Reservoir Limit Test (RLT) aims to obtain the volume of oil-in-place (VOIP), a valuable parameter at early stage of reservoir life. RLT is characterized by the well's production under pseudosteady state (PSS) flow regime. The basis for RLT in oil reservoirs without aquifer is well established, in theory and in practice, since long ago. But RLT in presence of aquifer still is not well understood, thus the need for further research.
This work presents the theoretical foundation of Reservoir Limit Test under aquifer influence based on material balance, new in the literature. It investigates how aquifer affects a RLT as the pseudosteady state flow takes place. Various aspects have been investigated, such as aquifer size, hydraulic connection and aquifer contribution to the total system compressibility.
Basic relations for RLT under influence of aquifer were developed and checked by significant number of simulated cases. These relations are similar to the classical ones published long ago, but now with the effect of aquifer incorporated. The differences are in the overall water and oil saturation, the total system compressibility, and the porous volume hydraulic connected to the well. The oil-aquifer system total compressibility is a function of average oil and water saturation in the entire porous volume including the aquifer. This new average total compressibility controls the pressure drop during pseudosteady state flow. Rigorous calculation of VOIP requires the ratio aquifer-water volume to oil volume, otherwise only estimates are possible. It is observed that given enough time, the pseudosteady state flow develops for oil-aquifer system for broad size range of confined aquifers. Even for a weak aquifer with reduced hydraulic connection to the oil zone, its presence is felt in the PSS and must be taken into account. Important to note, the VOIP value that results after taking the aquifer into account may be significantly smaller than the value calculated without considering the aquifer.
A Brazilian deepwater oilfield with aquifer case, with two wells equipped with permanent downhole gauge (PDG) is presented. A dozen RLTs are observed at early stage of production. Its data, analysis and results, illustrate and substantiate the observations and conclusions of this work.