ABSTRACT:

The reservoir characterisation of Karstified Fields are extremely complicated. The reasons are several, the most important are the following:

  • Complex lithology, high mineral variations of carbonates, caused by secondary burial processes, such as diagenesis;

  • Complex rock texture, the natural process of subaerial sediment exposure causes multiple drainage paths, leaching processes allowing channels, vugs, and caves to coexist;

  • Fracture network, the above processes together with the regional tectonic stresses causes fractures to be present in several ways and types;

  • Logging problems, borehole enlargements and cavings, bad pad tool contacts and common mud losses situation;

  • Porosity problems, without the proper knowledge for the correct matrix parameters, the porosity variations highly dependent upon pore geometry (m factor) can change the saturation values from oil to water.

Excellent examples of Karstified reserevoirs are present in the offshore Gulf of Valencia (Spain). These consist of mesozoic carbonates which were subaerealy exposed during Paleogene (Tertiary) and subsequently drawned and buried below thick Miocene to recent marine deposits. These reservoirs are difficult to log. In most of the situations due to the drilling fluid losses the risk of getting stuck increases. The ideal logging program has to be reduced, and the best tool to characterised the reservoir such as the FMI* is dropped from the logging program, mainly because high stucking risk in front of the large caves and vugs. The other tool that is also important to differentiate between oil and water when porosity measurements are dubious is the MDT*. Experience with this tool shows discouraging results for the same reasons. LWD resistivity imaging tools such as the RAB* combined with wireline services and thin section from cuttings, improves the characterisation of the complex Karst reservoir. This example shows the technical and economical benefits of this suite. We will use this example to show the best practice and benefits of the different interpretation technics, as well as the benefits of their integration for Karst reservoirs and for any other complex reservoir.* Trade Mark of Schlumberger

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