Method and system for precise drilling guidance of twin wells

A method to guide a drilling path of a second well in proximity to a first well including: applying a time-varying electrical current to a conductive casing or liner of the first well; from the drilling path of the second well, sensing an electromagnetic field generated by the current in the first well, and guiding the drilling path trajectory of the second well using the sensed electromagnetic field.

BACKGROUND OF THE INVENTION

The present invention relates to the field of well drilling guidance and, in particular, to guidance systems that use electromagnetic fields associated with an existing well casing to steer the drilling of a second well proximate to the first well casing.

There is often a need to drill a second well adjacent an existing well. For example, a pair of horizontal wells may be drilled to extract oil from a deposit of heavy oil or tar. A horizontal well includes well having a section that is truly horizontal through the earth and wells in which the “horizontal” section is slanted up or downhill to track the interface of an oil (or other resource) the producing formation in the earth. Thus, the horizontal portion of the well may not be geometrically horizontal and rather may follow a path that tracks a formation in the earth. Of the pair of wells, an upper well may inject steam into a subterranean deposit of heavy oil or tar while the lower well collects liquefied oil from the deposit. The pair of wells are to be positioned within a few meters of each other along their lengths, especially the lateral portions of the wells that typically extend horizontally. The wells are positioned proximate to each other so that, for example, the oil liquefied by the steam from the first well can be collected by the second well.

There is a long-felt need for methods to drill wells, e.g., a pair of wells, in juxtaposition. Aligning a second well with respect to a first well is difficult. The drilling path of the second well may be specified to be within a few meters, e.g., 4 to 10 meters, of the first well, and held to within a tolerance, for example, of plus or minus 1 meter, of the desired drilling path. Drilling guidance methods and system are needed to ensure that the drilling path of the second well remains properly aligned with the first well along the entire drilling path of the second well.

Surveying the drilling path at successive points along the path is a conventional drilling guidance method. A difficulty with typical surveying is that a cumulative error arises in the surveyed well path because small errors made at each successive survey point along the well path are introduced into the survey calculation made at subsequent survey points. The cumulative effect of these small errors may eventually cause the drilling path of the second well to drift outside the specified desired ranges of distance or direction relative to the first well.

U.S. Pat. Nos. 6,530,154; 5,435,069; 5,230,387; 5,512,830 and 3,725,777, and Published US Patent Application 2002/0112,856 disclose various drilling guidance methods and systems to provide drilling path guidance and to compensate for the cumulative effect of conventional survey errors. These known techniques include sensing a magnetic field generated by the magnetic properties of a well casing or a magnetic probe introduced into the well. These methods and systems may require the use a second rig or other device in the first well to push or pump down a magnetic signal source device. The magnetic fields from such a source are subject to magnetic attenuation and distortion by the first well casing, and may also generate a relatively weak magnetic field that is difficult to sense from the desired second well drilling path. In view of these difficulties, there remains a long felt need for a method and system to guide the trajectory of a second well such that it is aligned with an existing well.

BRIEF DESCRIPTION OF THE INVENTION

A system and method have been developed to precisely guide the drilling trajectory of a second well in a manner that ensures that the second well is properly aligned with a first well. In one embodiment, a metallic casing in the first well conducts an alternating current that generates an alternating magnetic field in the earth surrounding the first well. This magnetic field is substantially more predictable in magnitude than would be a magnetic field due solely to the static magnetic properties of the first well. The intended drilling trajectory of the second well is within the measurable magnetic field generated by the current in the first well. A magnetic detector is included within the drilling assembly used for guiding the boring of the second hole. The magnetic detector senses the magnetic field generated by the alternating current in the first well. Values measured of strength and direction of the magnetic field are used to align the trajectory of the drilling assembly drilling the hole for the second well.

The system may be used to guide a second horizontal well being drilled near a first horizontal well to enhance oil production from subterranean reservoirs of heavy oil or tar sands. The two parallel wells may be positioned one above the other and separated by a certain distance, e.g., within the range of 4 to 10 meters, through a horizontal section of a heavy oil or tar deposit. In one embodiment, the method guides a drilling path so that the second horizontal well is a consistent and short distance from the first well by: (1) causing a known electrical current to flow in the metallic casing or liner (collectively “casing”) of the first well to produce a continuous magnetic field in the region about the first well, and (2) using magnetic field sensing instruments in the second well while drilling to measure and calculate accurate distance and direction information relative to the first well so that the driller can correct the trajectory of the second well and position the second well in the desired relationship to the first well.

