WO2008150748A1

WO2008150748A1 - Method for determining the absolute orientation of subsea electromagnetic sensor arrays

Info

Publication number: WO2008150748A1
Application number: PCT/US2008/064821
Authority: WO
Inventors: Keith Vickery
Original assignee: Zupt, Llc
Priority date: 2007-05-31
Filing date: 2008-05-26
Publication date: 2008-12-11

Abstract

A method of determining the absolute orientation and attitude of seabed nodes used in electromagnetic sensing of hydrocarbon reservoirs, or seabed logging. The data from a series of acoustic, rotational rate, acceleration, and pressure sensors is integrated using a Kalman filter based software. The method allows for the determination of the depth of the seabed node, the attitude of the seabed node, and the orientation of an imaginary straight line drawn between the ends of the legs of the seabed node.

Description

METHOD FOR DETERMINING THE ABSOLUTE ORIENTATION OF SUBSEA ELECTROMAGNETIC SENSOR ARRAYS

FIELD OF THE INVENTION

[0001] The present invention relates generally to the field of electromagnetic sensing. More particularly, the present invention relates to the use of subsea electromagnetic sensor arrays to study hydrocarbon reservoirs. More particularly, the present invention relates to determination of the orientation and attitude of subsea electromagnetic sensor arrays used to study hydrocarbon reservoirs .

BACKGROUND OF THE INVENTION

[0002] As hydrocarbon reserves deplete worldwide, the oil industry must drill deeper water and in more challenging environments. For many years, the oil industry has relied on seismic surveying during exploration to determine the nature of geological formations beneath the earth's surface. While very useful, seismic surveying can not actually detect the presence of hydrocarbon deposits. Accordingly, oil exploration companies have had a very low success rate when drilling exploratory wells. The cost of drilling unsuccessful wells ("dry holes") is very high, and adds to the cost of oil and gas worldwide.

[0003] A new method of data acquisition has become more commonplace in the offshore oil and gas exploration industry. The business of electromagnetic sensing over subsurface reservoirs has spurned several new companies. Electromagnetic sensing, referred to as seabed logging, uses electromagnetic ("EM") energy to identify subsurface resistivity contrasts. An oil company will use seabed logging, in addition to traditional seismic data, to substantially reduce the risk of dry holes. For example, an oil company may have a prospective field in their portfolio, which seismic surveying suggests contains formations typical of a hydrocarbon reservoir. The oil company could then employ seabed logging techniques to identify subsurface resistivity contrasts, which would indicate whether or not there was actually a hydrocarbon deposit in the prospective field. Employing seabed logging would provide critical information to be considered when deciding whether to drill a well. Additionally, if it has been decided that the company will drill a well, the results of seabed logging will aid them in deciding where to drill the well.

[0004] Hydrocarbon reservoirs are electrically resistive. This can create conditions under which electromagnetic energy can be guided over distances of several kilometers. A powerful electromagnetic source can is towed close to the seabed and emits a low- frequency energy into the subsurface. The wave shape, current amplitude and timing are controlled to maximize the signal at the target. The source can be towed in water depths up to 4,000 meters. Lines or grids of seabed receivers (seabed nodes) deployed upon the seabed will detect electromagnetic energy that has propagated through the sea and the subsurface. Crucially, some of the energy is guided with low attenuation by resistive bodies, such as hydrocarbon reservoirs.

[0005] Processing and modeling is then used, including inversion and depth migration of seabed logging data results in maps, cross sections and three dimensional volumes that show the location and the depth of the restrictive bodies. This processing and modeling requires knowledge of the source position attitude as well as the receiver seabed node position. To date, most of the Electromagnetic data acquisition has been more comparable to two dimensional seismic data. The next generation of electromagnetic data acquisition will be equivalent to three dimensional seismic data. This will lead to a much higher resolution image of the subsurface reservoir. The next generation will require more precise positioning and attitude data for both the towed source and the seabed nodes.

[0006] A seabed logging operation requires that multiple seabed nodes be deployed to a specific grid (or array) on the seabed. These nodes are sometimes deployed with a spacing of approximately one kilometer between nodes. A single operation may have tens to potentially hundreds of nodes deployed. The operation is progressed by the nodes being deployed, data being acquired, and the nodes are recovered and then re-deployed as the operation moves along the prospective field. Typical operation durations may be several days to several weeks.

