|BRITAIN'S OFFSHORE OIL & GAS
Developing a Discovery
When promising amounts of oil and gas are found in an exploratory well, a programme of detailed field appraisal may begin. The size of the field must be established, and the most efficient production method worked out in order to assess whether it will repay, with profit, the huge costs of offshore development and day-to-day operation. Appraisal may take several years to complete and is itself very costly.
Appraisal draws together information from all available techniques. Detailed seismic surveys build up an accurate 3-dimensional image of the discovery, and appraisal wells are drilled to confirm the size and structure of the field (F75). Wireline logging in each new well yields data on porosity and fluid saturation and the thickness of the hydrocarbon-bearing rocks, while production testing yields hydrocarbon samples and information on reservoir productivity, temperatures and pressures. Oil, gas and reservoir rock samples are analysed in the laboratory. Most fields have both good and bad features which must be fully considered when deciding whether to develop.
Production may prove difficult and expensive if the reservoir rock is seriously disrupted by faulting or contains extensive areas of poor permeability. Porosity and permeability may vary dramatically where the reservoir rock consists of a variety of sediments (F78 - 80), and may be much reduced in areas where mineral growth has blocked the available pore spaces. Geologists compare core samples from the deeply buried reservoir rock with present-day sediments to identify the environment in which it accumulated. This environment is used to develop a geological model to help predict likely variations in the reservoir rock types and properties. If, for example, the best-quality reservoir rock is a dune sand or a beach sand, its likely extent and thickness can be estimated from the size and shape of a comparable modern dune complex or beach. The identification of microfossils that inhabited particular environments, such as shallow seas or brackish lagoons, helps confirm the model, as well as indicating the age of the reservoir rock (F76 & 77). Geologists and reservoir engineers use the geological model to select the best sites for production wells.
Studies in the Brent Field showed that the reservoir rocks most closely resemble the sediments deposited in a large delta. This geological model explained the interlayering of muddy, poorly permeable rocks with better quality reservoir sandstones. F78 shows the varied delta environments where these rocks may have accumulated, and F79 indicates the environments that produced the better quality reservoir rocks. Such analysis suggested the possibility of finding more oilfields within this ancient delta beneath the northern North Sea. Further exploration proved this to be so; these deltaic rocks are the most prolific oil reservoirs in the North Sea.