In another embodiment the invention is a method to guide a drilling path of a second well in proximity to a first well including: applying a time-varying electrical current to a conductor placed inside the casing of the first well; from the drilling path of the second well, sensing an electromagnetic field generated by the current in the conductor, and guiding the drilling path trajectory of the second well using the sensed electromagnetic field.

The inventive method may be a method to guide the drilling path of a second well in proximity to a first well comprising: drilling a third well towards a distal section of the first well and establishing a conductive path along the third well to the distal section of the first well; forming an electrical circuit comprising an electrical generator, a conductive casing of the first well and the conductive path along the third well, wherein said generator applies a time-varying electrical current to the circuit; from the drilling path of the second well, sensing an electromagnetic field generated by the current in the first well, and guiding the drilling path of the second well using the sensed electromagnetic field.

The invention may also be embodied as a drilling guidance system for guiding a drilling path of a second well in proximity to a first well, said system comprising: a first conductive path extending a length of the first well; a generator of electrical current connected to opposite ends of the first well to apply current to the first conductive path, and a magnetic field sensor placed within the drilling assembly of the second well and arranged to detect a field strength and direction of an electromagnetic field generated by the current applied to the first conductive path.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1schematically illustrates a typical well plan for drilling twin horizontal wells10,12. From the earth's surface14, the wells may be drilled from a single drilling platform16, where the second well is drilled from a second position of the drilling rig, located a short distance from the position from which the first well was drilled. After initially being drilled substantially vertically, the inclination angles of wells are built until they are horizontal, drilling into a desired deposit of, for example, heavy oil or tar. The first well12is typically drilled and cased before drilling commences on the second horizontal well10. The casing or slotted liner for a well is metallic and will conduct electric current. The horizontal portion of the first well may be below the second well by several meters, e.g., 4 to 10 meters.

A directional survey is made of the first well to locate the trajectory of the well and facilitate planning a surface location for a small, vertical borehole20which is a third well. This small borehole will preferably nearly intersect21the first well at the distal termination end of the first well. The small hole, with a temporary casing installed, preferably of a non-conductive material such as PVC installed, need only to be large enough to accommodate a special electrode22to be lowered to a position near the bottom and near to the first casing. The small vertical hole of the third well may be similar in size to a water well and may extend a few meters deeper than the first well.

In the embodiment shown inFIG. 1, a conductive path between the casing18in the first well10and the electrode in the third well may be enhanced if needed by pumping a suitable conductive fluid into the third well20. The electrode22is lowered into the vertical hole to provide a current path through the small well. The electrode22electrically connects the casing or liner18(collectively “casing”) of the first well to a conductive path, e.g. a wire, in the small bore hole20. The conductive path may include earth between the electrode22extending from the third well and the distal end of the casing18of the first well. By pumping a conductive fluid into the earth between the distal end of the first well and the distal end of the third well, the conductivity of that region of earth is increased to facilitate the flow of current between the electrode22and the casing18of the first well.

An above ground conductive path, e.g., wires24, connects the surface ends of the third well20and the casing or liner18of the first well10to an alternating-current (AC) electrical generator26, or other source of time varying current. A hoist27, with a depth measurement instrument, may lower and raise the wire and the electrode22in the third well. The hoist is connected to the insulated surface wire24and includes a spool of insulated wire to which the electrode22is attached. The hoist lowers the electrode22is preferably lowered to the depth of the first well casing. The electrical power from the generator drives a current28that flows through the wire24, the third well20, electrode22, casing or liner of the first well18and is returned to the generator.

The alternating-current28produces an electromagnetic field30in the earth surrounding the casing18of the first well. The characteristics of an electromagnetic field from an AC conductive path are well-known. The strength of the electromagnetic field30is proportional to the alternating current applied by the generator. The magnitude of current in the casing may be measured with precision by an amp meter, for example. Because the strength of the magnetic fields is proportional to the current, there is a well-defined relationship between the current, measured magnetic field strength at the new well and the distance between the new well and casing of the first well. The strength and direction of the magnetic field are indicative of the distance and direction to the casing of the first well.