[0007] The seabed nodes are prepared on the back deck of a deployment vessel, swung overboard, and then deployed. The node is held until it is just under the water's surface and then disconnected from the crane. The node then falls to the seabed. In deeper water this may take more than one hour as the node is only slightly negatively buoyant.

[0008] The seabed node contains logging receivers that are capable of measuring electromagnetic field strengths that vary greatly in magnitude, from very low level naturally occurring magnetotelluric signals to strong direct signals that were originated at the towed source. Precise measurement of the timing of the received signals is required for both the synchronization of the individual receiver data when recovered for post processing and deriving the electromagnetic phase with respect to the source. Receivers have four electric and two magnetic receivers that record vertical and horizontal field components. The combination of both the electric and magnetic fields allows these systems to work in shallow water.

[0009] The electric sensors are typically deployed on the ends of four meter-long arms. These arms are rugged and designed to flex during deployment in order to hold the electric sensor at a fixed distance and, hopefully, attitude from the center of the seabed node. The signal to noise of the complete receiving node depends on a fixed and repeatable offset between the sensors. [0010] A number of patents have issued that relate to the present invention. For example, U.S. Patent No. 5,770,945, issued on June 23, 1998, to Constable, teaches a magnetotelluric system for seafloor petroleum exploration comprising a first waterproof pressure case containing a processor, AC-coupled magnetic field post-amplifiers and electric field amplifiers (the "logger unit"), a second waterproof pressure case containing an acoustic navigation/release system, four silver-silver chloride (Ag-AgCl) electrodes mounted on booms and at least two magnetic induction coil sensors. These elements are mounted together on aplastic and aluminum frame along with flotation devices and an anchor for deployment to the seafloor. The acoustic navigation/release system serves to locate the system by responding to acoustic pings generated by a ship-board unit and receives a release command which initiates detachment from the anchor so that the buoyant package floats to the surface for recovery. The electrodes used to detect the electric field are configured as grounded dipole antennas. Booms by which the electrodes are mounted onto a frame are positioned in an "X" configuration to create two orthogonal dipoles, which are used to measure the complete vector electric field. The magnetic field sensors are multi-turn Mu-metal core coils which detect within the frequency range typically used for land-based magnetotelluric surveys. The magnetic field coils are encased in waterproof pressure cases and are connected to the logger package by high pressure waterproof cables. The logger unit includes the amplifiers for amplifying the signals received from the various sensors, which signals are then provided to the processor which controls timing, logging, storing and power switching operations. Temporary and mass storage is provided within and/or peripheral to the processor. [0011]U.S. Patent No. 6,842,006, issued on January 11, 2005, to Conti et al, describes a sea-floor electromagnetic measurement device for obtaining underwater measurements of earth formations including a central structure and arms attached to the central structure so that they can pivot relative to the central structure. An electrode is attached to the end of each of the arms or to the central structure, and/or magnetometers are attached to the arms. It also describes a method for undertaking sea-floor electromagnetic measurements of earth formations including measuring electric fields at a selected distance from a central structure of an electromagnetic measurement system. Magnetic fields are then measured at the same location.

[0012] U.S. Patent No. 7,026,819, issued on April 11, 2006, to Eidesmo et al, teaches a method for mapping a submarine or subterranean reservoir by conducting an electromagnetic survey using an electromagnetic field in the form of a wave. An electromagnetic field is applied by a transmitter on the seabed and detected by antennae. The nature of the detected reflected waves is used to determine whether the reservoir contains water or hydrocarbons, which may then be produced from a well that penetrates the reservoir.

[0013] U.S. Patent No. 6,900,639, issued on May 31, 2005, to Ellingsrud et al., describes a system for investigating subterranean strata. An electromagnetic field is applied using a dipole antenna transmitter and this is detected using a dipole antenna receiver. Phase information is extracted from a refracted wave response and used to identify the presence and/or nature of a subterranean reservoir. [0014] U.S. Patent No. 7,126,338, issued on October 24, 2006, to Eidesmo et al., teaches an electromagnetic survey method for surveying an area previously identified as potentially containing a subsea hydrocarbon reservoir, comprising obtaining first and second survey data sets with an electromagnetic source aligned end-on and broadside relative to the same or different receivers. The invention also relates to planning a survey using this method, and to analysis of survey data taken in combination allow the galvanic contribution to the signals collected at the receiver to be contrasted with the inductive effects, and the effects of signal attenuation, which are highly dependent on local properties of the rock formation, overlying water and air at the survey area. This is very important to the success of using electromagnetic surveying for identifying hydrocarbon reserves and distinguishing them from other classes of structure.

