1.1 Field of the Invention
This invention relates generally to the investigation of subsurface earth formations, and, more particularly, to techniques for monitoring formation properties and to place wells with greater accuracy using tilted or transverse magnetic dipole sources or sensors housed within a modified tubular. This invention is applicable to induction or propagation type measurements, i.e., at low and high frequencies.
1.2 Description of Related Art
Petroleum is usually produced from oil reservoirs sufficiently far below a gas cap and above an aquifer. As the oil zone is being produced and depleted, the gas cap starts coning downward and the aquifer coning upwards towards the oil-bearing zone. Such migration can adversely affect the extraction of petroleum by creating pockets that are missed by the producer and by saturating the oil deposits with water. As soon as either gas or water hits the well, its oil production usually ceases instantly.
Reservoirs are monitored for changes in saturation and early signatures of coning so that corrective action can be taken. One approach utilizes pulsed neutron measurements, which measure formation sigma (indicative of saline water) or carbon-oxygen ratios (indicative of the ratio of hydrocarbon to water). The primary disadvantages of such pulsed neutron measurements are shallow depth of investigation and low accuracy in low porosities. A shallow measurement can be fooled by water channeling behind casing, and shallow re-invasion of well fluids into the open zones (e.g., the perforated zones) when the well is not flowing.
Measuring the electrical resistivity near a borehole has long been used to determine production zones in oil and gas fields and to map sand and shale layers. Electrical resistivity depends directly on porosity, pore-fluid resistivity, and saturation. Porous formations having high resistivity generally indicate the presence of hydrocarbons, while low-resistivity formations are generally water saturated.
Cross-well monitoring is an approach to monitoring changes in reservoir saturation. This technique is an outgrowth of radar experiments conducted in the early 1970s. See Michael Wilt, Exploring Oil Fields with Crosshole Electromagnetic Induction, SCIENCE AND TECHNOLOGY REVIEW, August 1996 (available at  less than http://www.llnl.gov/str/Wilt.htm greater than ); See also Q. Zhou et al., Reservoir Monitoring with Interwell Electromagnetic Imaging, SPWLA FORTIETH ANNUAL LOGGING SYMPOSIUM, May 30Jun. 3, 1999. With this technique, a transmitter is deployed in one well and a receiver is deployed in a second well. At the receiver borehole, the receiver detects components of the transmitted and induced currents for determination of the reservoir characteristics between the wells.
This approach has been studied for wells with fiberglass casing. The technique was used to monitor water-saturation changes in heavy oil zones undergoing steam flooding. See Michael Wilt et al., Crosshole electromagnetic topography: A new technology for oil field characterization, THE LEADING EDGE, March 1995, at 173-77. However, this technique is presently limited to closely spaced wells with either open-hole completions or cased with insulating composites. The disadvantages of these systems include the fragility and expense of fiberglass casing, making the technique impractical for use in production wells. Moreover, drilling a special well for monitoring is very expensive and therefore rarely done.
Another proposal for surveying and monitoring a reservoir is to deploy electrodes on the exterior of the casing. U.S. Pat. No. 5,642,051 (assigned to the present assignee) describes a casing, which has external insulation, electrodes, and cables for use in the completion. Its disadvantages include: the fragility of the external hardware and cable, the difficulty of running a complex completion into the well, the logistics of running a wire outside of the casing from surface to downhole, the inability to repair damaged or malfunctioning components, the difficulty to guarantee a hydraulic seal between the casing and the formation with external cables present, the possibility of cross-talk between these cables, the difficulty to place preamplifiers and other electronics near the electrodes, electrode impedance effects, and the influence of the cement annulus on resistivity.
Downhole techniques have been proposed utilizing slotted tubes or slotted liners. U.S. Pat. No. 5,372,208 describes the use of slotted tube sections as part of a drill string to sample ground water during drilling. A Slotted liner is a completion method used to prevent the wellbore from collapsing, and may also be used to prevent sand grains and other small particles from entering the wellbore and forming debris piles which may restrict fluid flow. A slotted liner is most often used in a horizontal well and is within a single producing formation. It is an alternative to leaving the hole completely open (i.e., with no casing), when an open hole may collapse or become blocked with debris. However, these types of slotted tubes or liners are not cemented in the wellbore, and do not provide hydraulic isolation from one well section to another. Slotted liners may be obtained from manufacturers such as Valley Perforating Co. of Bakersfield, Calif. (information available at  less than http://www.valleyperf.com/perf.htm greater than ). See also James J. Smolen, Production Logging In Horizontal Wells, SPWLA THIRTY-FIFTH ANNUAL SYMPOSIUM, workshop notes, Tulsa, Okla., Jun. 19, 1994.
