Abstract:
The invention relates to DAS observation has been proven to be useful for monitoring hydraulic fracturing operations. While published literature has shown focus on the high-frequency components (&gt;1 Hz) of the data, this invention discloses that much of the usable information may reside in the very low frequency band (0-50 milliHz). Due to the large volume of a DAS dataset, an efficient workflow has been developed to process the data by utilizing the parallel computing and the data storage. The processing approach enhances the signal while decreases the data size by 10000 times, thereby enabling easier consumption by other multi-disciplinary groups for further analysis and interpretation. The polarity changes as seen from the high signal to noise ratio (SNR) low frequency DAS images are currently being utilized for interpretation of completions efficiency monitoring in hydraulically stimulated wells.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a non-provisional application which claims benefit under 35 USC §119(e) to U.S. Provisional Applications Ser. No. 62/305,758 filed Mar. 9, 2016, entitled “ LOW FREQUENCY DISTRIBUTED ACOUSTIC SENSING,” and Ser. No. 62/305,777 filed Mar. 9, 2016, entitled “PRODUCTION LOGS FROM DISTRIBUTED ACOUSTIC SENSORS,” which are incorporated herein in their entirety. 
     
    
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH 
       [0002]    None. 
       FIELD OF THE INVENTION 
       [0003]    The present invention relates generally to the use of distributed fiber optic sensing including Distributed Acoustic Sensing (DAS) and Distributed Thermal Sensing (DTS) in subterranean wellbores for hydrocarbon production. In particular this application is directed to detailed completion operations and production monitoring utilizing low frequency monitoring of fiber optic signals including DAS and DTS signals. 
       BACKGROUND OF THE INVENTION 
       [0004]    Fiber-optic cables can be installed in vertical and horizontal wells, which can be treatment wells, injector wells or observation wells. Within the cable there are often both single mode fibers for DAS and multi-mode fibers for DTS. Multiple fibers within one cable can offer redundancy and the ability to interrogate with different instrumentation simultaneously. 
         [0005]    DAS is the measure of Rayleigh scatter distributed along the fiber optic cable. A coherent laser pulse is sent along the optic fiber, and scattering sites within the fiber cause the fiber to act as a distributed interferometer with a pre-set gauge length. The intensity and phase of the reflected light is measured as a function of time after transmission of the laser pulse. When the pulse has had time to travel the full length of the fiber and back, the next laser pulse can be sent along the fiber. Changes in the reflected intensity and phase of successive pulses from the same region of fiber are caused by changes in the optical path length of that section of fiber. This type of system is very sensitive to both strain and temperature variations of the fiber and measurements can be made almost simultaneously at all sections of the fiber. 
         [0006]    Raw DAS data are usually in the form of optical phase, with a range from −pi to +pi. The optical phase is defined by the interference pattern of the back-scattered laser energy at two locations separated by a certain length (gauge length) along the fiber. The phase varies linearly with a small length change between these two locations, which can be interpreted as axial strain change of the fiber in between. Depending on the vender, the measured optical phase is sometimes differentiated in time before it is stored. In this case, the DAS data can be considered as linear scaled fiber strain rates. 
         [0007]    DAS has been used to monitor hydraulic fracturing operation. The applications include injection fluid allocation (e.g. Broone et al. 2015), hydraulic fracture detection (e.g. Webster et al. 2013), and production allocation (e.g. Paleja et al. 2015). However, these applications focus on the DAS signals that are in high frequency bands (&gt;1 Hz), and some applications only use the “intensity” of the signal (waterfall plot), which is obtained through a RMS averaging operation. 
         [0008]    DAS has been used extensively to measure strain in hydrocarbon wells. Hill, et al., (U.S. Pat. No. 8,950,482) monitor hydraulic fracturing during oil/gas well formation. Tubel, et al., (US20060272809) control production operations using fiber optic devices. Hartog, et al., (US20090114386) use an optical fiber as a distributed interferometer that may be used to monitor the conduit, wellbore or reservoir. Minchau (US20130298665) provides an in-situ permanent method for measuring formation strain in a volume around a treatment well. McEwen-King (US20130233537) acoustic data from distributed acoustic sensing is processed together with flow properties data to provide an indication of at least one fracture characteristic. This is in no way an all-encompassing review of the technology. A recent review was published by Webster (2013) and the field continues to advance rapidly. 
