This disclosure generally relates to a method and apparatus for the detection of a material within a region of the Earth.
This section is intended to introduce various aspects of the art, which may be associated with one or more embodiments of the present disclosure. This discussion is believed to assist in providing a framework to facilitate a better understanding of particular aspects of the present disclosure. Accordingly, it should be understood that this section should be read in this light, and not necessarily as admissions of prior art.
Oil production or transfer in ice-prone marine or freshwater locations could result in a subsurface release—for example from a well blowout or leaking pipeline—that results in oil trapped within or beneath ice. Oil-spill countermeasures will require that this oil is accurately located and mapped.
The detection of oil within or under ice has been of concern since the exploration and production for hydrocarbon resources in the Arctic began in the early 1970's. There have been numerous attempts to detect oil under ice using acoustics, optical/UV excitation, and ground penetrating radar (for a review, see “Detection and Tracking of Oil under Ice”, D. F. Dickins, report submitted to the US Department of the Interior Minerals Management Service, Oct. 6, 2000). All of these techniques have shown the capability to detect oil under ice with some success; however, they have not been used in the field. The methods proposed to date have a limited range of applicability and are susceptible to false positive results. They also have only a limited ability to “see” or detect oil through a layer of ice and require contact with the ice surface.
Notably, all three of these methods require access and traverse across the ice surface, some require the removal of snow cover, and special care must be taken to ensure good ice contact with the sensor. The surface access limitation presents both logistic and safety concerns such as breakthrough, and limits the coverage to a small area per day.
Dickins, et al. (2006) successfully detected oil under ice using ground-penetrating radar using a skid-mounted unit pulled along the ice surface (see “2006 Experimental Spill to Study Spill Detection and Oil Behavior in Ice”, D. F. Dickins, P. J. Brandvik, L. G. Faksness, J. Bradfor, and L. Liberty; report submitted to the US Department of the Interior Minerals Management Service, Dec. 15, 2006, contract number 1435-0106CT-3925). Tests with the system mounted in a helicopter were less conclusive although additional research is mentioned as being planned.
Nuclear magnetic resonance (“NMR”) is a tool used for the characterization of the molecular composition of liquids and solids. More particularly, in some applications NMR is used to distinguish between a solid (e.g. rock in the Earth) and a liquid (e.g. ground water or oil). NMR molecular characterization works by placing a sample in a static magnetic field to align the magnetic moments of the protons with the field. The proton magnetic moments are then perturbed using one or more radio frequency (RF) excitation signals. The energy released or emitted as these magnetic moments return to equilibrium is monitored by a receiver.
In the oil and gas industry, NMR is applied in reservoir characterization in the field for well logging measurements and in laboratory analysis of rock cores. The NMR logging tool technology is capable of directly detecting the signals from fluids in the rock pore space and differentiating between different types and phases of fluids. In well logging, a magnet and a radio frequency transmitter/receiver is lowered into the bore hole. NMR well logging tools, such as those in commercial use by oilfield service companies such as Schlumberger, Halliburton and BakerHughes, detect fluids in the pore space over a volume on the order of several cubic decimeters (dm3). An example of such a tool is CMR PLUS™ or MR SCANNER™ by Schlumberger.
NMR has also been used to detect aquifers (e.g. an underground formation including ground water). Such instruments typically utilize the Earth's magnetic field as the static magnetic field, detect a larger volume than the downhole devices (cubic meters (m3) rather than dm3), and are placed on the Earth's surface for operation. Examples of such a system are NUMIS™ and NUMIS PLUS™ by Iris Instruments and GMR™ by Vista Clara. These devices typically utilize a 100 meter diameter wire loop coil placed flat on the ground as the transmitting/receiving coil. The large loop coil permits the sensing of aquifers over a larger volume than the downhole devices (m3 v. dm3) and depths (up to 150 m). A measurement time of one hour or more per detection volume is typically required. Current NMR research in geophysical applications addresses difficulties that arise when attempting to identify liquids located in pores or at a surface between a liquid and a solid. See, e.g., PAPE, et al., Pore Geometry of Sandstone Derived from Pulsed Field Gradient NMR, J. of Applied Geophysics 58, pp. 232-252 (2006).
In U.S. Pat. No. 8,436,609, NMR is described in an application to detect liquid under a surface, in particular oil under ice or snow, using the Earth's magnetic field. The NMR coil is mounted to a helicopter to remotely detect the presence of oil under ice or snow.
While NMR tools have been used for a variety of applications, it is desired to improve the signal intensity including signal-to-noise ratio and ability to detect materials within a region of the Earth.
Other useful information may be found in the following references: U.S. Pat. No. 3,019,383; U.S. Pat. No. 4,022,276; U.S. Pat. No. 4,769,602; U.S. Pat. No. 4,868,500; Gev, et al., Detection of the Water Level of Fractured Phreatic Aquifers Using Nuclear Magnetic Resonance (NMR) Geophysical Measurements, J. of Applied Geophysics 34, pp. 277-282 (1994); SLICHTER, CHARLES P., Principles of Magnetic Resonance, 2nd Edition Springer Series in Solid-State Sciences, (1996); LEGCHENKO, et al., Nuclear Magnetic Resonance as a Geophysical Tool for Hydrogeologists, J. of Applied Geophysics 50, pp. 21-46 (2002); WEICHMAN, et al., Study of Surface Nuclear Magnetic Resonance Inverse Problems, J. of Applied Geophysics 50, pp. MOHNKE, et al., Smooth and Block Inversion of Surface NMR Amplitudes and Decay Times Using Simulated Annealing, J. of Applied Geophysics 50, pp. 163-177 (2002); SHUSHAKOV, et al., Hydrocarbon Contamination of Aquifers by SNMR Detection, WM'04 Conference, Feb. 29-Mar. 4, 2004, Tucson, Ariz.