field studies and recompletions (Fig 4). One reason for these results, amongst others, is the maturity of the area, since most of the valid structures identified by the standard geological methods have been drilled. As a result, it is now difficult to find new structures with the same methodology in spite of the abundant data collected during the past 36 years of hydrocarbon exploration and development at Roma. The fact that several reservoirs are producing better than expected imply two things: first, that the current geological models do not fully explain the reservoir character. Second, that in the Roma area there are untapped reserves associated with the current pools that are yet to be found. The finding of these untapped reserves would probably not be possible with the current seismic alone since the structure and stratigraphy are beyond seismic resolution. Therefore, this additional reserves would more likely be found by the application of new theories and a combined effort of reservoir engineering and geology. P. 397