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Today, many machine learning techniques are regularly employed in petrophysical modelling such as cluster analysis, neural networks, fuzzy logic, self-organising maps, genetic algorithm, principal component analysis etc. While each of these methods has its strengths and weaknesses, one of the challenges to most of the existing techniques is how to best handle the variety of dynamic ranges present in petrophysical input data. Mixing input data with logarithmic variation (such as resistivity) and linear variation (such as gamma ray) while effectively balancing the weight of each variable can be particularly difficult to manage.
A novel method - Domain Transfer Analysis (DTA) - has been developed which uses a non-linear partial differential equation solver for predicting log curves, enabling more effective integration of disparate data types. DTA is conceived based on extensive research conducted in the field of CFD (Computational Fluid Dynamics).
This paper is focused on the application of DTA to petrophysics and its fundamental distinction from various other statistical methods adopted in the industry. Case studies are shown, predicting porosity and permeability for a variety of scenarios using the DTA method and other techniques. The results from the various methods are compared, and the robustness of DTA is illustrated. The example datasets are drawn from public databases within the Norwegian and Dutch sectors of the North Sea, and Western Australia, some of which have a rich set of input data including logs, core, and reservoir characterisation from which to build a model, while others have relatively sparse data available allowing for an analysis of the effectiveness of the method when both rich and poor training data are available.
The paper concludes with recommendations on the best way to use DTA in real-time to predict porosity and permeability. The future and ongoing applications of DTA for petrophysical analysis encompasses saturation, TOC, mineral volumes, and brittleness from the data that are available at varying stages of the drilling and completions process.
The Torosa gas field forms part of the proposed Browse LNG Development1, a project to commercialise three gas and condensate fields located on the outer continental shelf of northwest Australia, approximately 400km north of Broome. Two of these fields (the Brecknock and Calliance fields) are located in deep water and have been fully imaged by modern 3D towed streamer seismic surveys. However, the Torosa field partly underlies Scott Reef, which consists of two coral atolls separated by a deep channel.
The only permanently emergent land at Scott Reef is a small sand cay (Sandy Islet - Figure 1), although the reef crests of both atolls are exposed at low tide. Outside the reefs the seafloor drops away rapidly, with water depths of about 350m to the east, increasing to more than 1000m to the west. South Scott Reef lagoon is open to the north, with water depths increasing to about 50m before deepening abruptly into the channel between the two reefs. North Scott Reef lagoon is shallower - generally less than 25m - and is connected to the ocean by two narrow channels. Semi-diurnal tides with a range of up to 4.6m produce strong tidal currents in and near these channels. Small, steep-sided coral heads, or ‘bommies', are common throughout the lagoons, especially in water less than 25m deep.
Since 2005 Woodside Energy Ltd has acquired five seismic surveys over the Torosa field using a range of acquisition techniques. These surveys were important steps towards acquiring full seismic coverage over the Torosa gas field (Figure 1). The first of these surveys, the Torosa 3D marine seismic survey (MSS), was a conventional deep water 3D towed streamer marine seismic survey acquired in late 2005, which covered the deep water portion of the Torosa Field located northeast of Scott Reef. Four further seismic surveys have extended the area of 3D seismic coverage into the south Scott Reef lagoon and across north Scott Reef.
Prior to commencing the seismic programme over the shallow-water parts of the Torosa Field, Woodside carried out an extensive airborne bathymetric survey of the entire Scott Reef system in early 2006. Additional bathymetric surveying was undertaken to fill some data gaps within the airborne bathymetry survey, resulting in a comprehensive, highly-detailed data set on a 4m2 grid with vertical resolution of 0.1m covering both north and south Scott Reef to depths of about 50m. Woodside also undertook extensive metocean work including both tide and current modelling and model verification. These datasets were critical for the planning of the subsequent seismic surveys.
Summary In late 2011 Woodside Energy Ltd (Woodside), as operator of the proposed Browse LNG Development, acquired the Tridacna 3D Ocean Bottom Cable (OBC) seismic survey (Tridacna survey) over north Scott Reef. The remote offshore location, environmental sensitivity, tidally-emergent reef crests and a semi-diurnal macro-tidal setting imposed significant operational limitations at Scott Reef. The ocean bottom cable technique was selected as the most appropriate technological solution for 3D seismic acquisition in this setting. The survey design incorporated the technical requirements for the acquisition of good-quality seismic data necessary for reservoir imaging whilst cognisant of the operational realities associated with contractor and equipment availability, a shallow restricted marine survey location, complex environmental approval conditions and cost/timing considerations. The survey operations comprised a wide range of activities, operational restrictions and personnel not normally part of conventional offshore towed streamer seismic surveys, and required highly-detailed operational planning.