Oilfields produced by dissipating high pore fluid pressures maybe subject to increased compressibility and decreased permeability of the reservoir rock if yield occurs during production. Reservoir sandstones deformed along the Ko stress path show an initial elastic pre-yield behaviour, a peak strength followed by a condition of steady state flow which progresses to a stiffening of the material. Permeability measurements made during various phases of the deformation show little change prior to yield, but exponential decline during the post-yield region, where the permeability decrease can exceed an order of magnitude. This paper describes the mechanisms, prevalent during deformation, that are responsible for the permeability decline. Experimental results for reservoir sandstones deformed along the Ko stress path are discussed in relation to understanding the dynamics of a deforming reservoir.
Production of hydrocarbons resulting in the dissipation of pore pressure causes the reservoir rock matrix to support a greater proportion of the overburden stress (Jones, Leddra and Addis 1987; Potts, Jones and Berget 1988). This additional effective load will cause the reservoir rock to deform, and if the pore pressure is decreased sufficiently, this deformation may proceed to the extent that yield occurs. Such deformation of the reservoir formation (compaction) will increase the driving energy for production but may also lead to increased solids production in wells, well casing collapse, permeability decline, pore volume reduction and land surface subsidence. Localised displacements of weaker stratigraphic horizons within the reservoir are suspected of causing casing collapse, whilst the rate of pore pressure drawdown is thought to contribute to the local mobilisation, and consequent in- Vasion of the reservoir rock through perforations in the well casing (Leddra and Jones 1989). Volumetric deformation of the rock matrix via pore collapse and subsequent primary consolidation have been demonstrated to be the major mechanisms responsible for reservoir compaction (Potts, Jones and Berget 1988), and the related closing of the pore throat size may cause a reduction in the permeability of the formation (Leddra et al. 1989). Recognition of the settlement of the sea floor, over the Ekofisk oilfield, by Phillips Petroleum Co, Norway in 1984 (Sulak and Danielsen 1988), highlighted the necessity of geotechnical investigations with respect to the reservoir and overburden lithologies. Such occurrences prompted other operating companies and the responsible government bodies in both Norway and the UK to initiate detailed analyses of the compaction caused by reservoir pressure depletion. Prediction of possible deformations, both over the entire field, and associated with individual wells, has become a priority where depletion is considered as a possible production mechanism. This paper presents the results of a study carried out to evaluate the compressibility of weak Jurassic sandstones from the North Sea, and discusses the implications of these data for hydrocarbon production. The emphasis of the study lies in relating permeability decline to the compressibility during the progressive deformation of the material.
Deformation of a sandstone reservoir rock occurs in response to a change in the effective stress, either by an alteration in the total stress, or because of change in pore fluid pressure (Terzarghi 1936; 1943):- where: σ is the effective stress; U is the pore fluid pressure; and σ is the total stress.
