Chemical Evolution of Formation Waters in the Palm Valley Gas Field, Northern Territory: Impact on Field Management

Abstract

Abstract The chemical composition and evolution of formation waters associated with gas production in the Palm Valley field, Northern Territory, has important implications for reservoir management, saline water disposal, and gas reserve calculations. Historically, the occurrence of saline formation water in gas fields has been the subject of considerable debate. At Palm Valley gas field there were no occurrences of mobile water early in the development of the field and, only after the gas production had reduced the reservoir pressure, was saline formation water produced. Initially this was in small quantities but has increased dramatically with time, particularly after the installation of compression in November 1996. Produced waters from the field have been chemically and isotopically characterized in order to investigate the origin of the extreme salinity observed in some cases. The produced waters range from highly saline (TDS=>300,000), with unusual enrichments in Ca, Ba and Sr, to very low salinity fluids that may represent condensate waters. The Sr isotopic compositions of the waters are also variable but do not correlate closely with major and trace element abundances. The formation waters preserve chemical and isotopic heterogeneities and are thus not well mixed. The high salinity brines have Sr isotopic compositions and other geochemical characteristics more consistent with long term residence within the reservoir rocks than with present-day derivation from a more distal pool of brines associated with underlying evaporites. This conclusion is important in that the brines, if locally derived, may be less significant volumetrically than might be expected if there was present-day hydrological continuity between the brines and the evaporites. Introduction The Palm Valley gas field is situated in the Palm Valley anticline, central northern Amadeus Basin of central Australia1. The field supplies natural gas via pipelines to Alice Springs (120 km) and Darwin (1500 km) (Fig. 1)2. Palm Valley is a classic Type 2 naturally fractured reservoir2 where fractures provide essential reservoir permeability in the tight matrix rock. These reservoir effective fractures have yielded gas flow rates up to 137 MMcf/D (3.88 Mm3/D). The gas is reservoired in low porosity (4-5%), low permeability (0.01-0.03 md) matrix rock and is producible only because of the presence of a complex network of natural fractures. The gas is relatively dry, averaging 88% methane and 8% ethane, and the primary drive mechanism is gas expansion.