Method for identifying formation fractures surrounding a well casing

Abstract: An acoustic borehole logging tool traverses a fluid-filled cased well with an acoustic energy transmitter. Pressure waves created in the well fluid by the transmitter generate tube waves in the well casing. A closely spaced receiver detects tube waves that have travelled along the cased wellbore and been reflected by fracture interfaces. These reflected tube waves are recorded and used to identify the top and bottom of any fracture interval in the formation behind the well casing. This method can also be appoied to determination of the effectiveness of a hydraulic fracturing operation in a subsurface formation behind a well casing.

Description: The present invention relates in general to a method for identifying formation fractures surrounding a well casing and, more particularly, to a method for detecting the presence of hydraulically induced fractures behind such well casing. The invention also relates to a method of detecting the effectiveness of a hydraulic fracturing operation in a subsurface formation behind a well casing. It has long been known to acoustically log open wellbores to determine the velocities of compression ("P") waves and shear ("S") waves travelling through rock formations located in the wellbore region, and tube waves ("T") travelling along the wellbore interface. Logging devices have been used for this purpose which normally comprise a sound source (transmitter) and one or more receivers disposed at pre-selected distances from the sound source. By timing the travel of compression waves, shear waves, and/or tube waves between the transmitter and each receiver, it is normally possible to determine the nature of surrounding rock formations including natural fracture identification. Descriptions of various logging techniques for collecting and analyzing compression wave, shear wave, tube wave, and secondary wave data, can be found in U.S. Patent Nos. 3 333 238 (Caldwell); 3 356 117 (Loren); 3 362 011 (Zemanek, Jr.); Reissue No. 24 446 (Summers); 4 383 308 (Caldwell) and 4 715 019 (Medlin et al.); and in "The Correlation of Tube Wave Events With Open Fractures in Fluid-Filled Boreholes" by Huang and Hunter in Geological Survey of Canada, pages 336-376, 1981. In each of the foregoing references, the acoustic waves are generated in the formation of response to an acoustic energy transmission from within an open wellbore. However, the disclosure of such references is not applicable to the identification of induced fractures in formations which are traversed by well casing, that is, well pipe cement bonded to the formation. It is an object of the present invention to provide for a method of logging a cased well to detect hydraulically induced fractures, in the formations surrounding the cased well. According to the essence of the invention there is provided a method of acoustic well logging for identifying formation fractures behind well casing according to claim 1. Further features of the invention are defined in the dependent claims. Reference is now made to the accompanying drawings, in which: Fig. 1 illustrates an acoustic borehole logging system for use in carrying out the method of the present invention. Figs. 2 and 3 illustrate tube wave reflection logs recorded by the borehole logging system of Fig. 1. Fig. 4 is a schematic block diagram of electronic circuitry utilized in the operation of the borehole logging system of Fig. 1. The present invention is directed to a method for generating tube waves in a cased well and measuring reflections of such tube waves as they travel along the well casing to identify fracture intervals behind the well casing. Referring now to Fig. 1, there is illustrated an acoustic logging system that is used in accordance with the present invention for generating and receiving acoustic tube waves in well casing. The logging system includes an elongated logging tool 10 which is suspended from a cable 11 over a roll 25 within a cased well 12 which traverses a subsurface formation of interest 14. The cased well 12 comprising a casing 13 and a cement sheath 15 is filled with a fluid, such as a completion fluid 16. The logging tool 10 comprises an acoustic transmitter 17 and an acoustic receiver 19. Signals from the logging tool 10 are transmitted uphole by conductors within cable 11 to any suitable utilization system at the surface. For example, the utilization system is illustrated as comprising an uphole analysis and control circuit 22 and a recorder 24 in order that the output from circuit 22 may be correlated with depth. The logging tool 10 may be moved through the borehole at any suitable rate while it is operated to generate and receive acoustic pulses. Typically, the tool will be lowered to the bottom of the interval to be logged and then pulled upwardly during the logging measurements at a speed of at least 3,3m (10 feet) per minute. Somewhat greater logging speeds, e.g., 7m (20 feet) per minute, normally can be used. To carry out the method of the present invention, the acoustic transmitter 17 comprises at least one characteristic resonance frequency between a few hundred hertz and several kilohertz, such as between about 200 hertz and 2 kilohertz for example. Such a resonator type transmitter may preferably take the form of a double bender-type transducer, a flexure disc-type transducer or a Helmholtz type transducer. A suitable double bender-type transducer is described in U.S. Patent No. 4 718 046 to Medlin. A suitable flexure disc-type transducer is described in U.S. Patent Nos. 3 363 118 to Sims and 4 742 495 to Medlin and Slover. A suitable Helmholtz type transducer is described in U.S. Patent Nos 4 432 077 to Alhilali and Zemanek and 4 674 067 to Zemanek. Receiver 19 may be any of several types of hydrophones or ceramic devices conventionally used in acoustic borehole logging. Such receiver should preferably have a flat response over the acoustic frequency band and is closely spaced from the transmitter 17, such as no more than about 1 to 2m (3 to 5 feet) away. Transmitter 17 is utilized to generate acoustic energy imparting a pressure wave in the well fill-fluid 16. The transmitter 17 is excited to resonance either by a transient, high-voltage pulse or by a fixed-frequency, sine wave tone burst of about 3 cycles applied across its active elements. In logging operations, the transmitter is excited repetitively at a rate which is typically in the range 1-10 per second. After each excitation, the pressure waves of the resonant acoustic energy cause outward and inward deflections of the well casing. These deflections traverse both upwardly and downwardly from the transmitter along the cased wellbore as tube waves. Direct arrivals