Reservoir Characterisation

The key elements of reservoir behaviour are its storage capacity and ability to permit fluids to flow. Sigra specialises in determining the factors that control fluid flow, using field and laboratory techniques to do so. Some characterisation can be achieved by short term testing while some other factors can only be determined by long term monitoring of reservoir behaviour.

The key storage term is always the volume of the reservoir followed by pressure. In petroleum terms, the next most important factors in storage are the compressibility and porosity, unless the reservoir stores gas by absorption, in which case the sorption pressure (pressure at which gas is desorbed) and sorption isotherm control storage.

In hydrogeological terms, the storage is described by the volume per unit area released per unit reduction in head, with the term storativity being used for confined aquifers, and specific yield being used for unconfined cases where a phreatic surface defines the upper boundary of the aquifer.

The flow of fluids through a reservoir is governed by the fluid potential gradient, the thickness of the reservoir and its permeability.

Fluid potential can be either described in terms of pressure (petroleum) or head (hydrogeological).

Permeability is in consistent terms the dimensions of length squared but using the hydrogeological term for fluid potential hydraulic conductivity is the correct term. This has the dimensions of length/time. Permeability may not be a constant throughout the life of a reservoir but may be dependent on the effective stress in the reservoir and the relative proportion of fluid phases that exist therein.

In reservoirs that exhibit sorbative behaviour the flow rate from the reservoir will be influenced by the rate of diffusion. This is a function of the concentration gradient of the sorbate, the diffusion coefficient and the dimension of the block of reservoir being considered. This is controlled by cleating or jointing in a natural reservoir.