SUBSURFACE FRACTURE ANALYSIS, PALM VALLEY GAS FIELD

Abstract

A comprehensive set of directional fracture data was obtained from the Palm Valley-7 well, central Australia, which has greatly added to the knowledge of the open fracture system which is present in and largely determines production rate from the Palm Valley gas field. The high quality of the Palm Valley-7 fracture data is attributed to the use of the recently introduced tools and techniques of recording down-hole core orientation data with an electronic survey tool, of measuring core fracture orientation at surface with an electronic goniometer linked to a lap-top computer and of acquiring wire-line microresistivity (dipmeter) data which can be analysed interactively to identify fractures and calculate their orientation. Open fractures at Palm Valley-7 have measured apertures of up to 3 mm. Inferred apertures of up to 10 mm are thought to exist. Open fractures occur to depths of at least 1800 m and dip at angles variably greater than 40° towards the south-southwest and west. Many fractures are wholly or partly mineralised; the mineral assemblage is complex, with dominant ankerite (ferroan dolomite), subordinate calcite and quartz, minor barite and traces of pyrite and iron oxides. Fracture orientation and density may vary considerably laterally between wells, vertically within a well and, at a particular point in a well, between different types of fractures. This variability is to some extent controlled by host rock lithology; for example, open fractures are largely confined to highly competent sandstones and dolomitic limestones and bedding plane fractures appear to be confined to less competent shales and shaley siltstones. Results of this study, when incorporated into an ongoing, larger field fracture study, should lead to an improved structural model for the field and an improved ability to site development wells to maximise the likelihood of such wells encountering open fractures in the reservoir section.