Deformation of chalk through compaction and flow
Hydrocarbon bearing chalks are of significant economic importance within the Central Graben of the North Sea. The reservoirs have formed within predominantly allochthonous deposits which exhibit a range of sedimentary and diagenetic features not necessarily found in their onshore equivalents. Reservoir quality is closely related to the preservation of high porosity and is associated with high pore pressures. Today, during hydrocarbon production, change in pore fluid pressure has led to both compactional deformation of the reservoir and localised flow of chalk through perforations into production wells. This project has been undertaken to investigate, in the laboratory, the compaction and flow characteristics of chalks of medium and high porosity. The data obtained are used to evaluate the present day compaction and flow characteristics of chalk from the standpoint of reservoir engineering, and also to interpret the mechanical conditions that prevailed during allochthonous chalk deposition. This experimental investigation has show that the mechanical behaviour of all porous chalks is similar. Quantitatively, this behaviour is dependent on a number of parameters, the most important of which is the pre-deformational porosity. The experimental study, utilised high pressure triaxial equipment to determine the mechanical characteristics of a number of different chalks with pre-deformational porosities in the range 19-49%. Behaviour during loading under undrained triaxial and uniaxial strain conditions has been investigated. The former experiments provide data of importance to evaluating flow, both today, due to pore pressure drawdown in hydrocarbon production wells, and in the past during mobilisation and redeposition of the Central Graben chalks in Cretaceous and Palaeocene times. The experiments using the uniaxial strain path were conducted, primarily, to determine the compactional characteristics of the chalk for computer modelling of reservoir compaction and associated sea-floor subsidence using the finite element method. Compaction associated factors such as permeability change, and possible instability of chalk during sea water injection have also been investigated. A number of experiments were conducted at slow strain rates in an attempt to determine the influence of strain rate on the magnitude of the deformation.