Initial saturations of hydrocarbons and water, existing in porous media, control reserves in place and, in many cases, the deliverability/recovery of these reserves. Hence, their accurate determination is essential for proper economic and reservoir engineering evaluation and optimization. Electric logging techniques are commonly used to estimate initial fluid saturations, but may suffer from problems with accurate determination of the log calibration constants for a given reservoir rock, as well as the accurate determination of in situ water resistivity in some situations. Advanced logging techniques, such as magnetic resonance, have been used in recent years to estimate the saturation of bound and free water. The use of these techniques is increasing, but cost and other factors have limited their widespread usage. Various types of reactive tracers and other in situ techniques for the determination of initial fluid saturations have also been used to attempt to determine initial fluid saturations with varying degrees of success.
Another family of techniques, on which this paper concentrates, is the actual measurement of in situ initial or swept zone fluid saturations on samples of appropriately obtained, preserved, handled and analysed core material. Different coring techniques and fluid and coring procedures will be discussed, along with the elative merits and advantages/disadvantages of each. Illustration of the results of various techniques will be given with respect to their cost, effectiveness, and accuracy of the data generated.
Initial fluid saturations, defined as the fraction of the interstitial space in a pore system occupied by oil, water, and gas, are key factors in the determination of initial reserves of actual and recoverable hydrocarbons in place and dominate reservoir flow properties due to the strong influence they exhibit on relative permeability (1–5). Surprisingly, in many reservoirs, initial fluid saturations are virtually unknown or improperly measured, resulting in gross over or under estimation of oil or gas reserves in place. The improper determination of these initial saturations may also greatly affect thepotential for formation damage due to phase trapping, resulting in a poor appraisal of deliverability. In some cases, improper initial saturation estimates can lead to the bypassing of potentially productive pay zones, resulting in significant lost reserves or the erroneous completion of ineffective pay and wet or no production from the completed zone.
Improper determination of the initial oil, water or gas saturations existing in porous media may often lead to expensive mistakes in the development of a field. In some cases, large amounts of capital are invested where minimal reserves are present or marginal flow is obtained. In other cases, viable pay is overlooked due to a perceived belief, from improper saturation evaluations, that the pay will be wet or non-productive. The hazards involved with an inadequate understanding of initial saturation conditions can generally be grouped into three categories.
Oil or gas in place is based on a simple volumetric calculation of hydrocarbon volume present in the effective porosity of the system.