[0015] It is an object of the present invention to provide a method of precisely determining the orientation and attitude of seabed nodes used in seabed logging. [0016] It is another object of the present invention to provide a method of determining the orientation and attitude of seabed nodes which improves the capability to process logged electromagnetic data.

[0017] It is another object of the present invention to provide a method of determining the orientation and attitude of seabed nodes which yields an improved picture of a hydrocarbon reservoir.

[0018] It is yet another object of the present invention to provide a method of determining the orientation and attitude of seabed nodes which speeds up the process of seabed logging.

[0019] These and other objects and advantages of the present invention will become apparent from the reading of the attached specification.

BRIEF SUMMARY OF THE INVENTION

[0020] The present invention is amethod of determining the attitude and orientation of seabed nodes used in electromagnetic sensing of subsea hydrocarbon reservoirs. The method utilizes a combination of acoustic sensors, rotational rate sensors, acceleration sensors, and pressure sensors, all of which are associated with very precise timing procedures. Using these sensors and software utilizing a Kalman filter, the unique method allows for the determination of the absolute orientation of an imaginary line drawn between the ends of two opposite arms of the four arms of a seabed node.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS [0021] Figure 1 is a plan view of the configuration of a seabed node. [0022] Figure 2 is a plan view diagram showing the operation of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

[0023] FIG. 1 shows a standard configuration of a seabed node 10. The seabed node 10 has four flexible arms 14, which extend outwardly perpendicularly from housing 22. In the present example, the flexible arms 14 each extend four meters from the center of the housing 22. Contained within the housing 22 are battery 18, receiver electronics 20, and magnetic receiver 16. Generally, seabed node 10 has two magnetic receivers 16, which may be alternatively located on the flexible arms 14 or extending outwardly perpendicularly from housing 22.

[0024] The seabed node 10 has four electrical receivers 12. As shown in FIG. 1 , electrical receivers 12 are located at the ends 24 of the each of the flexible arms 14. One of the key issues for the seabed logging industry to be operationally effective in future developments will be the availability of precise seabed node attitude and orientation. This is more accurately defined as the absolute - with respect to North - orientation of the receiving elements (electrical receivers 12) of the seabed node 10.

[0025] The electrical receivers 12 which are opposite each other are separated by eight meters. The knowledge of the absolute orientation with respect to North of these receivers 12 will significantly improve the capability to process the logged electromagnetic data to give a very focused picture of the reservoir. A precise knowledge of the absolute orientation of the electrical receivers 12 will allow the positioning and timing accuracy of the collected data to be reduced significantly, which would speed up the data acquisition process in the field. If the seabed node 10 were to rest on the seabed as shown in FIG. 1, the absolute orientation of the housing 22 would allow for a relatively simple determination of the orientation of the receivers 12 because the flexible arms 14 would be totally perpendicular to the housing 22. However, these arms are necessarily flexible, and they will not always remain perpendicular to the housing 22.

[0026] FIG.2 shows the seabed node 10 in position on the seabed 30. As can be seen, the arms 14 are in a flexed state and have not remained perpendicular to the housing 22. The present invention allows for a determination of the orientation of imaginary lines 32 (shown as dotted lines) drawn between the ends 24 of the opposite sets of flexible arms 14. There are shown two imaginary lines 32 corresponding to two sets of two flexible arms 14.