FIG. 2is a schematic view of the first and second wells at a cross-sectional plane along the vertical sections through the wells. The electromagnetic field30emanates from the casing18of the first well10and into the surrounding earth. The second well12is shown as the upper well however the position of the first and second well may be reversed depending on the drilling application. A sensor assembly40in the second well senses the earth's magnetic and gravity fields, and the electromagnetic field emanating from the first well.

The acceptable drilling path of the second well is defined by a typical acceptable zone32that is shown in cross-section inFIG. 2. The acceptable zone32may be a region that is usually centered in the range of 4 to 10 meters from the first well. The zone32may have a short axis along a radius drawn from the upper well and a long axis perpendicular to a vertical plane through the upper well. The dimensions of the acceptable zone may be plus or minus one meter along the short axis and plus or minus two meters along the long axis of the zone. The shape and dimensions of the acceptable zone are known for each drilling application, but may differ depending on the application.

The drilling trajectory for the second well should remain within the acceptable zone32for the entire length of the horizontal portion of the two wells. The drilling guidance system, which includes the sensor assembly40, is used to maintain the drilling trajectory of the second well within the acceptable zone. Whether the drilling trajectory of the second well12is within the acceptable zone32is determined based on the direction and strength of the electromagnetic field30along the second well path as sensed by the sensor assembly40.

Measurements of the field intensity and field direction by the sensor assembly40, in the second well provide information sufficient to determine the direction to the first well and the distance between the two wells. This information is provided to the driller in a convenient form so that he can take appropriate action to maintain the trajectories of the two wells in the proper relationship. The sensor assembly40is incorporated into the down hole probe of a wireline steering tool or MWD system for drilling the second well12. The sensor assembly thus guides the drilling of the second well for directional control of the drill path trajectory.

As alternating current flows in the conductive casing18of the first well, the alternating electromagnetic fields produced in the region surrounding the conductor are predictable in terms of their field strength, distribution and polarity. The magnetic field (B) produced by a long straight conductor, such as the well casing, is proportional to the current (I) in the conductor and inversely proportional to the perpendicular distance (r) from the conductor. The relationship between magnetic field, current and distance is set forth in Biot-Savart's Law which states:
B=•I/(2πr)

Where • is the magnetic permeability of the region surrounding the conductor and is constant. The distance (r) of the second bore hole from the casing of the first well can thus be determined based on the measurement of the current (I) in the casing and the magnetic field strength (B) at the second bore hole.

FIG. 3is a schematic diagram of a component-type sensor assembly40(shown in a cut-away view) having the ability to discriminate field direction. Component-type magnetic sensors, e.g., magnetometers, and accelerometers, are directional and survey sensors conventionally used in measurement-while-drilling (MWD) measurements. The sensor assembly40moves through the second bore hole typically a few meters behind the drill bit and associated drilling equipment. The sensor assembly40collects data used to determine the location of the second bore hole. This information issues to guide the drill bit along a desired drilling trajectory of the second well.

The sensor assembly40includes both standard orientation sensors, such as three orthogonal magnetometers48(to measure the magnetic field of the earth), three orthogonal accelerometers51(to measure the gravity field of the earth), and three highly-sensitive orthogonal alternating-field magnetic sensors44,46,52for detection of the electro magnetic field about the first (reference or producer) well. The magnetic sensors, have a component response pattern and are most sensitive to alternating magnetic field intensity corresponding to the frequency of the alternating current source. These sensors are mounted in a fixed relative orientation in the housing for the sensor assembly.

A pair of radial component-magnetic sensors4446and52(typically three sensors) are arranged in the sensor assembly40such that their magnetically sensitive axes are mutually orthogonal. Each component sensor44,46and52measures the relative magnetic field (B) strengths at the second well. The sensors will each detect different field strengths due to their orthogonal orientations. The direction on the field (B) may be determined by the inverse tangent (tan−1) of the ratio of the field strength sensed by the radial sensors44,46. The frame of reference for the radial sensors44,46is the earth's gravity and magnetic north, determined by the conventional magnetic sensors48and the gravity sensors51. The direction to the conductor of current is calculated by adding 90 degrees to the direction of the field at the point of measurement. The direction from the sensors to the first well and the perpendicular distance between the sensors and the first well provides sufficient information to guide the trajectory of the second well in the acceptable zone32.