of these tube waves are received at the receiver 19 within about a millisecond after excitation of the transmitter 17. As the tube wave move upwardly and downwardly in the cased wellbore from the transmitter and receiver they may be subject to partial reflection back along the cased wellbore to the receiver by the presence of tube wave impedance changes created by the effects of a hydraulically induced fracture behind the perforated well casing. The tube wave reflections start and end at the top and bottom of the fractured interval and may appear within this interval if there are local changes in fracture volume. These tube wave reflections are recorded and displayed in correlation with depth so as to identify the height of each fractured interval. With a tube wave frequency below about 2 kilohertz the principal tube wave reflections are from the fractured interval and the wellbore bottom. Above about 2 kilohertz the casing collars begin to act as prominent reflectors in intervals of poor cement bond. Consequently, the transmitter 17 is preferably energized to resonate below about 2 kilohertz. In addition to using a tube wave log of recorded reflections travelling through the well casing to locate hydraulically induced fractures and determine fracture heights or intervals, the present invention may be utilized to determine the effectiveness of a hydraulic fracturing operation through perforations in the well casing. Prior to hydraulic fracturing, the tube wave log is run in a cased well traversing the formation to be fractured. The formation is then fractured hydraulically at selected formation intervals through perforations in the well casing. After the hydraulic fracture treatment is complete, the tube wave log is again run in the cased well. The difference in these two tube wave logs indicates the effectiveness of the hydraulic fracture treatment. Typical hydraulic fracturing treatments are described in U.S. Patent Nos. 4 687 061 and 4 714 115 to D.C. Uhri. A typical tube wave log run in a cased well following a hydraulic fracturing operation is illustrated in Fig. 2 for identifying the top of a formation fracture. In this example, the receiver is shown as being monitored for 100 milliseconds after each transmitter excitation. At each excitation, the tube wave energy moves in both directions along the well casing. The downgoing tube wave produces a strong direct arrival wavelet as it passes the receiver closely spaced below the transmitter. Further along, it reaches the top of a fracture located below the logging tool and is partially reflected. The reflected tube wave energy travels back up the well casing and is detected as a weaker wavelet arriving some time later, such as about 90 milliseconds later, when the transmitter and receiver are located at about 348m (1140 feet). In similar manner, but not shown in Fig. 2, upgoing tube waves would be reflected downward from the bottom of a fracture located above the logging tool. As can be seen in Fig. 2, the reflected tube waves arrive in the receiver at progressively later times as the logging tool moves up the wellbore away from the fracture top. By projecting an event in each of the tube wave reflection wavelets, such as the first arrival, to the corresponding event in the direct wave, the fracture top is identified. Such a projection of first arrivals, is shown in Fig. 2. As noted in Fig. 2 such projection intersects the corresponding event on the direct arrival wavelets at a depth. slightly below 408m (1340 feet). It is this depth point that is identified in Fig. 2 as the fracture top. A complete log of a formation interval will identify both the top and bottom of each fracture interval. Such a log is shown in Fig. 3. The fracture top is identified from the intersection of the projected selected tube wave reflection event and the corresponding direct arrival event at about 415m (1360 feet). The fracture bottom is identified from the intersection of the projected selected tube wave reflection event and the corresponding direct arrival event at about 448M (1470 feet). The tube wave reflection event may be selected from any of several common features on each tube wave reflection, such as the first arrival as clearly seen in Fig. 2, a maximum amplitude or an amplitude envelope peak. In recording the tube wave reflection log, the circuitry of Fig. 4 may be employed. A control circuit 22 includes a depth encoder or controller 28, a surface control panel 20, a signal digitizer 27, and an uphole signal amplifier 26. A downhole tone burst generator and power amplifier 21 in the logging tool 10 provides preferably a gated sine wave excitation to the transmitter 17. Transmitter 17 responds with a characteristic resonant vibration. A commercial version of the generator and power amplifier 21 is the Model 7060 Generator supplied by Exact Electronics, Hillsboro, Oregon with a Model MC 2500 Power Amplifier supplied by McIntosh Laboratory, Binghamton, NY. Amplitudes are in the range of 100-150 volts. This voltage is sufficient for regenerating acoustic source levels from the transmitter which will provide detectable acoustic signals in conventional borehole logging tools. The output of receiver 19 is amplified by a downhole signal amplifier 23 and carried over cable 11 to the uphole signal amplifier 26. A continuous measurement of the receiver 19 output amplitude is provided by the amplifier 26. Such detector provides a way to sense and record amplitudes much more rapidly than with a conventional signal analyzer. This allows successive traces to be recorded at one-foot intervals while moving the logging tool along a borehole at normal logging speeds. Foot-by-foot recordings of traces such as shown in Fig. 3 constitute the tube wave reflection log. Tone burst excitation of the acoustic transmitter at one-foot intervals is triggered by the depth encoder 28 of Fig. 4. This encoder produces a triggering pulse at each one-foot interval of depth as a tool moves up the borehole. This pulse is also used to condition the signal digitizer 27 to start digitizing the output of amplifier 26. The output of signal digitizer 27 is sent to recorder 24. This cycle is repeated when the depth encoder produces the next triggering pulse for the next one-foot depth point. Since the transmitter-to-receiver spacing is small the delay due to travel of tube waves from the transmitter to the receiver can be neglected. To properly display the large range of amplitude variations in each trace by this method of recording requires an extended vertical scale. Very long records are needed to cover borehole intervals of practical interest. It will be understood that numerous modifications or alterations may be made within the scope of the appended claims.

