Method for preventing the drilling of a new well into one of a plurality of production wells

A logging sonde injects alternating current into the well casings of a plurality of producing wells at desired subsurface depth points. The resulting magnetic fields created around the well casings of the producing wells are detected by a magnetometer in a new well being drilled to prevent drilling into one of the production wells.

BACKGROUND OF THE INVENTION 
The borehole of producing oil and gas wells is typically lined from top to 
bottom with steel casing anchored by a sheath of cement that is securely 
and circumferentially bonded to both the casing and the wall of the 
wellbore. Oftentimes production wells are drilled at extreme depths with 
the drilled wellbore deviating substantially from the vertical. There is, 
therefore, the need for a reliable method of locating the wellbore of such 
production wells, especially when a new well is being drilled in the 
vicinity of the producing well to prevent the drilling into such producing 
well. 
In one conventional drilling practice, the direction and position of the 
production wellbore was determined at the time of drilling by running hole 
direction surveys and processing the directional data for wellbore 
position. The directional measurements are commonly made using a magnetic 
compass and inclinometer. The standard deviation of these directional 
measurements is not accurately known, but is probably greater than the 
resolution of the instruments. There are other random errors due to 
instrument friction and mounting instability, tool alignment in the 
wellbore, gravity pertubations, magnetic variation uncertainty, magnetic 
disturbance of the drill pipe, etc. In addition to these random errors, 
there are bias errors. The largest bias error is probably in the compass 
reading caused mostly by poor calibration and the drillpipe magnetic 
disturbance. The total directional bias error can range up to a few 
degrees or even more. 
Another method that has been used in such well location efforts is by 
searching with a magnetometer in the well being drilled for the magnetic 
anomaly created by the well casing of the producing well, as taught in 
U.S. Pat. No. 4,458,767 to Hoehn. The natural magnetization of the well 
casing due to the earth's magnetic field produces an anomaly in the total 
magnetic field which may be detected with a proton magnetometer at 
distances up to a few hundred feet. Also, a well casing may be magnetized 
by means of an internal magnetizer being advanced through the well casing 
to create a magnetic anomaly along the well casing, as taught in U.S. Pat. 
No. 4,465,140 to Hoehn. 
In U.S. Pat. No. 4,372,398 to Kuckes, there is described a system wherein 
the well being drilled contains a device for magnetizing a different well 
through a current injection by way of the formation between the well being 
drilled and such different well. 
SUMMARY OF THE INVENTION 
In accordance with the present invention, there is provided a method for 
preventing the drilling of a new well into any one of a plurality of 
existing wells, such as production wells. 
A borehole tool is lowered into one of the plurality of existing wells and 
an alternating current flow is injected into the well casing by way of the 
borehole tool at a selected depth point. A first magnetic field is created 
above the current injection point around the well due to the difference in 
the opposing current flows downward through the borehole tool and its 
support cable to the point of injection and upwardly through the well 
casing from the point of injection. A second magnetic field is created 
below the current injection point around the well casing due to the 
downward current flow through the well casing from the injection point. 
These first and second magnetic fields are both equal in direction and 
magnitude so as to create a uniform magnetic field along the length of the 
well casing both above and below the current injection point. This step of 
creating a detectable magnetic field around the well is repeated for each 
of the plurality of existing wells, each such step utilizing a differing 
alternating current frequency for each of the existing wells so as to 
create a differing detectable magnetic field around each of the existing 
wells. 
A magnetometer is located downhole in a non-magnetic section of the drill 
string of the new well being drilled. Such magnetometer detects the 
differing magnetic fields around the existing wells. Drilling of the new 
well is redirected when the measurement of the differing magnetic fields 
indicates that the continued drilling of the new well would result in the 
intersection of the new well with one of the existing wells.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
A typical wellbore 10 is shown in FIG. 1 lined with steel casing 11 and 
anchored by a sheath of cement 12 which is circumferentially bored to both 
the casing 11 and the wall of the wellbore 13. A current sonde 14 with 
releasable contact pads 15 and 16 is lowered through the casing 11 to a 
desired depth by means of the insulated cable 17. At the desired location, 
the pads 15 and 16 are released by the spring loaded arm members 18 and 
19. Pads 15 and 16 are equipped with current emitting electrodes which 
make contact with the well casing 11. Current supplied from the uphole 
power supply 22 passes through the sonde 14 and the contact pads 15 and 16 
into the well casing 11 at the desired depth location. At this depth 
location about one-half the current flows up the casing 11 while about 
one-half the current flows down the casing 11 as illustrated in FIG. 2. 
