Measuring and testing – With fluid pressure
Reexamination Certificate
2002-05-28
2004-04-06
Williams, Hezron (Department: 2856)
Measuring and testing
With fluid pressure
C073S038000
Reexamination Certificate
active
06715341
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to a measurement apparatus and a method of measuring the gas permeability of rock and other geological materials. In particular, though not exclusively, the invention is for use in the petroleum, geotechnical, built environment and groundwater industries.
The measurement of rock permeability, porosity and fluid saturation from core samples in oil and gas exploration work gives vital information of the state of the reservoir under consideration, its potential flow capacity and can also provide insight into the most efficient methods of oil and/or gas extraction. Taking measurements of the gas permeability of rocks provides a method of classification of the rock type and quality. Permeability data are used both as absolute values and to provide correlation statistics.
At present, permeability has been measured utilising either a core plug drilled from the rock sample at predetermined intervals, or by a contact method. For contact methods a probe is normally used. The probe is commonly in the form of a pipe having an orifice of known diameter at one end, with a sealing ring at this end for forming a seal with the surface with which the probe end makes contact. In use, the probe end is placed in direct contact with the rock whose permeability is to be measured. A fixed rate flow of gas is then flowed through the pipe (at either a predetermined or continuously varying pressure). A pressure drop is caused by gas entering the rock, the pressure drop being dependent on the, permeability of the rock.
Point sampled measurements of this pressure drop can be used to obtain a measure of the local permeability of the rock material.
The main problem with such contact measurement methods is the errors produced by variation of the coupling of the probe to the rock, and statistical errors induced by an inappropriate sampling regime when the natural heterogeneity of the rock material and/or its surface is considered. Both the volume of rock being investigated, its relationship to the theoretical statistical support volume and the restrictive frequency of the point sampling nature of these experiments, lead to potential aliasing of the frequency of natural geological variability, and have thus inhibited the utility and application of the method. With core plug sampling these potential errors are compounded by the enforcing of a strict one-dimensional pressure field upon the sample. The currently used methods are also very time consuming since the probe needs to be lifted up and down (i.e. out of contact with the sample and back into contact with the sample) between each measurement.
SUMMARY OF THE INVENTION
The present invention seeks to avoid or minimise one or more of the foregoing disadvantages. In particular, the invention seeks to increase accuracy of the measurement of the permeability, while also speeding up data acquisition.
Accordingly, the present invention provides an apparatus suitable for use in measuring permeability of a material, the apparatus comprising: a non-contact probe comprising a first conduit and a second conduit which are arranged so that their open ends are contiguous; a gas inlet for admitting a flow of gas into a first space inside said first conduit; and a pressure difference measuring system for measuring a pressure difference between said first space and a second space inside said second conduit; wherein there is provided a probe support formed and arranged for supporting the open end of the probe at a predetermined height above a surface of said material, in use of the apparatus; and wherein said apparatus includes processor means programmed for converting a said pressure difference measurement into a permeability value, in use of the apparatus. Normally the second conduit would be blind i.e. the end remote from its open end, would be closed.
Various different forms of conduits may be used in the apparatus of the invention. Advantageously, the conduits are configured so as to provide a relatively large degree of contiguity therebetween. Conveniently the conduits are configured so that one of said conduits extends along opposite sides of the other conduit. Most conveniently one of said conduits surrounds the other conduit, said conduits being defined by substantially coaxial pipes. Other possible configurations comprise coaxial elongate, generally slot-form, rectangular cross-section pipes. Where coaxial pipes are used, it will be appreciated that either the outer or the inner conduit may be used for the gas supply.
In a particularly simple form of the invention, the probe support comprises a spacer element projecting below the open end of the probe whereby said open end may be held above the material surface by contacting the spacer with the material surface.
Advantageously the probe is mounted on a moveable head assembly and the apparatus further includes a position control system formed and arranged for controlling the movement of the head assembly so as to cause the probe to scan across a surface of the material to be analysed, in use of the apparatus, while maintaining the probe at a constant distance from said surface, whereby pressure difference measurements from a plurality of points across said surface, using said pressure difference measuring system, may be readily collected by scanning of said surface by said probe.
Thus, in a preferred aspect the present invention provides an apparatus suitable for use in measuring permeability of a material, the apparatus comprising: a non-contact probe comprising an inner pipe and an outer pipe which are arranged coaxially; a gas inlet for admitting a flow of gas into a first space defined between the inner and outer pipes; and a pressure difference measuring system for measuring a pressure difference between said first space and a second space comprising the interior of the inner pipe; wherein the probe is mounted on a moveable head assembly and the apparatus further includes a position control system formed and arranged for controlling the movement of the head assembly so as to cause the probe to scan across a surface of the material to be analysed, in use of the apparatus, while maintaining the probe at a constant distance from said surface, for collecting pressure difference measurements from a plurality of points across said surface, using said pressure difference measuring system, during scanning of said surface by said probe.
The control means preferably includes a distance-measuring means i.e. rangefinder device for measuring, preferably substantially continuously in time, the separation between the probe and the surface of the material being analysed i.e. the height of the probe above the surface. The distance measuring means preferably provides a feedback signal, representing the amount of any change in said measured distance, to the control means which is adapted to control the spacing of the head assembly from the surface of said material so as to maintain the probe at a constant height above the surface of said material. Preferably, the distance-measuring means is a distance-measuring laser.
In general the apparatus is formed and arranged so that the probe is supported at from 0.1 to 0.5 mm above the material surface, advantageously from 0.2 to 0.3 mm above the surface. It is important that the probe end is supported at a constant height above the material surface. Preferably the height should be kept with +/−10%, most preferably +/−5%, desirably +/−3% of a predetermined height. It is generally preferred though to limit the size of the probe and diameter in order to maintain a reasonable spatial resolution for permeability measurements across the surface of the material. Typically there is used an overall probe end diameter in the range from 2 to 5 mm.
The apparatus may include a data processing device for calculating a value representing the permeability of said material to said gas, for each collected pressure difference measurement, and preferably immediately following collection of that pressure difference measurement
Bowen David Gordon
Smart Brian George Davidson
Somerville James McLean
Heriot-Watt University
Jackson André K.
Voorhees Catherine M.
Williams Hezron
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