--> Fit-for-Purpose Log Conditioning as Applied on the F-A Gas Field in the Bredasdorp Basin, Offshore South Africa.

2018 AAPG International Conference and Exhibition

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Fit-for-Purpose Log Conditioning as Applied on the F-A Gas Field in the Bredasdorp Basin, Offshore South Africa.

Abstract

The F-A gas field is situated on the north flank of the Bredasdorp Basin in Block 9, Offshore South Africa. The field is producing gas from Early Cretaceous, Lower Valanginian, shallow marine sandstone reservoir since 1992. Since the field was discovered in the early 1980’s, a single reservoir model was built in 1999. To assist in optimising field and reservoir management, an alternative reservoir model is built, integrating up to date production data. The scope of the reservoir modelling project is to review all data and apply an integrated workflow from log conditioning to the final dynamic simulation and history matching. The data set used comprises wireline and logging while drilling logs, acquired in 27 wells of which 13 are deviated and 9 of the deviated wells are converted into production wells. Compressional sonic logs are acquired in all the wells and bulk density are acquired in most of the wells. Gamma ray and resistivity are logged in all the wells and most of the wells are logged with neutron porosity, caliper, spontaneous potential and photoelectric factor measuring tools. For this discourse, a focus is placed on log conditioning applied during this project. Examples show how, using fairly simple methods, logs are conditioned, which are then used to generate synthetic seismic ties. The initial focus is to produce logs that are suitable for well to seismic ties. Good quality seismic ties are an important part of the workflow and essential to improve the time to depth relationship at well locations. A good tie has the effect of increasing confidence placed on the subsequent reservoir model. Acoustic impedance is the main input to well to seismic ties hence the bulk density and compressional sonic logs, used to calculate acoustic impedance are the focus of this work. The basic premise of the above mentioned log conditioning is based on two basic workflows: Correcting the compressional sonic by relating resistivity to a sonic log, unaffected by the compaction trend, as described by Burch (2002). Simplifying a method of relating compressional velocity, clay volume and bulk density as used by Adrian and Robles (2011). Examples are used to illustrate the method used and the results. The results of this work shows how simple mathematical processes and a little interpretation by the log analyst can be used to improve log quality based on existing log data.