Communications – electrical: acoustic wave systems and devices – Seismic prospecting – Well logging
Reexamination Certificate
2000-07-14
2002-11-19
Toatley, Jr., Gregory J. (Department: 2838)
Communications, electrical: acoustic wave systems and devices
Seismic prospecting
Well logging
Reexamination Certificate
active
06483777
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates generally to methods and apparatus for ultrasonically imaging cased wells. More specifically, the invention relates to methods and apparatus for imaging and characterizing, with azimuthal resolution, (i) the annular region between the casing and the earth formations surrounding the casing and (ii) the wall surface of such earth formations.
2. Description of the Prior Art
In a well completion, a string of casing or pipe is set in a wellbore, and a fill material (typically cement) is forced into the annulus between the casing and the earth formations. The primary purpose of such cement is to separate oil- and gas-producing layers from each other, and from water-bearing strata.
If the cement fails to provide isolation of one zone from another, fluids under pressure may migrate from one zone to another, reducing production efficiency. In particular, migration of water into a hydrocarbon-bearing zone can, in some circumstances, render a well non-commercial. Also, migration of hydrocarbons into aquifers is environmentally and economically undesirable. Thus, imaging the annulus content, and, in particular, detecting interfaces between cement and a fluid-channel and/or between cement and the formation, is important for reliable determination of the hydraulic isolation of the different strata of a formation.
Current open-hole logging procedures—using electrical devices, such as Schlumberger's Fullbore Formation MicroImager (FMI), or acoustic devices, such as Schlumberger's Ultrasonic Borehole Imager (UBI)—stress the importance of imaging the formation wall. These imaging techniques allow for identification of hydrocarbon-bearing beds within the earth formations, and for detection of fractures, breakouts, and washouts, to help assess well stability; however, they do not work through casing.
It is widely reported that a significant percentage of existing cased wells were never imaged prior to encasement. There may be several reasons why such wells were never imaged prior to encasement, e.g., lack of adequate imaging technology, cost, etc. Today, however, imaging of existing cased wells can be desirable for, among other things, detection and identification of so-called bypassed pay (i.e., hydrocarbon-bearing) zones.
Another need for through-the-casing imaging exists in the process of hydraulic fracturing, which typically takes place after a well has been cased, and is used to stimulate the well for production. Often, the fracturing process is accompanied by sanding, whereby certain strata of the formation release fine sand that flows through casing perforations into the well, and then up to the surface, where it can damage production equipment. This problem can be remedied if the sand-producing zones are detected—as could be done, for example, with an imaging technology capable of operating through the casing.
Generally speaking, a cased well includes a number of interfaces at the junctures of the differing materials within the wellbore. A “first interface” exists at the juncture of the borehole fluid in the casing and the casing. (The casing is generally referred to as a “first material” and is typically comprised of steel.) A “second interface” is formed between the casing and a second material adjacent to the exterior of the casing. If cement is properly placed in the annulus, the “second interface” exists between the casing (i.e., the first material) and the cement (i.e., the second material). A “third interface” also exists between the cement and a “third material” (i.e., the formation).
The problem of investigating the fill material within the annulus has motivated a variety of cement evaluation techniques using acoustic energy. These techniques generally fall into two classes: (i) sonic cement evaluation and (ii) ultrasonic cement evaluation.
One sonic cement evaluation technique, described in U.S. Pat. No. 3,401,773, to Synott, et al., uses a logging tool employing a conventional, longitudinally spaced sonic transmitter and receiver. The received signal is processed to extract the portion affected by the presence or absence of cement. The extracted portion is then analyzed to provide a measurement of its energy, as an indication of the presence or absence of cement outside the casing. This technique provides useful information about cement detects at the second interface. However, sonic techniques have several limitations, such as: (i) poor azimuthal and axial resolutions, and (ii) strong sensitivity to the bond quality between the casing and the cement, thus requiring, in the cases of poor bond quality, internal pressurization of the casing, which, itself, can degrade cement integrity.
Ultrasonic cement evaluation tools, such as Schlumberger's Cement Evaluation Tool (CET) and UltraSonic Imager (USI) Tool, concentrate on the second interface to determine whether cement or mud is adjacent to the casing. in the annulus between the casing and the earth formation. The measurement obtained by these tools is based on a pulse echo technique, whereby a single transducer, pulsed with a broad-band signal (ie., 200-600 kHz), insonifies the casing at near-normal incidence, and receives reflected echoes. The method of measurement is based on exciting a casing resonance, measuring the temporal period and amplitude decay rate, and interpreting the data to determine whether cement or undisplaced mud lies adjacent to the casing. Such ultrasonic techniques, optimized to yield information about casing thickness, are described in U.S. Pat. No. 2,538,114 to Mason and U.S. Pat. No. 4,255,798 to Havira. The main limitation of these pulse echo techniques is that little of the acoustic energy (i.e., typically less than 10 percent) is transmitted through the casing to probe the annulus.
U.S. Pat. No. 5,011,676 to Broding purports to address the problem of primary and multiple reflections from a well casing interfering with formation reflection signals. Broding suggests elimination of interfering casing reflections by using one or several acoustic transducers directed on the casing at incident angles that fall between the compressional and shear critical angles of (borehole fluid)-steel interface. such that only shear waves are excited within the casing and no compressional waves propagate therein. The method relies on the fact that no signal is received so long as the cement-casing interface is regular, the annulus contains no channels or discontinuities, and the cement-formation interface is also smooth. Hence, when a signal is received by the transducer, one or more than one of these conditions is violated. And Broding does not offer a methodology on how to relate the received signal to the scatterer responsible for establishing it. Moreover, the Broding disclosure also states that when the transducer energy is directed at an angle larger than the shear critical angle, no energy is transmitted through the casing and into the annulus. Applicant herein has found this to be incorrect.
EP 0549 419 B1 to Stanke et al. disclose a method and apparatus to determine hydraulic isolation of oilfield casings by considering the entire volume of the annulus between the casing and the earth formation, and characterizing the third interface formed at the juncture of a second material, contacting the outside of the casing, and a third material adjacent to, and outside, the second material. Interrogation of the “third interface” is performed by directing an acoustic pulse at a segment of the casing. Ultrasonic transducers aligned along the casing axis—at angles of incidence, with respect to the casing inner wall, falling within the compressional and shear critical angles of a water-steel interface, i.e., about 14 to 27 degrees—such that shear signals within the casing are optimized and compressional signals within the casing are excluded. To effectively track a third-interface echo as the annulus thickness varies, a receiver array and complex signal processing algorithm are required. Additionally, the measurement would be a
Batzer William B.
Ryberg John J.
Schlumberger Technology Corporation
Toatley , Jr. Gregory J.
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