In order to increase the production of hydrocarbons (e.g., oil and gas) from formations, various fracturing techniques may be used. For example, in “hydraulic fracturing” a fracturing fluid is forced, under high pressure, down a cased, perforated borehole. The fracturing fluid enters the formation surrounding the borehole and creates and/or opens fractures within the formation. In some cases, a “proppant” is included with the fracturing fluid. When the pressure of the fracturing fluid is released, and the proppants remain in the fractures of the formation to keep the fractures propped open.
It is desirable to know the extent that fracturing has occurred within a formation, particularly the “vertical” extent of the fracturing (i.e., the distance in relation to the axis of the borehole). While tools exist to estimate the extent of the fracturing, any technique that can more accurately determine the extent of the fracturing provides a competitive advantage.
For a detailed description of exemplary embodiments, reference will now be made to the accompanying drawings in which:
Certain terms are used throughout the following description and claims to refer to particular system components. As one skilled in the art will appreciate, oilfield service companies may refer to a component by different names. This document does not intend to distinguish between components that differ in name but not function.
In the following discussion and in the claims, the terms “including” and comprising” are used in an open-ended fashion, and thus should be interpreted to mean “including, but not limited to . . . .” Also, the term “couple” or “couples” is intended to mean either an indirect or direct connection. Thus, if a first device couples to a second device, that connection may be through a direct connection or through an indirect connection via other devices and connections.
“Gamma” or “gammas” shall mean photon energy created and/or released due to neutron interaction with atoms, and in particular atomic nuclei, and shall include such energy whether such energy is considered a particle (i.e., gamma particle) or a wave (i.e., gamma ray or wave).
“Spectrum” shall mean the expected and/or measured counts of gammas having particular energy or energies, the gammas created by decay of a single type of radioactive element.
“Basis matrix” shall mean a plurality of spectra, for example, one spectrum for each type of radioactive element within an earth formation.
“Measured spectrum” shall mean, for a particular gamma detector, a plurality of count values of energies of gammas, each count value based on gammas counted having energies within a particular energy range or window.
The following discussion is directed to various embodiments of the invention. Although one or more of these embodiments may be preferred, the embodiments disclosed should not be interpreted, or otherwise used, as limiting the scope of the disclosure, including the claims. In addition, one skilled in the art will understand that the following description has broad application, and the discussion of any embodiment is meant only to be exemplary of that embodiment, and not intended to intimate that the scope of the disclosure, including the claims, is limited to that embodiment.
The various embodiments were developed in the context of wireline logging tools measuring the extent of fracturing caused by a fracturing (or “frac”) operation, and thus the description that follows is based on the developmental context; however, the various systems and methods find application not only in wireline logging tools regardless of the number of cables used in the logging tools, but may also find application in measuring-while-drilling (MWD), logging-while-drilling tools (LWD), and slickline (memory) logging. Thus, the developmental context shall not be construed as a limitation as to the applicability of the various embodiments.
In some embodiments the neutron source 210 is an Americium-Beryllium (AmBe) source, which release neutrons having average energies of about 4 Mega-electron Volts (MeV). However, any neutron source capable of producing and/or releasing neutrons with sufficient energy (e.g., a deuterium/tritium source, or a Californium (Cf) source) may be equivalently used. An AmBe source and Cf source release neutrons at all times (i.e., neutron release is not controllable). In cases where release of neutrons from the neutron source is controllable, under command from surface computer 22 or computer system 206 the tool may selectively produce and/or release the energetic neutrons. In order to reduce the irradiation of the gamma detectors 204 and other devices by energetic neutrons from the neutron source 210, neutron shield 208 (e.g., HEVIMET® available from General Electric Company of Fairfield, Conn.) may be disposed between the neutron source 210 from the gamma detectors 204.
Because of the speed of the energetic neutrons, and because of collisions of the neutrons with atomic nuclei that change the direction of movement of the neutrons, a neutron flux is created around the logging tool 10 that extends into the formation 14. The interactions of the neutrons with materials in the borehole and in the formation 14 produce gammas. Some gammas are created by way of inelastic collisions and/or thermal capture that result in prompt gamma production. Gammas created by way of inelastic collision and/or thermal capture that result in prompt gamma production are not of interest in the various embodiments. However, the interaction of the neutrons with the materials in the formation may also make some otherwise non-radioactive materials radioactive, with the newly radioactive elements decaying with particular half lives by release of gammas. The energy of the gamma resulting from the decay is indicative of the material from which the gamma is released. Moreover, other materials in the formation (e.g., Potassium, Uranium and Thorium), while not made radioactive by the neutron interaction, are nonetheless radioactive, and these naturally radioactive materials also decay, with particular half lives, by producing gammas.