FIG. 4is a schematic illustration of an exemplary electrode22lowered into the small vertical hole20to the zone where conductive fluid has been introduced. The electrode22includes metallic bow springs50e.g., an expandable mesh, that expand to contact the walls of the open borehole of the well20. The spring elements50also retract to a size which slides through the temporary casing53of the vertical well20. The temporary casing insures that the material around the borehole does not slough into the hole. The electrode22is positioned near the first casing18at the nearest to a point of intersection21of the two wells. A conductive fluid in the third well20seeps into the earth56surrounding the intersection21between wells. The conductive fluid enhances the electrical connectivity of the earth between the first casing and the electrode in the third well. The electrode is connected to the insulated conductor wire54that extends through the well20and to the surface. The wire54is connected via wire24to the return side of the generator.

FIG. 5is a side view of an exemplary drilling guidance system60forming an electrical path62through a region of earth63between an ground surface electrode64and an electrode66extending beyond the end of an existing underground well casing68.

The electrode66extends a few meters, e.g., ten or more, beyond the distal end of the well casing68. The distance between the electrode66and the end of the well casing should be sufficient to avoid current flowing from the electrode66, up through the casing of the first well and to the surface electrode.

Well casings are conventionally metallic and have slots to allow steam and other gases to vent to the earth. Electromagnetic fields generated by the low frequency of the AC current source, e.g., preferably below 10 Hertz and most preferably at 5 Hertz, are not significantly attenuated by the slotted metallic casings in conventional wells. The electromagnetic fields generated by the current in the insulated wire passes through the slots in the casing and into the earth. Eddy currents on the casing that could interfere with the electromagnetic field are not significant due to the low frequency of the AC source.

An alternating current (AC) source70applies an AC current to the return ground electrode64and to the underground electrode66to form an electrical current path including62, e.g., producing a diffuse electrical field, through the earth63between the ground electrode64at or near the surface and the underground electrode66. A wire74with an insulated covering extends from the AC power source70, through the entire length (S) of the well casing68and through the extended borehole a distance past the distal end of the well casing to the electrode66, contacting the earth. The current path62through the earth and to the return ground electrode64completes an electrical circuit that includes the AC source70, wire74and electrode66.

The current path62through the earth and to the return ground electrode64completes an electrical circuit that includes the AC source70, wire74and electrode66. Preferably, the wire74extending down through the first well casing to the underground electrode66is insulated and has steel armor to provide mechanical strength to the wire. Electromagnetic fields from the wire74pass through insulation, armor and the well casing68and into the earth. The steel armor provides mechanical strength to the wire.

The surface wire75to the wire74and the surface wire24and wire112extending down the third well may have shielding to prevent electromagnetic fields from these wires from generating spurious electromagnetic fields that enter the earth. Further, the connections between the current source and the wire74and the current source and surface wire78are established to avoid current leakage to ground. Care is taken in setting up the electrical circuit for the drilling guidance system to ensure that current does not unintentionally leak to ground and that unwanted electromagnetic fields are not created that may affect the data collected by the sensors88.

The alternating current in the wire74generates an electromagnetic field that extends around and beyond the casing68of the first well. A known current value is applied to the wire74and electrode66. Knowing the current in the wire74, a calculation, e.g. an application of Ampere's Law, can be made to estimate the electromagnetic field at any given distance from the wire74and the well casing68. This calculated distance can be used to guide the drilling of a second well.

FIG. 6is a side view of the drilling guidance system60in which a second well80is being drilled parallel to the first well68. A drilling rig82, which may be the same rig used for the first well, guides a drill head84forming the second well along a trajectory86that is parallel to the first well casing68. Electromagnetic sensors88in the second well and behind the drill head detect the electromagnetic field from the first well68and wire80in the well. A current path90extends from the AC current source70, along the wire74extending the length of the first well casing68and out from the distal end of that casing to the electrode66, through the diffuse electrical path62in the earth63between the electrode66and return ground electrode64, and from the return ground electrode along the return wire92to the source70.

