Communications: directive radio wave systems and devices (e.g. – Transmission through media other than air or free space
Reexamination Certificate
2001-02-28
2004-03-30
Gregory, Bernarr E. (Department: 3662)
Communications: directive radio wave systems and devices (e.g.,
Transmission through media other than air or free space
C342S195000, C342S196000, C324S323000, C324S332000, C324S344000
Reexamination Certificate
active
06714153
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention concerns a radar-like device for use in production wells, that is arranged for detecting the oil/water contact in a reservoir rock.
More specifically the invention comprises: (a) a transmitter antenna for electromagnetic waves that is placed fixedly by a production tubing inside a geological formation; and (b) receiver antennas, also placed near a production tubing. This radar-like device is capable of detecting reflectors which constitute by electrically conductive surfaces inside the reservoir. One such surface of particular importance is the oil/water contact. The water front, in most instances, constitutes a relatively sharp transition between oil-bearing sand with high resistivity, to water-filled sand with low resistivity, thereby consisting a reflector.
2. Description of the Related Art
Borehole logging tools utilizing the radar principle are known from U.S. Pat. No. 4,670,717, U.S. Pat. No. 4,814,768, U.S. Pat. No. 4,297,699, U.S. Pat. No. 4,430,653, and GB 2,030,414. Some of these patents use methods where one must estimate a wave propagation speed in order to be able to interpret the radar signals.
Schlumbergers U.S. Pat. No. 5,530,359 “Borehole logging tools and methods using reflected electromagnetic signals” describes a logging tool with pulsed radar signals being transmitted from a transmitter antenna in a separate vertical section. The logging tool freely hangs in the borehole from a cable or in a coiled tubing. Linear antenna elements are arranged parallel to the tool's long z axis. Electromagnetic pulses are emitted with a center frequency of 40 MHz and with a highest frequency of 120 MHz. This pulse is radiated in all directions in the formation and is reflected by structures in the formation back to the tool in the borehole. The transit time for the pulse out to the structure and back to the tool is used for determining the distance between the reflecting structure and the borehole. Directional information is achieved by arranging receiver antennas about the entire periphery of the tool so that the reflecting structure's direction may be found by taking differences between the reflected signals. These differences may be calculated by means of electronic circuits, or subtraction may be performed by directly differentially coupled receiver antennas. One method for calculating the reflected signals' directions is given. One disadvantage with Schlumberger's patent U.S. Pat. No. 5,530,359 is that the instrument applies pulsed electromagnetic waves. this entails spreading of frequency components already in the emitted signal, and thereby the emitted signal pulse has a continuously varying group velocity. The reflected signal becomes smeared out causing an unclear image of the reflecting structures. Close reflecting structures will also dominate over the more remotely reflecting structures remote structures may be difficult to detect if the closer rocks have a relatively high conductivity/low resistivity. Another disadvantage of the Schlumberger instrument is that as it is not fixedly arranged by the geological formation, it is impossible to trace changes of the electrical parameters in the formation over a period of time, e.g. from one data to another. The instrument is also not capable of being applied in production wells or in injection wells.
Another apparatus is described in U.S. Pat. No. 5,552,786 “Method and apparatus for logging underground formations using radar”, (Schlumberger). U.S. Pat. No. 3,552,786 describes a logging tool which partially solves the problem with the electromagnetic wave speed in the formations which are to be logged. The apparatus transmits an electromagnetic pulse insert a comma in close contact a borehole wall, into the formations and receives the direct wave in a predetermined distance along the borehole string from the transmitter. Thus the wave speed for the direct wave through the rocks (which may be invaded by borehole mud) can be calculated and the reflectors distances from the transmitter/receiver system may be calculated more exactly than if one had only an estimate of the wave speed.
U.S. Pat. No. 4,504,833 “Synthetic pulse radar system and method” describes a synthetically pulsed radar generating a plurality of signals of different frequencies simultaneously. The response from the formation those different frequencies simulates parts of the Fourier spectrum which would have been measured if one emitted a very short pulse which, according to the mathematical background, should be very broad in frequency spectrum. The system can however be used on board a vehicle because it is able to generate all the component signals simultaneously.
U.S. Pat. No. 4,275,787 “Method for monitoring subsurface combustion and gasification processes in coal seams” describes a radar for detecting a combustion front in a geological formation, for example a coal bearing formation. Because resistivity generally increases with temperature, such a combustion front will display high resistivity and constitute a very large contrast with respect to the coal bearing formation which normally will display low resistivity. The attenuation: (a) exceeds 100 dB/wave length in the combustion front; (b) is one dB/wave length in “Pittsburgh coal”; and (c) is 3 dB/wave length in “British coal.” A detection range of the combustion front is 100 meters, an unrealistically large distance when one takes in consideration the conditions in an oil well with the attenuation of the signal being much higher and where it is very difficult to detect reflecting surfaces only one to two out in the reservoir. A swept signal varying continuously between a lowest and a highest frequency is emitted. Whereas the combustion front is displaced, one will by subtraction of the received signals be able to see a change corresponding to the difference between the signals. However, that patent does not consider the need for tuning the transmitter antennas when the transmitter antennas are situated very close (e.g. a few millimeters) to a metallic tubing surface (e.g., the linear tubing or a completion tubing.
SUMMARY
The invention is made partially on the background of the potential problems which could arise in connection with petroleum production on the Troll oilfield in the North Sea. As described below the resistivities in the actual geological formations are relatively lower with respect to the conditions described in the known art. Thus, it is not feasible to perform detection by means of electromagnetic waves according to the known art.
1. Expected Resistivity
A map of the Troll Oilfield generally covering the licence blocks
31
/
2
,
31
/
3
,
31
/
5
and
31
/
6
is shown in
FIG. 3
a.
Resistivity data are available from five wells:
31
/
2
-
2
(
FIG. 3
b
),
31
/
2
-
4
(
FIG. 3
c
),
31
/
2
-
5
(
FIG. 3
d
),
31
/
2
-
6
(
FIG. 3
e
), and
31
/
2
-
7
(
FIG. 3
f
). The graphs display resistivity in &OHgr;m as a function of logging depth in generally vertical wells through the reservoir rocks. The oil/water contact, hereafter called “OWC,” is defined in the wells by the depths marked in the respective graphs. The distribution of resistivity with respect to depth is markedly different from well to well. In
31
/
2
-
2
the resistivity Rt varies between about 3 &OHgr;m and 1.3 &OHgr;m over the OWC while Rt in well
31
/
2
-
4
decreases from 100 &OHgr;m to 1 &OHgr;m over the OWC. In well
31
/
2
-
5
the resistivity varies between 40 &OHgr;m and 80 &OHgr;m before it starts to decrease monotonously, about 1 meter above the OWC. By the OWC the resistivity falls by about 7 &OHgr;m. The development in well
31
/
2
-
6
is characterized by a relatively strong “ripple” between 8 &OHgr;m and 14 &OHgr;m, even though the resistivity drop is clear by the OWC. Well
31
/
2
-
7
has a low and relatively little varying Rt in the area between 7 meters above the OWC and down to the OWC, with a maximum of approximately 2 &OHgr;m and falling to approximately 0.4 &OHgr;m just before the OWC.
The resistivity cu
Eidesmo Terje
Ellingsrud Svein
Kong Fan-Nian
Sten-Halvorsen Vidar
Stølen Einar
Den Norske Stats Oljeselskap A.S.
Foley & Lardner
Gregory Bernarr E.
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