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

MAGIC: A GIS database of marine gas seeps and seep indicators

A geographically related database called MAGIC has been developed, using GIS (Geographic Information System) technology, for MArine Gas seeps and seep IndiCators. A complementary bibliographic database (GASREF) stores details of related publications. The databases include data relating to natural seabed gas seeps and features such as pockmarks, cold seep communities, and methane-derived carbonates which are known to be found in association with seeps. The databases are compiled from published reports (so far restricted to those written in English), and users are able to interrogate the system for specified features from user-defined areas

Reservoir compaction and surface subsidence due to hydrocarbon extraction

36.50Available from British Library Document Supply Centre- DSC:6244.38(OTH--87-276) / BLDSC - British Library Document Supply CentreSIGLEGBUnited Kingdo

On the collapse behaviour of oil reservoir chalk

Oil exploitation in North Sea Ekofisk oilfield started in 1971, the reservoir is located in a 150 m thick layer of porous chalk (n = 40-50%) at a 3000 m depth. Enhanced oil recovery procedure by sea water injection (waterflooding) was initiated in 1987. Starting from this date, seabed subsidence due to chalk compaction evolves at a fairly constant rate (i.e. 40 cm/year). Nowadays, the decrease of the seafloor level is of about 10 m. Reservoir management and production strategies are at the origin of the growing interest of petroleum industry in disposing of a comprehensive description of the chalk mechanical behaviour. In this note the subsidence due to waterflooding is interpreted within a framework taken from the mechanics of unsaturated soils. By considering oil as the non-wetting fluid and water as the wetting fluid, chalk compaction is depicted as a collapse phenomenon due to oil-water suction decrease. A series of suction controlled tests in the osmotic oedometer cell are presented. Water weakening effects and chalk compaction (collapse) seem likely to occur through the lost of strength of the inter-granular links existing in the oil saturated sample. The nature of these links includes both capillary and physico-chemical fluids-chalk interactions, and is well characterised by the oil-water suction