         [0009]    Current visualization and interpretation of DAS data as-provided by vendors and as seen from publications (e.g. Webster et al., 2013) are based on high-frequency (&gt;1 Hz) intensity analysis that contains useful but limited information. The high frequency information has been used by others for proppant allocation estimation, stimulation operation monitoring, production logging, and other useful observations. Unfortunately, the use of high frequency DAS signals for analysis, had led to the removal of lower frequency and ultra-low frequency signals from the data by many vendors. 
       BRIEF SUMMARY OF THE DISCLOSURE 
       [0010]    The DAS signal in the ultra-low frequency band from 0-50 milliHz (mHz) has the potential to explain the strain-field response due to the completions (in-well) and fracturing process (cross-well) at monitor wells in greater details ( FIGS. 3 and 4 ). Preliminary analysis indicates that DAS low frequency signals may help us understand some key characteristics of the hydraulic fracturing process: stage isolation, injection allocation, cluster efficiency and spacing, fracture geometry and network, stimulated reservoir volume, and the like. 
         [0011]    In one embodiment, a process for monitoring hydrocarbon production is described with the following steps:
   a) installing fiber optic cables along a wellbore in a hydrocarbon formation;   b) attaching an interrogator on the fiber optic cable;   c) interrogating the fiber optic cable with an interrogation signal;   d) obtaining one or more datasets from the interrogator;   e) converting the datasets from the interrogator into a continuous record;   f) transforming the continuous record with a low-pass filter with corer frequency at 1-50 milliHz while down sampling the data into a transformed well signal;   g) interpreting the transformed well signal; and   h) improving hydrocarbon production from said hydrocarbon formation.   
 
         [0020]    In another embodiment, a method of monitoring hydrocarbon production comprises, installing fiber optic cables along the wellbore in a hydrocarbon formation; interrogating the fiber optic cable with an interrogator; obtaining one or more datasets from the interrogator; converting the datasets from the interrogator into a continuous record; transforming the continuous record with a low-pass filter from 0-50 milliHz while down sampling the data into a transformed well signal; interpreting the transformed well signal; and monitoring hydrocarbon production from said hydrocarbon formation. 
         [0021]    In an additional embodiment, the effectiveness of a hydrocarbon well treatment is determined by:
   a) installing fiber optic cables along a wellbore in a hydrocarbon formation;   b) installing one or more interrogators on the fiber optic cables;   c) interrogating the fiber optic cable with an interrogation signal before, during, and/or after a well treatment;   d) obtaining one or more datasets from the interrogator;   e) converting the datasets from the interrogator into a continuous record;   f) transforming the continuous record with a low-pass filter with corner frequency at 1-50 milliHz while down sampling the data into a transformed well signal;   g) interpreting the transformed well signal; and   h) improving hydrocarbon production from said hydrocarbon formation.   
 
         [0030]    The transformed well signal may be an in-well signal from the wellbore of interest consisting of a production well or an injection well. Alternatively, the transformed well signal may be a cross-well signal from a wellbore that is adjacent to the well of interest consisting of a production well, and injection well, a monitoring well, a vertical well, or other well within the formation. 
         [0031]    The hydrocarbon formation may be monitored via fiber optic signals while it is undergoing hydraulic fracturing, start-up, shut-in, enhanced oil recovery, waterflood, steamflood, water alternating gas treatment, or other well treatment. 
         [0032]    The continuous record may be transformed with an ultra-low frequency filter by removing signals above 1 mHz, 5 mHz, 10 mHz, 15 mHz, 20 mHz, 30 mHz, 45 mHz, or 50 mHz. Additionally, the continuous record may be downsampled to 1, 3, 5, 10, 15, 20, 25, 50, 75, or 100 second intervals. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0033]    A more complete understanding of the present invention and benefits thereof may be acquired by referring to the follow description taken in conjunction with the accompanying drawings in which: 
           [0034]      FIG. 1  is a standard industry waterfall DAS plot; 
           [0035]      FIG. 2  is a Low Frequency filtered DAS plot; 
           [0036]      FIG. 3  is a DAS Low-frequency Cross-well Signal; and 
           [0037]      FIG. 4  is DAS Low-frequency In-well Signal. 