[0027] The method of the present invention utilizes a combination of acoustic sensors, rotational rate sensors, acceleration sensors, and pressure sensors associated with very precise timing to allow the absolute orientation of the electrical receivers 12 to be determined. There are shown four acoustic sensors 34 located at the corners of the housing 22. These acoustic sensors 34 are used in conjunction with the electrical receivers 12 (as shown in FIG. 1) to determine the location of the receivers 12 at the far ends of the arms 14 with respect to the nearest edges of the housing 22 through a process of range-range acoustic positioning triangulation 36. The triangles of triangulation 36 are shown by dashed lines each having two corners corresponding to adjacent corners of the housing 22 and a third corner corresponding to an end 24 of a flexible arm 14. [0028] Through the use of inertial sensors contained in the housing 22, the method of the present invention allows for the determination of the orientation of the housing 22 with respect to North; more specifically, the orientation with respect to the earth's rotation and hence, North. The inertial sensors (not shown) contained in housing 22 include rate and acceleration sensors. In addition, a pressure sensor (also not show) contained in the housing 22 allows for determination of the absolute depth of the seabed node 10. The accelerometers allow for adjust the software for local gravity and hence are able to provide attitude (i.e. the pitch and roll) information for the seabed node 10 as well. [0029] The information provided by the sensors will be uniquely integrated with specific software utilizing a Kalman filter. The results produced from the software will be the absolute orientation of the imaginary lines 32, the depth of the seabed node 10, and the attitude of the node 10. [0030] The foregoing disclosure and description of the invention is illustrative and explanatory thereof. Various changes in the details of the described method can be made within the scope of the appended claims without departing from the true spirit of the invention.

Claims

CLAIMS I claim:

1. A method of determining absolute orientation and attitude of seabed nodes used in electromagnetic sensing of hydrocarbon reservoirs or seabed logging, said method comprising the steps of: positioning a seabed node (10) on a seabed (30), said seabed node (10) having a housing (22) and four flexible arms (14) in a flexed state and in a non-perpendicular extended relation to said housing (22), the flexible arms (14) being paired in sets and positioned opposite each other and having ends (24) extended from said housing (22); determining orientation of imaginary lines (32) drawn between the ends (24) of opposite sets of the flexible arms (14), each end (24) having a receiver (12); determining location of the receivers (12) at far ends of the flexible arms (14) with respect to nearest edges of the housing (22) through a process of range-range acoustic positioning triangulation (36), said housing (22) having acoustic sensors (34) located at corners of the housing (22) and used in conjunction with the electrical receivers (12); determining orientation of the housing (22) with respect to North by rotation of Earth through use of inertial sensors contained in the housing (22); determining absolute orientation of the electrical receivers (12) utilizing a combination of acoustic sensors, rotational rate sensors, acceleration sensors, and pressure sensors; and utilizing a Kalman filter with software to process information provided by the sensors .

2. The method of determining absolute orientation and attitude of seabed nodes according to Claim 1, said step of determining location of the receivers (12) further comprising: forming said triangulation (36) by two corners corresponding to adjacent corners of the housing (22) and a third corner corresponding to an end (24) of a flexible arm (14).

3. The method of determining absolute orientation and attitude of seabed nodes according to Claim 1, said step of determining absolute orientation of the electrical receivers (12), further comprising: pressure sensors contained in the housing (22) determining absolute depth of the seabed node (10).

4. The method of determining absolute orientation and attitude of seabed nodes according to Claim 1, said step of determining absolute orientation of the electrical receivers (12), further comprising: acceleration sensors adjusting software for local gravity, providing attitude information for the seabed node (10).

5. The method of determining absolute orientation and attitude of seabed nodes according to Claim 1 , wherein said information is comprised of absolute orientation of the imaginary lines (32), depth of the seabed node (10), and attitude of the seabed node (10)in the step of utilizing a Kalman filter with software.

6. A seabed node (10) comprising: four flexible arms (14); a housing (22), the flexible arms extending outwardly perpendicularly from said housing (22); a battery (18), contained in said housing (22); receiver electronics (20), contained in said housing (22); a magnetic receiver (16), contained in said housing (22); and four electrical receivers (12), being located at ends 24 of the each of the flexible arms (14) and having an absolute orientation with respect to North.

7. The seabed node (10), according to Claim 6, wherein the flexible arms (14) each extend four meters from a center of the housing (22).

8. The seabed node (10), according to Claim 6, wherein said magnetic receiver (16) is comprised of two receivers, being alternatively located on the flexible arms (14) or extending outwardly perpendicularly from said housing (22).

9. The seabed node (10), according to Claim 6, wherein said electrical receivers (12) are positioned opposite each other and separated by eight meters.

10. The seabed node (10), according to Claim 6, further comprising: four acoustic sensors (34) located at corners of the housing (22).