The Effect of Filtrate Invasion and Formation Wettability on Repeat Formation Tester Measurements

Abstract

ABSTRACT This study addresses some of the problems (conceptual and practical) associated with the Repeat Formation Tester (RFT)* tool. It is shown that although the RFT detects the filtrate phase pressure at the sandface, this may differ from the oil phase pressure in the undisturbed formation as a result of capillary interface effects, formation wettability effects and supercharging (the pressure superimposed on theinitial formation pressure as a result of the viscous flow of filtrate). It is shown that whilst capillary driven flow may be significant in high permeability formations, the effect of capillary gradients in low permeability formations may be neglected and the Buckley-Leverett immiscible displacement equation used to model the invasion process.From this basis a modified theory of supercharging is developed. The theory recognises that supercharging can be a two phase problem and that the difference between the invaded and uninvaded zone mobilities should be taken into account. It is found that depending on the stability of the displacement, supercharging in a hydrocarbon zone can be enhanced or reduced by the presence of mud filtrate. A field example is analysed in an attempt to validate the proposed theory, as well as to demonstrate how an estimate of the downhole static filtration rate may be determined from RFT measurements. Consideration of formation wettability characteristics leads to the conclusion that most formations possess some degree of mixed wettability and that consequently the free water level (FWL) should lie above the oil-water contact (OWC). It is suggested that if such a reservoir were analysed on the assumption that it was water wet, then the reversal of the FWL and OWC from the water wet case would probably notbe observed. In addition the estimated formation pressure would be too high and the estimated FWL to low.