Everest Well ET-14z: The Complexity of Produced Water Production Revealed

Abstract

Abstract The Everest Field is a gas condensate field produced by natural depletion from the Forties and Mey/Maureen reservoirs. The Everest Field reservoirs display complex geology which sets up the possibility of variability in formation water compositions and a recent evaluation of the produced water analyses (mixtures of formation water and condensation water) for the field identified five compositionally distinct formation waters in the reservoirs (FW 1, FW 2 (Ba-rich), FW 2 (Ba-depleted), FW A and FW A(SA)) (McCartney and Ross, 2021). In that study, the compositions of the formation waters were estimated from the produced water analyses after correction for their condensation water fraction. This information has been used to obtain predictions of scaling risks to the production wells which has aided the development of scale mitigation plans for the field. However, for one well, ET-14z, it has been challenging to obtain good quality produced water samples from it because it produces water at very low rates. This has raised questions over the type of formation water being produced from the well; most samples indicate that FW 1 is being produced from the well, but some suggest that FW 2 (Ba-rich) is being produced. In addition, possibly due to slugging in the test separator, it has not been possible to correct the produced water analyses for their condensation water fraction to obtain a reliable estimate of the composition of FW 1. Therefore, there are uncertainties over the predicted scaling risks to this well. To try to reduce these uncertainties, a well testing procedure has been developed with the specific purpose of obtaining more reliable produced water samples from this well. This objective was achieved when the well test was implemented, and the resulting sample compositions showed that ET-14z initially produces only FW 2 (Ba-rich), then both FW 1 and FW 2 (Ba-rich) with the fraction of the latter decreasing over time. Throughout, condensation water dominates produced water production (>86%) whilst produced formation water rates are very low (<1 m3/day). A conceptual model to explain this behaviour has been developed in which the type of formation water being produced is determined by (a) differences in pressure between the Mey and Maureen Formations, (b) the duration of ET-14z shut-in and (c) the duration of subsequent production. The influence of formation pressures on the composition of commingled produced water has also been observed in the Veslefrikk Field, Norwegian North Sea. The results of the well test and associated model have allowed the range of production scaling risks for this well to be confirmed with more certainty.