GEOLOGICAL INTERPRETATION OF WELL TEST ANALYSIS:

A CASE STUDY FROM A FLUVIAL RESERVOIR IN THE GULF OF THAILAND

S.Y. Zheng1*, P.W.M. Corbett1 and A. Emery2

1 Department of Petroleum Engineering, Heriot-Watt University, Edinburgh, EH14 4AS.

2 Western Geophysical, London.

*Author for correspondence: shiyi.zheng@pet.hw.ac.uk

One problem with the inversion of transient well test data is that it can yield non-unique solutions. The uncertainty resulting from this type of approach can only be resolved by considering information from another source such as geology. Geological information will help to define the interpretation model which will ensure the correct analysis of the well test data. The results of well test analyses are of little value to reservoir characterisation and modelling unless they can be explained from a geological point of view. This last step is what we refer to here as geological interpretation. Other sources of information which can help with well test analyses come from seismic surveys and petrophysics. Modern well test interpretation therefore consists of two major steps: analysis of the well test data; and interpretation of the results. In detail, this should include the following:

(1) definition of an interpretation model -- this requires the integration of geological, seismic and petrophysical data with transient pressure data;

(2) analysis of the well test data based on the interpretation model defined;

(3) geological interpretation of the results, which is necessary in order to explain or give meaning to the results.

In this paper, we present a case study from a fluvial gas reservoir in the Gulf of Thailand which demonstrates these procedures. In the context of a defined geological environment, a transient pressure test has been fully analysed. Newly-developed software based on the finite element method has been used to forward model the test scenarios. This allowed the results of seismic and petrophysical analyses to be integrated into the well test model.

This case study illustrates the integrated use of geological, petrophysical, well test and seismic attribute data in defining a reservoir model which respects both the reservoir geometry at some distance from the well location and also the reservoirís heterogeneity. We focus on a particular well in the Pattani Basin at which conventional well test analyses have been conducted. By considering the results of these analyses, forward modelling was carried out in which the drainage area was "cut" out of the structural map defined by seismic interpretation; also, the formationís internal heterogeneity was modelled according to well logs and petrophysical analyses. Finally, analytical and simulation results were compared with the transient pressure data.

We conclude that the integration of geological, seismic, petrophysical and well test data greatly reduced uncertainties in well test interpretation. The consistency of the results and the fact that they satisfied all the relevant disciplines meant that much more confidence could be given to their interpretation.

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