The AC sensors88are positioned approximately 18 or 20 meters behind the bit, thus will not be affected by the more concentrated current in the region where the current leaves the electrode and becomes more and more diffused as it moves away from the electrode. In practice, the AC sensors in the Injector well will be located some 40 or more meters behind the electrode at the closest point, which will be near the termination of drilling of the (lower) Injector well.

The calculation of the estimated electromagnetic field strength at a distance from the first well casing is used to estimate the distance from the first well casing of a second well trajectory86being drilled parallel to the first well casing68. Because the strength of the magnetic field at any distance from first well casing can be calculated, the measured field strength from the sensors88can be used to determine the distance between the second well and the first well. This information regarding the distance between the positions of the electromagnetic sensors88in the second well will be used to guide the trajectory of the drilling head84along a path parallel to the first well casing.

The calculation of the electromagnetic field around the first well casing may also account for other elements of the AC circuit that contribute to the magnetic field detected by the sensors88in the second well. For example, electromagnetic fields that extend into the ground may be produced by the surface mounted return wire92carrying current between the AC power source70and the return ground electrode64, e.g., a rod. Similarly, the current-conducting wire74in the vertical section94of the first well casing68produces an electromagnetic field in the earth. These additional electromagnetic fields should preferably be taken into account in calculating an expected field intensity in the region of the earth near the horizontal portion of the first well. Calculations of expected electrical field strength from a variety of current sources, e.g., wire92, the vertical portion84of wire74and the diffuse electrical current62in the earth region63, can be accomplished with known computational techniques for calculating electrical field strengths. Preferably, the calculations of the expected field intensity and the measurement of the field intensity by sensors in the second well are conducted in real time and substantially simultaneously.

The current62in the region of earth63between the electrode and the ground rod is so thoroughly diffused that the field resulting from this current will not be detected at by the AC sensors88at their positions in the second well. Thus, the current62can be ignored for purposes of calculating the electromagnetic field around the casing of the first well. The electromagnetic field strength of the current62in the earth63may relatively strong in the vicinity of the distal end of the first well. However, it is not needed to measure the field at the distal end of the first well because this point is at or near the end of the second drilling path86. At the end of the path there is likely to much less need, if any, to monitor the field because the drilling path is nearly complete and the trajectory will not significantly change further.

Deployment of the electrode outside the first well (the Producer well)68casing into open hole may be done in a variety of ways. The electrode may be pumped down through whatever tubular is used to run it into the hole, pushed into position with an extension of the tubing or drill pipe used to lower it into the hole, or it may be pushed into place with an extended well tractor. Yet another possibility is the use of coiled tubing to push it into place.

Assuming that a suitable method of deployment is developed, this method may well be more accurate than the three-well method because of the lossless current conduction by the wire inside the pipe, with no loss of accuracy due to poor information about the conductivity of formations surrounding the casing.

FIG. 7is a side view of another exemplary drilling guidance system100in which current flows along the entire length of a conductive casing102of a first well, through a region of earth104between a distal end106of the casing and a return ground electrode108lowered into a third well casing110extending near to but not intersecting with the casing102of the first well. A current source70applies current directly to the conductive casing102of the first well and to a conductive return wire112extending along the surface from the source70to and down the third well110to the return ground electrode108. The return ground electrode108extends beyond the distal end of the casing of the third well into open borehole in the earth and is connected to the return wire that extends through the casing, which is preferably non-conductive, of the third well.

A diffuse electrical current path115is formed in the earth between the return electrode108and the casing of the first well. This electrical path is included in the current path114extending from the source70, casing102of the first well, return electrode108and return wire112. The return electrode is positioned close to the first well casing (and preferably in contact with the casing) to reduce the electrical path through earth between the casing and the return electrode.

The current path114includes the current in a horizontal portion of the casing102of the first well which generates an electromagnetic field around the casing that is detected by sensors88in a second well80being drilled by a drill head84following a desired drilling trajectory86. By measuring the electromagnetic field at the sensors88and knowing the current in the casing of the first well, the distance between these sensors in the second well80can be used to calculate the distance between the first well and the second well, from the location of the sensors.