Measuring and testing – Borehole or drilling – Formation logging
Patent
1998-04-29
1999-10-19
Williams, Hezron
Measuring and testing
Borehole or drilling
Formation logging
7315206, 436 31, 436150, E21B 4900, G01N 3324, G01N 2706
Patent
active
059692405
DESCRIPTION:
BRIEF SUMMARY
FIELD OF THE INVENTION
The present invention relates to a method for determining irreducible water resistivity in an oil well site and to methods for estimating oil reserves in the oil well site from calculated resistivity.
BACKGROUND ART
Oil well sites often occur in porous rock formations (eg. sub-sea formations) whereby oil and water (and often gas) are stored or retained in the pores of the rock (eg. between grains of granular rocks such as sandstone). Specifically, the water is retained in the pore spaces as a film between any oil and, for granular rocks, individual grains and has low mobility. This water is referred to in the art as "irreducible". In other words, it is held in the rock in the pore spaces through capillary forces and is not removed when oil is retrieved from the site.
It is known that an estimation of oil reserves in a rock site can be made if the amount of irreducible water can be determined. One technique for determining this amount is to calculate and/or measure the water's resistivity (ie. electrical resistivity) and from this the water saturation (and thence oil saturation) of the oil well can be determined. However, existing tests for determining irreducible water resistivity are highly prone to error.
For example, resistivity manual measurement techniques involve the retrieval of a granular rock sample and the direct measurement of resistivity on water extracted from the rock sample but such tests are effected by mud filtrate contamination (ie. mud filtrate is captured when the sample is retrieved and contaminates the irreducible water in the sample).
To overcome introduced inaccuracies, calculating methods have been evolved which are not effected by mud filtrate contamination. One preferred technique is referred to as the "sp method" (spontaneous potential log method) which is a calculative technique based upon the voltage effect between the drilling mud and the formation water in a porous rock. The sign of the voltage switches polarity depending on whether the drilling mud is more or less saline than the irreducible water. The SP method is, however, notoriously uncertain and is effected by clay minerals and other influences not related to salinity. It would be advantageous to provide a technique that is not effected by such variables.
SUMMARY OF THE INVENTION
The present invention provides a method for determining the resistivity of water in an oil well site that includes rock that is capable of water entrapment, including the steps of:
Entrapped water is irreducible water which has been trapped intragranularly during, for example, formation of the rock or crystallisation of minerals within a rock. The method of the present invention can be employed for water that is entrapped as intracrystalline inclusions in "granular-type" rocks (eg. sandstone) within grains themselves of the rock. Alternatively, the method can be employed for other rocks in oil well sites (eg. carbonate type rocks) where the irreducible water is entrapped as intracrystalline inclusions in the rock so as to be permanently separated off from surrounding irreducible water.
Thus, the entrapped water is a pristine sample of surrounding irreducible water. The entrapped water is not affected during sampling as it is protected within the rock (eg. within the grain). In most cases the entrapped water is entrapped with oil and the entrapped water can thus be identified by firstly locating the entrapped oil.
In this specification, the terms "oil well" and "oil well site" include a site suspected of containing oil (eg. a site located during oil exploration).
Preferably, in step (c) the resistivity is determined from a determination of salinity of the entrapped water. A preferred method for determining salinity is:
Salinity S is preferably calculated in step (4) according to the formula: -0.00037T.sub.ice.sup.3).times.10000(1)
Once salinity has been calculated resistivity is preferably calculated according to the formula: ##EQU1## where Rw.sub.FT is the resistivity of irreducible water at a reservoir (or format
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Asquith, George, Gibson, Charles, Basic Well Log Analysis for Geologists, The American Association of Petroleum Geologists, p. 102. Oct. 1982.
Eadington Peter John
Krieger Francis William
Lisk Mark
Commonwealth Scientific and Industrial Research Organisation
Soliz Chad
Williams Hezron
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