Electricity: measuring and testing – Of geophysical surface or subsurface in situ – Using electrode arrays – circuits – structure – or supports
Reexamination Certificate
2001-12-28
2003-11-25
Patidar, Jay (Department: 2862)
Electricity: measuring and testing
Of geophysical surface or subsurface in situ
Using electrode arrays, circuits, structure, or supports
C324S375000
Reexamination Certificate
active
06653839
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to an electrical measurement apparatus and method for measuring an electrical characteristic of an earth formation, and in particular to an apparatus and method for measuring the electrical resistance of rock surrounding a drilled hole. The apparatus and method are likely to find their greatest utility in holes drilled for the exploration for hydrocarbons, and the following description will relate primarily to such use; however, it is to be understood that he invention can be used in other applications also.
BACKGROUND OF THE INVENTION
It has been found that the electrical resistivity or resistance of rock surrounding a drilled hole can be used as a very good indicator of the structure of the rock, i.e. the electrical resistance is very sensitive to the structure of the rock. For example, rock itself, and rock containing oil, has a relatively high electrical resistance, whilst rock containing water and dissolved salts (e.g. brine) has a relatively low resistance.
Much work has been undertaken in recent decades to utilise the changes in electrical resistance adjacent to a drilled hole (and also deeper within the surrounding rock) to determine the structure of the rock, and in particular the likely presence of oil bearing strata therein.
Measurement tools and methods have been developed to measure the electrical resistance in the immediate vicinity of the drilled hole, i.e. within the few centimeters adjacent the drilled hole, and also deeper within the rock surrounding the drilled hole. The tools and methods for the latter type of measurements are typically less precise than those for the former measurements, i.e. in the latter case the measurements cover a larger volume of rock and are therefore less sensitive to small variations within only a part of that rock. The present invention is particularly suited to the former measurements, and is intended to provide very precise measurements (though it could, if desired, also be utilized with the latter measurements).
It is a requirement of measurements in the immediate vicinity of the drilled hole that the measurements be as precise as possible, with a resolution, for example, of as little as 0.1 inches (approximately 2.5 mm).
It will be understood that when measurements are to be taken within a drilled hole, the measurement tool is first introduced into the hole, and moved to the distal end of the zone of interest. The tool is typically connected by a cable to a winch at the surface, and the measurements are taken as the tool is pulled out of the hole, past the region of interest. It is not economic to allow the tool to stop for each measurement, nor is it desirable since if the tool stops it is likely to stick in position (within the mud which will typically be present within the hole, which mud can readily cause the tool to stick to the wall of the hole). A typical rate of movement of the tool during measurement is approximately 30 feet per minute (approximately 0.167 meters/second), and this speed is generally accepted as a realistic compromise between economics (the desire to take the measurements as quickly as possible so that, for example, drilling can subsequently be continued), the ability to take sufficient measurements sufficiently quickly, and the likelihood of the tool sticking.
To take a measurement every 0.1 inches whilst the tool is travelling at 30 feet per minute requires a measurement to be taken every one sixtieth of a second.
It is also known that in more precise measurements several sensor elements can be arranged on a single sensor pad, a measurement being taken from each sensor element. A known design of sensor pad has twenty five sensor elements, for example.
When taking resistance measurements in these applications, it is typical to utilize an alternating applied voltage. This has the advantage that electrolytic and other contact-induced electrical effects between the sensor elements and the rock can be ignored, it being understood that those effects induce DC voltages, or at least voltages which are sufficiently invariant to be considered to be DC. There is, however, a practical upper limit to the frequency which can be used, since higher frequencies attenuate more within the rock, and are prone to phase shifts between the applied voltage and measured current. Generally, frequencies in the range from 5 kHz to 20 kHz can be used, with the embodiment described herein using a frequency of around 7.5 kHz.
FIG. 1
demonstrates the principle involved in taking an electrical resistance measurement of the rock surrounding a drilled hole, which principle underlies the measurement methods used in many prior art applications, and also within the present invention. In
FIG. 1
, a hole
2
has been drilled within formation
4
. A measurement tool (not shown) has been inserted into the hole, and includes a sensor element
6
, which sensor element is surrounded by a guard element
8
. The sensor element
6
and guard element
8
are connected to a voltage generator
10
, supplying an alternating voltage. The electrical circuit is completed by an electrode
12
in contact with the rock remote from the sensor element
6
and guard element
8
. Since the electrode
12
is remote from the sensor element
6
and guard element
8
, it is typically considered as electrical infinity.
FIG. 1
also shows, in dashed outline, a representation of the current flow through the rock, i.e. between the sensor element
6
and guard element
8
, and the electrode
12
. It is desired that the electrode
12
be sufficiently far from the sensor element
6
that the current flow is substantially perpendicular to the rock surface for a distance within the rock, so that the current flow through an imaginary cylinder
14
is substantially linear and uniform.
If the current flow through the imaginary cylinder
14
is linear and uniform, the current flowing through the cylinder will be dependent upon the electrical resistance of the rock within the cylinder
14
, and this current corresponds to the current flowing through the line
16
.
It will be understood that the guard element
8
serves to reduce (and hopefully eliminate) the edge effects of the sensor element
6
. It is desired that the voltage of the guard element
8
matches the voltage of the sensor element
6
at all times, so that no current flows through the rock between the sensor element and the guard element. This will also help to ensure that the current flowing though the imaginary cylinder
14
is linear and uniform.
The design of the sensor element and guard element, as well as other characteristics of the apparatus, which seeks to ensure that the current flow is substantially perpendicular to the rock surface adjacent the sensor element
6
is known in this art as “focussing”, and a properly focussed apparatus can be used to determine the current flow through the imaginary cylinder
14
by determining the current flow through the line
16
.
From a measurement of the current flowing through the line
16
, and a knowledge of the voltage applied by the generator
10
, the resistance of the electrical circuit (including the rock within the imaginary cylinder) can be determined from Ohms law. However, it is necessary to apply a calibration factor so that the resistance of the rock within the imaginary cylinder
14
can be determined. The calibration factor will depend upon the particular apparatus, and in particular its geometry and componentry, but once established for a tool will not change unless the apparatus geometry or componentry is changed. In addition, absolute resistance values for the formation
4
are seldom required, but variations in the resistance at different locations within the formation are particularly useful.
DESCRIPTION OF THE PRIOR ART
Early workers in this field utilized a resistor in the line
16
so that a voltage drop across the resistor could be measured and the current flow calculated. However, that method had the disadvantage that the resistor caused a difference in the voltage between the sensor element and
Jewell Andrew David
Yuratich Michael Andrew
Computalog USA Inc.
Kinder Darrell
Mantooth Geoffrey A.
Patidar Jay
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