At least some of the gammas created by radioactive decay of various materials are incident upon the gamma detectors 204. Referring to gamma detector 204A as indicative of both gamma detectors 204, a gamma detector comprises an enclosure 212, and within the enclosure 212 resides; a crystal 216 (e.g., scintillation crystal); a photo multiplier tube 218 in operational relationship to the crystal 216; and a processor 220 coupled to the photomultiplier tube 218. As gammas are incident upon/within the crystal 216, the gammas interact with the crystal 216 and flashes of light are emitted. Each flash of light itself is indicative of an arrival of a gamma, and the intensity of light is indicative of the energy of the gamma. The output of the photomultiplier tube 218 is proportional to the intensity of the light associated with each gamma arrival, and the processor 220 quantifies the output as gamma energy and relays the information to the surface computer 22 (
Illustrative count values for each energy bin are shown in
During the fracturing process, the fractures may propagate throughout the formation. For example, if not carefully controlled, a fracture may extend beyond a zone of interest into zones that are not productive. If the fracture extends into non-productive zones (e.g., water bearing zones), adverse economic consequences may result. Thus, the various embodiments are related to logging systems that are used to ensure that the fractures do not extend beyond the zone of interest.
In accordance with various embodiments, the logging system 400 determines in the axial extent (i.e., the distance in relation to the axis of the borehole) of travel of the formation treatment materials used to create the fractures 32. In particular, system 400 comprises a logging tool 40 comprising a pressure vessel 16 that houses a neutron source 210, a gamma detector 204, and in some embodiments a computer system 206. The gamma detector 204 may be on the order of 6 feet from the neutron source 210; however, other spacing may be equivalently used. In the particular embodiment, the logging tool 20 is initially lowered into the borehole 12 by way of the cable 18 before the formation 14 is fractured. Because the neutron source 210 of the logging tool 50 is above the gamma detector 204, logging initially occurs during downward travel of the logging tool 50. Because of the distance between the source 210 and the detector 204, and the logging speed, the single gamma detector 204 of these embodiments senses gammas produced only by naturally radioactive materials present in the formation 14. In alternative embodiments, logging may occur during upward travel if the neutron source is below the gamma detector. The surface computer 22 (or the computer system 206) generates a measured spectrum of the energies of the gammas sensed. The measured spectrum is analyzed with a basis matrix to determine background elemental concentrations of the naturally radioactive materials present in the formation (e.g., Potassium, Uranium and Thorium).
In some embodiment, the background elemental concentrations of the naturally radioactive materials present in the formation 14 may be determined by a spectral fitting technique, such as a weighted least squares technique. The surface computer 22 (or the computer system 206) uses the weighted least squares technique to determine the elemental concentrations of the materials in the formation 14 based on the following equation:
[C]=[A]·[M] (1)
where [C] is the measured spectrum of the energies of the gammas sensed by the detector, [A] is a basis matrix, and [A] is the elemental concentrations of the materials in the formation. In some embodiments, [C] is a i-by-1 matrix that represents the measured spectrum of the energies of the gammas sensed by the detector, [A] is a i-by-j matrix of the elemental spectra, and [A] is a j-by-1 matrix of the elemental concentrations of the materials in the formation. In the particular embodiment, i is the number channels the spectrum is divided into (see
In the particular embodiment of
After the logging tool 40 has read the spectrum for the naturally occurring radioactive materials, the logging tool 40 is either lowered to a particular borehole depth or removed. Whether lowered to a particular borehole depth or removed, thereafter, the formation 14 is fractured using a fracturing technique. In particular, the fracturing technique uses formation treatment materials (e.g., a fracturing fluid, an acidizing fluid or a proppant) comprising radiation activated materials. The radiation activated materials are initially inactive (i.e., non-radioactive), but when the radiation activated materials interact with the neutron released by the neutron source 210, the radiation activated materials will become radioactive and produce gammas.
In alternative embodiments, the formation 14 is fractured with radiation activated materials in the treatment materials prior to lowering the logging tool 40 to determine the background elemental concentrations of naturally radioactive materials present in the formation 14. Thus, the logging tool 40, by logging away from the neutron source 210, may be used to determine the background elemental concentrations of naturally radioactive materials present in the formation 14 after the fracturing.
In particular embodiment of
After the formation 14 is fractured using formation treatment materials comprising radiation activated materials, the logging tool 40 is raised at a predetermined rate (e.g., 5 feet per minute) by way of the cable 18. The neutrons interact with materials in the formation 14 and the materials in the formation produce gammas whose energies are sensed by the gamma detector 204. Because the logging tool 40 is raised at a slow speed (e.g., 5 feet per minute), by the time the detector 204 arrives at a particular borehole depth previously passed by the neutron source, the prompt gamma production has substantially died away, and the single gamma detector 204 of these embodiments senses gammas produced by naturally radioactive materials present in the formation 14 and gammas produced by materials made radioactive by the neutron radiation. In some embodiments, the surface computer 22 (or the computer system 206) generates a measured spectrum of the energies of the gammas sensed. The measured spectrum is analyzed with the background elemental concentrations using a spectral fitting technique to determine elemental concentration of radiation activated materials, and the axial extent (i.e., the distance in relation to the axis of the borehole) the formation treatment materials with radiation activated materials have migrated within the formation is determined based on the elemental concentration of radiation activated materials.