There is also a small leakage current to the formation per unit length of 
casing. As can be further seen in FIG. 2, the upward current flow in 
casing 11 subtracted from the cable current I generates a magnetic field H 
in a first direction about the casing while the downward current flow 
generates a magnetic field H' approximately equal to H in the same 
direction about the casing. In this manner, the entire magnetic field, 
both above and below the current injection point, is in one direction 
about the well casing. Such magnetic fields can be generated about a 
production well at a desired depth or deviation of direction and can be 
detected by a magnetometer located in a non-magnetic section of the drill 
string in a new well being drilled nearby to prevent any inadvertent 
drilling of the new well into the production well at such depth or 
deviation of direction. Current injection techniques of the type described 
in the aforementioned U.S. Patent to Kuckes would not be acceptable since 
a current injection through the formation from an adjacent well would 
create equal, but opposing magnetic fields at the current injection point 
by the equal and opposing current flows above and below the current 
injection point. These opposing magnetic fields cancel each other along a 
portion of the well casing on either side of the current injection point, 
thereby the drilling into the well at such point where there is no 
detectable magnetic field would be possible. 
The current injection point should be as close to the likely intersection 
point between the two wells as possible. The further the injection point 
is from the desired depth point, the weaker the magnetic field will be at 
the desired depth point. If the current were injected at a point on the 
casing near the surface of the earth, the current would be attenuated 
rapidly with depth due to leakage through the surrounding formations. For 
example, with one ampere of current injected into the casing at the 
surface of the earth, about 10.sup.-7 amperes would be flowing in the 
casing at a depth of about 10,000 feet. The magnetic field created at 
10,000 feet is thus greatly reduced. However with one ampere of current 
injected at the 10,000 foot depth in accordance with the present 
invention, about one-half ampere of current will be flowing in each 
direction in the casing with a much larger resulting magnetic field. 
Referring to FIG. 3, to prevent the drilling of a new well 31 into any one 
of a plurality of production wells (two such production wells 33 and 35 
being shown for simplicity) current is injected into the casings of said 
production wells at a selected depth point in the manner described above. 
A magnetometer 21 is located in a non-magnetic section 23 of the drill 
string 25 of the new well 31 and the magnetic fields 29 and 37 around the 
well casings of the production wells 33 and 35 are measured by the 
magnetometer 21. Drilling of the new well is redirected when the 
measurement of the magnetic field indicates that the continued drilling of 
the new well would result in the intersection of the new well with one of 
said production wells. To aid in such measurement, a different frequency 
alternating current is injected into each production well by the 
alternating current sources 40 and 41, thereby creating differing 
detectable magnetic fields for each of the producing wells. Preferable 
frequencies will be in the range of about 0.1 Hz to about 30 Hz. The phase 
of each of the injected currents is continuously compared with the 
magnetometer output by the phase detectors 42 and 43. A valid phase 
comparison from one of the phase detectors will identify which of the 
production wells is in danger of being intersected by the new well and 
further drilling of the new well can be redirected away from the 
identified production well. This distinguishes the present invention over 
the aforementioned U.S. Patent to Kuckes in that if two wells are involved 
and a third well is being drilled between them, the current injection 
technique of the Kuckes system would cause cancelling magnetic fields to 
be generated in each of the two wells making it impossible to determine 
the position of one or the other of the two wells. In the present 
invention, this problem is completely avoided since each well is 
individually energized using different frequencies such that a 
magnetometer in a well being drilled can easily distinguish the various 
well magnetic fields and accurately determine its position relative to a 
predetermined energized well. An additional feature of the invention is 
that injecting a current directly into the well casing causes a larger 
magnetic field to be created by a given amount of power than can be 
created with the aforementioned Kuckes system which must rely on currents 
flowing through the earth in order to magnetize the well casing. 
It is to be understood that the foregoing merely describes one embodiment 
of the present invention. Various modifications, as well as alternate 
embodiments, may become apparent to one skilled in the art without 
departing from the scope and spirit of the invention as hereinafter 
defined by the appended claims.