Mechanisms of sediment compaction responsible for oil field subsidence
During the production of a hydrocarbon reservoir, the compaction of weakly cemented sedimentary materials can result from increases in effective stress, and lead to surface subsidence. Such a phenomenon has recently been observed in the oil and gas bearing chalk fields in the Central North Sea. In order to evaluate the compaction potential of sedimentary materials during exploitation of a reservoir, laboratory experiments- were performed on chalks and clays. These experiments were predominantly K0 (zero lateral strain) tests. The tests were performed in a high pressure triaxial cell, the development of which continued throughout the experimental 'programme. Tests performed on chalks from the Central North Sea, and from two onshore sites in southern -England showed similar deformational trends. The analyses of these results concentrated on the variables of testing and the possible errors resulting from the use of laboratory data in the modelling of field situations. The analyses of the tests also include a comparison between the experimental methods and the interpretation of the results of this study and those of other workers on the subject of reservoir compaction. A parametric description of the compaction of chalk is presented as a summary to these tests. Two compaction tests on clay samples from the Central North Sea were also undertaken. The clays were uncemented and show contrasting behaviour to the chalks. These tests were performed to evaluate the amount of compaction likely to occur in the overburden of a hydrocarbon reservoir during production. The results of the deformational trends obtained from this study. are compared to those obtained by other workers, with particular reference to the changes in physical parameters during compaction. This follows a literature survey into hydrocarbon reservoir compaction. The K0 tests performed in this study are thought to represent the condition of sediment burial in a tectonically inactive basin. This subject is briefly reviewed, and the relevance of the results presented earlier discussed in terms of the prediction of stresses existing within differing lithologies. The analysis of the results have been performed using parameters commonly used in soil mechanics, this seemed to be appropriate for the deformations undergone by the materials used in this study.