Trapped Gas Saturation Measurements: New Perspectives

Abstract

Abstract When a gas reservoir is overlying an aquifer, water will imbibe into the gas-saturated zone with the onset of gas production, trapping a certain amount of gas. This mechanism is expected to occur in a super giant field in Mozambique that hosts over 80 TCF of natural gas in place. The observed laboratory variations in trapped gas saturation represent one of the main uncertainties in the recoverable reserves of the field. Two laboratory methods for estimating this quantity are compared: one is the classic test in which core samples are immersed in a liquid and undergo a spontaneous imbibition measurement. The other is a new method that involves a series of forced imbibitions and combines centrifuge and NMR measurements. While the standard method produces countercurrent imbibition data, the new method generates co-current results. These are more representative of the reservoir process and tend to be more optimistic. The data analysis process involves the estimation of two critical Bond numbers: one is the classic value for the mobilization of trapped blobs of residual gas, the other sets the boundary between countercurrent and co-current imbibition regimes. Representative trapped gas saturation results are obtained only for Bond numbers lying between the two critical values. The new method is validated by the log data acquired in the two reservoirs under examination below the gas water contact, where immobile gas saturations are observed. Being caused by the rise of a paleo-contact, the observed saturations are considered to be representative of the trapped saturations that will form in the reservoir when production starts and taken as a reference for the lab measurements.