In the particular embodiment of
In alternative embodiments, during the first pass the surface computer 22 (or the computer system 206) generates (at plurality of borehole depths) the measured spectrum [C]b of the energies of the gammas produced by naturally radioactive materials in formation 14, and the measured spectrum [C]b is stored in a memory of the surface computer 22 (or the computer system 206) with the corresponding borehole depth. Thereafter, during the second pass of the logging tool 40 the surface computer 22 (or the computer system 206) generates (at the plurality of the borehole depths) the measured spectrum [C]v. The surface computer 22 (or the computer system 206) access the measured spectrum [C]b from the memory and simultaneously analyzes measured spectrums [C]b and [C]v (e.g., using the weighted least squares technique and equation (1)) to determine the axial extent of the formation treatment materials with radiation activated materials in the formation 14.
In some embodiments, the logging system 500 may determine, in real-time and in a single pass through the borehole 12, the axial extent of the formation treatment materials with radiation activated materials used to create the fractures 32. Because the neutron source 210 of the logging tool 50 is below the gamma detectors 204, logging occurs during downward travel of the logging tool 50 at a predetermined rate (e.g., 5 feet per minute). As the logging tool 50 is lowered in the borehole 12, the neutron source 210 (e.g., a Cf source) continuously releases neutrons into the formation. The neutrons interact with materials in the formation 14 and the materials in the formation produce gammas whose energies are sensed by the detectors 204. The materials in the formation 14 comprise naturally radioactive materials present (e.g., Potassium, Uranium and Thorium) in the formation 14, and radiation activated material in the formation as delivered by the formation treatment materials.
Like the embodiments of
For a particular borehole depth, the surface computer 22 (or the computer system 206) generates a measured spectrum of the energies of the gammas sensed by the “far” gamma detector 204A, and determines the elemental concentrations of Potassium, Uranium and Thorium using the weighted least squares technique and equation (1). In this case, the basis matrix [A]b is a combination of spectra of Potassium, Uranium and Thorium. The surface computer 22 (or the computer system 206) analyzes the measured spectrum of the energies sensed by the gamma detector 204A [C]f with the basis spectrum [A]b to determine [M]b in equation (1), the elemental concentrations of Potassium, Uranium and Thorium. In the particular embodiment of
The “near” gamma detector 204B senses energies of gammas produced by naturally radioactive materials present in the formation (e.g., Potassium, Uranium and Thorium) and radiation activated material. The surface computer 22 (or the computer system 206) generates a measured spectrum [C]n of the energies of the gammas sensed by the “near” gamma detector 204B. The basis matrix [A]v is a combination of the spectra of Potassium, Uranium and Thorium, and spectra of radiation activated material. The surface computer 22 (or the computer system 206) analyzes the measured spectrum of the energies sensed by the gamma detector 204B [C]n with the basis spectrum [A]v at the particular borehole depth to determine [M]v (i.e., elemental concentration of radiation activated material) in equation (1). The surface computer 22 (or the computer system 206) uses the weighted least squares technique and constrains the elemental concentrations of Potassium, Uranium and Thorium in [M]v, to determine the elemental concentration of the radiation activated material. The surface computer 22 (or the computer system 206) calculates the axial extent of the formation treatment materials with radiation activated materials in the formation 14 at the particular borehole depth based on the elemental concentration of the radiation activated material. In particular embodiment of
The energies of the gammas produced by decay of Potassium and Vanadium are close, approximately 1.46 MeV and 1.43 MeV respectively. Because the Vanadium and Potassium spectra are very similar and cannot be easily distinguished the various embodiments so far are related to determining axial extent by utilizing a logging tool with one gamma detector and two passes through the borehole, and a logging tool with two gamma detectors and one pass through the borehole.
The various embodiments discussed so far are related to creating fractures in the formation using formation treatment materials comprising radiation activated material Vanadium. In alternative embodiments, radiation activated material Indium may equivalently be used in formation treatment materials to create fracture in the formation. Returning to
In the particular embodiment, the basis matrix [A] is a combination of spectra of Potassium, Uranium, Thorium and Indium. The surface computer 22 (or the computer system 206) analyzes (e.g., using the weighted least squares technique) the measured spectrum of the energies sensed by the gamma detector 204 [C] at the particular borehole depth with the basis spectrum [A] to determine [M] in equation (1), the elemental concentrations of Potassium, Uranium, Thorium and Indium. Because of the spectral uniqueness of Indium, the elemental concentrations of Potassium, Uranium and Thorium in [M] are not constrained. The surface computer 22 (or the computer system 206) calculates the axial extent of the formation treatment materials with radiation activated material Indium in the formation 14 based on the elemental concentration of Indium.
From the description provided herein, those skilled in the art are readily able to combine software created as described with appropriate computer hardware to create a special purpose computer system and/or special purpose computer sub-components in accordance with the various embodiments, to create a special purpose computer system and/or computer sub-components for carrying out the methods of the various embodiments and/or to create a computer-readable media that stores a software program to implement the method aspects of the various embodiments.
The above discussion is meant to be illustrative of the principles and various embodiments of the present invention. Numerous variations and modifications will become apparent to those skilled in the art once the above disclosure is fully appreciated. For example, in the particular embodiment of
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20100224773 A1 | Sep 2010 | US |