       
    
    
     DETAILED DESCRIPTION 
       [0038]    Turning now to the detailed description of the preferred arrangement or arrangements of the present invention, it should be understood that the inventive features and concepts may be manifested in other arrangements and that the scope of the invention is not limited to the embodiments described or illustrated. The scope of the invention is intended only to be limited by the scope of the claims that follow. 
         [0039]    Previously, Friehauf and Dean (US-2014-0358444, 2014) identify fractures in a formation having a wellbore including: (a) positioning a sensor within the wellbore, wherein the sensor generates a feedback signal representing at least one of a temperature and pressure measured by the sensor; (b) injecting a fluid into the wellbore and into at least a portion of the formation adjacent the sensor; (c) waiting a predetermined period of time; (d) generating a standstill simulated model representing at least one simulated temperature characteristic and at least one pressure characteristic of the formation during and after fluid injection; (e) shutting-in the wellbore for a pre-determined shut-in period; (f) generating a shut-in simulated model representing at least one simulated temperature characteristic and at least one pressure characteristic of the formation during the shut-in period; (g) generating a data model representing the standstill simulated model and the shut-in simulated model, wherein the data model is derived from the feedback signal; and (h) observing the data model for presence of fractures within the wellbore, wherein fractures are present when the temperature characteristics are lower than the temperature characteristics of other sections of the wellbore. 
         [0040]    Additionally, Grubb and Friehauf (US-2016-0003032, 2016) provide a logging tool to determine temperature of produced fluid flowing into or within a wellbore including: a core structure; an arm extendibly and pivotally mounted to the core structure, the arm is extended away from the core structure and is near the inner surface of the wellbore, wherein the arm pivots in one plane relative to the core structure; a data transfer device connected to the core structure for receiving, processing and storing data; and at least one temperature sensors attached to the arm, wherein the temperature sensor is located at a tip of the arm, wherein when the arm is extended away from the core structure the temperature sensor is at or near the inner surface of the wellbore. 
         [0041]    Unlike prior methods, this method uses transient temperature and strain changes that occur during the completion operation and production period. The signal can be caused by wellbore fluid movement, fracture propagation in the formation, and/or change of reservoir properties. 
         [0042]    Scattered light pulses from nearby segments of the fiber are combined in an interferometer. The phase differences between the pulses is then a measure of their relative separations of their origins. A change in separation as small as one one-hundredth of a wavelength can easily and accurately be measured. This typically amounts to a 10E-7 percentage change in fiber length (strain). Although DAS was originally designed to record fiber strains due to acoustic waves of frequencies up to 10 kHz, its response also extends to very low frequencies (&lt;0.05 Hz). It is these low-end frequency components in the DAS response that are caused by the mechanic strain or the thermal expansion from localized temperature variations that this invention utilizes. 
       Data Acquisition 
       [0043]    The fiber-optic cables are installed out of the casing and cemented in place. Numerous interrogators are available to record optical signals including single laser, dual laser, multiple laser, PINNACLE™ Gen-1 and Phase-1, HALLIBURTON™ FIBERWATCH®, PROXIMION™ FBG-Interrogator, NATIONAL INSTRUMENTS™ PXI, LUNA™, or other interrogator. The raw data may be sampled from 5-20 kHz continuously at more than 6000 locations (frequently referred as “channels”) with a variety of spatial separations from centimeters to meters along the fiber dependent upon length and required sensitivity. The gauge length may also be varied dependent upon fiber length and the interrogator used, from 1-100 meters, including 1, 5, 10, 25, 50, 75 and 100 meters. When recording, the measured optical phase is differentiated in time, so the raw DAS data is linearly associated with strain rate along the fiber. 
       Data Processing 
       [0044]    The data are down-sampled to 1 s after the anti-aliasing filter (0-0.5 Hz) is applied. It is then median filtered to remove the spiky noise. Then a low-pass filter with a corner frequency of 0.05 Hz is applied. At this stage, a DC component with an amplitude around 0.1 rad/s can be detected in the signal. The DC value is stable and does not vary significantly with operations, and is probably associated with the interrogator noises. For the cross-well applications, we determine the DC value by taking the median value of the channels that are out of range of interested signal at each time interval, and subtract it from the data. For the in-well measurements during the injection, as the signal strength is much higher and the interested signal covers the whole fiber, this DC value is ignored. 