Claims: A method of acoustic well logging for identifying formation fracture interfaces behind well casing comprising the steps of: (a) traversing a fluid-filled cased well penetrating a subsurface formation with a well logging tool containing an acoustic energy transmitter; (b) exciting said transmitter with a plurality of tone bursts of fixed frequency to drive said transmitter to resonate and produce acoustic energy tube waves in said cased wellbore; (c) receiving at a receiver, which is closely spaced to said transmitter, tube waves which have travelled through said cased well bore directly from said transmitter toward said receiver. (d) receiving at said receiver reflected tube waves which have travelled along said cased wellbore away from said transmitter and receiver and have been partially reflected back toward said receiver from a point along said cased wellbore adjacent a hydraulically induced fracture; (e) recording the arrivals of said directly received tube waves and said reflected tube waves at said receiver; and (f) identifying said hydraulically induced fracture behind the cased wellbore as being located at a depth point at which a common event on the recording of said reflected tube waves is projected to coincide with a corresponding event on the recording of said directly received tube waves. A method according to claim 1, wherein the frequency of said tone bursts is selected to minimize tube wave reflections from well casing. A method according to claim 1 or 2, wherein the frequency of said tone burst is no greater than substantially 2 kilohertz. A method according to claim 1, 2 or 3, wherein said recording of said directly received tube waves and said reflected tube waves is in the form of a variable density log. A method according to claim 1, 2, 3 or 4, wherein said common event in said reflected tube waves is one of the peak amplitudes of said reflected tube waves. A method according to claim 1, 2, 3 or 4, wherein said common event is a first arrival. A method according to any preceding claim, further comprising the step of identifying the bottom of a fracture interval from said reflected tube waves, which have initially travelled up the cased wellbore from the transmitter and receiver and thereafter have been reflected down the cased wellbore toward the receiver when the transducer and receiver are outside the fracture interval. A method according to any preceding claim, further comprising the step of identifying the top of a fracture interval from said reflected tube waves which have initially travelled down the cased wellbore from the transmitter and receiver and thereafter have been reflected back up the cased wellbore when the transducer and receiver are outside the fracture interval. A method according to any preceding claim, wherein said tone bursts are in the order of 3 cycles. A method of determining the effectiveness of a hydraulic fracturing operation in a subsurface formation behind a well casing using the method of acoustic well logging according to any of claims 1 to 9, comprising the further steps of : (g) after step (e) hydraulically fracturing said subsurface formation through perforations in said cased wellbore; (h) after step (g) repeating steps (a) through (e) following the hydraulic fracturing of said cased wellbore; (i) after step (h) comparing the recording of said directly received tube waves and said reflected tube waves before and after the hydraulic fracturing of said subsurface formation to identify those of said reflected tube waves occurring only after said hydraulic fracturing; (j) after step (i) projecting a reflected tube wave event common to each of the reflected tube waves occurring after hydraulic fracturing from the point of origin of said common event on the recording of said reflected tube waves to a corresponding event on the recording of said directly received tube waves; and (k) as step (f), identifying a formation fracture tip behind the well casing as being located at a depth point at which said corresponding event occurs on the recording of said directly received tube waves.

Inventors: MEDLIN WILLIAM LOUIS, MEDLIN, WILLIAM LOUIS
Applicants: MOBIL OIL CORP, MOBIL OIL CORPORATION
Application year: 1990
Main classification: G01V1
IPC codes: G01V1