These technologies have not been readily applicable to measurement and monitoring techniques using steel-cased production wells. The steel casing dramatically attenuates electromagnetic (EM) signals, restricting the possible use of known techniques primarily to qualitative detection of high resistive zones, but not for quantitative saturation measurements.
Conventional downhole measurement tools are implemented with transmitter and receiver arrays consisting of a set of coil antennas mounted on a support and axially spaced from each other in the direction of the borehole. A coil carrying a current can be represented as a magnetic dipole having a magnetic moment proportional to the current and the area encompassed by the coil. The direction and strength of the magnetic dipole moment can be represented by a vector perpendicular to the area encompassed by the coil. Typical well tools are equipped with coils of the cylindrical solenoid type comprised of one or more turns of insulated conductor wire.
In conventional induction and propagation logging systems, the transmitter and receiver antennas are generally mounted with their axes parallel to the longitudinal axis of the support member. Thus, these systems are implemented with antennas having longitudinal magnetic dipoles (LMD).
An emerging technique in the field of well logging is the use of tools incorporating antennas having tilted or transverse coils, i.e., where the coil""s axis is not parallel to the longitudinal axis of the support. These tools are thus implemented with antennas having a transverse or tilted magnetic dipole (TMD). One particular implementation uses a set of three antennas having non-parallel axes (referred to herein as tri-axial). The aim of these TMD configurations is to provide EM measurements with directional sensitivity to the formation properties, including information about resistivity anisotropy in vertical wells and directional sensitivity to bed boundaries that can be used for navigation. Logging instruments equipped with TMDs are described in U.S. Pat. Nos. 6,163,155, 6,147,496, 5,757,191, 5,115,198, 4,319,191, 5,508,616, 5,757,191, 5,781,436, 6,044,325, 4,264,862 and 6,147,496.
It is desirable to implement a through-casing system for monitoring the properties of subterranean formations using TMD antennas. It is also desired to implement a TMD antenna system for through-casing signal transfer to place wells with greater accuracy.
The invention provides a system for monitoring a property of a subsurface formation surrounding a borehole. The system comprises a tubular disposed within the borehole, the tubular being adapted for long-term or permanent disposal within the borehole, the tubular having a longitudinal axis with a central bore and tubular walls, the tubular including at least one slot formed therein such that the slot fully penetrates the tubular wall; and at least one antenna disposed within the tubular, the antenna being adapted to generate a magnetic dipole moment with a transverse or controllable orientation; wherein the antenna is positioned near the slot on the tubular.
The invention also provides a system for placing a borehole in the vicinity of a well in an earth formation. The system comprises a tubular disposed within the well, the tubular being adapted for long-term or permanent disposal within the well, the tubular having a longitudinal axis with a central bore and tubular walls, the tubular including at least one slot formed therein such that the slot fully penetrates the tubular wall; at least one antenna disposed within the tubular, the antenna being adapted to generate a magnetic dipole moment with a transverse or controllable orientation, wherein the antenna is positioned near the slot on the tubular; a drilling tool adapted to drill a borehole in the formation, the tool being connected at one end of a drill string; and at least one antenna disposed on the drilling tool or on the drill string.
The invention also provides a method for monitoring a property of a subsurface formation surrounding a borehole. The method comprises disposing a tubular within the borehole, the tubular being adapted for long-term or permanent disposal within the borehole, the tubular having a longitudinal axis with a central bore and tubular walls, the tubular including at least one slot formed therein such that the slot fully penetrates the tubular wall; disposing at least one antenna within the tubular, the antenna being adapted to generate a magnetic dipole moment with a transverse or controllable orientation; positioning the at least one antenna near the slot on the tubular; and transmitting or receiving a signal with the antenna to determine the formation property.
The invention also provides a method for placing a borehole in the vicinity of a well in an earth formation. The method comprises disposing a tubular within the well, the tubular being adapted for long-term or permanent disposal within the well, the tubular having a longitudinal axis with a central bore and tubular walls, the tubular including at least one slot formed therein such that the slot fully penetrates the tubular wall; disposing at least one antenna within the tubular, the antenna being adapted to generate a magnetic dipole moment with a transverse or controllable orientation; positioning the at least one antenna near the slot on the tubular; deploying a drilling tool connected at one end of a drill string within the formation, the tool or the string including at least one antenna adapted to transmit or receive a signal; transmitting or receiving a signal between the at least one antenna disposed within the tubular and the antenna on the drilling tool or the drill string; and using the transmitted or received signal to guide the placement of the borehole.