         [0045]    A comparison between the conventional waterfall plot  FIG. 1  and the processed low-frequency signal is shown in  FIG. 2 . The low-frequency processing not only increases the signal-to-noise ratio of the cross-well signal, but also recovers the polarity of the strain rate, which is critical for later interpretation. The strain change recorded by DAS at this frequency band can be caused by thermal extension/contraction or by mechanic strain change in the formation. 
         [0046]    The DAS signal in the low-frequency band (0-0.05 Hz) also carries critical information that is related to the hydraulic fracture development during the stimulation process. This information has the potential to answer key questions to improve the efficiency of completion operations in unconventional reservoirs. This signal has not been publicly reported by others. In this invention, we develop a workflow to efficiently manage and process the DAS data to retrieve this low-frequency signal for further interpretation. 
         [0047]    DAS data delivered by acquisition contractors are sampled at very high frequency (10 k Hz), and segmented into small time intervals. The data volume ranges from 100 TB to 1000 TB for a single project. Utilizing parallel computing and data storage infrastructure created for data intensive projects like seismic data processing can easily handle the completed data set once transformed. A simple process has been developed:
       DAS data are converted from vendor&#39;s format into a continuous record   A low-pass filter transforms the data to 0.05 Hz and down sampled to 1 s intervals       
 
         [0050]    This process significantly increase the signal-to-noise ratio of the desired signal while decrease the data size by 10000 fold. The low frequency filtered data also maintains the polarity of the DAS strain-rate measurements (compression or extension), which will have a significant impact for later interpretation ( FIG. 2 ). The polarity change in the low-frequency DAS response is well correlated with stimulation operations ( FIG. 3,4 ). 
         [0051]      FIG. 1  shows a standard DAS dataset as provided by the vendor. Although activity can be observed that correlate with different activities performed during operation, the data is noisy and difficult to interpret.  FIG. 2  shows low-frequency DAS data after being transformed with a low-pass filer and down sampled to is intervals. The extension and compression can be clearly seen and differentiated in this plot and directly correlated with well events. 
         [0052]    The following examples of certain embodiments of the invention are given. Each example is provided by way of explanation of the invention, one of many embodiments of the invention, and the following examples should not be read to limit, or define, the scope of the invention. 
       EXAMPLE 1 
     In-Well Signal During Injection 
       [0053]    In example 1, fiber-optic cables are installed outside of the casing and cemented in place along the length of the well during a procedure, in this case during a stage of fracturing. Numerous interrogators are available to record optical signals. In one embodiment, Pinnacle Gen-1 Phase-1 interrogators are used for the recording. The fiber optic cables are interrogated with multiple high-speed laser pulses while monitoring the reflected light signal (Rayleigh scatter) along the fiber. This is repeated in a variety of patterns dependent upon the data being collected. The raw data are sampled at 10 kHz continuously at more than 6000 locations (referred as “channels” in this study) with 1 m spatial separation along the fiber. The gauge length is set to 5 m. When recording, the measured optical phase is differentiated in time, so the raw DAS data is linearly associated with strain rate along the fiber. 
         [0054]      FIG. 4  shows a Low-Frequency In-well signal. The data is obtained from a horizontal production well that is being fractured. A single stage from approximately 15,900 feet to 16,150 feet is shown. Pumping is initiated after 0.5 hours (A) and compression may be seen in the previous stage, between 16,300 feet to 16,500 feet, until the ball seats (B). Once the ball is seated, the heel pressure increases and a large amount of compression occurs at the target depth, approximately 15,900 to 16,150 feet. After the initial compression, extension begins to occur and is sustained for approximated 2 hrs until pumping stops (C). The plug depth is labeled vertically (D) to emphasize the current perforation stage. 
         [0055]    This application can be used to detect the stage isolation condition in real time to provide advice for the field operators. 
       EXAMPLE 2 
     Cross-Well Signal During Hydraulic Fracturing 
       [0056]    In example 2, fiber-optic cables are installed outside of the casing and cemented in place along the length of an adjacent well. Numerous interrogators are available to record optical signals. In one embodiment, Pinnacle Gen-1 Phase-1 interrogators are used for the recording. The fiber optic cables are interrogated with multiple high-speed laser pulses while monitoring the reflected light signal (Rayleigh scatter) along the fiber. This is repeated in a variety of patterns dependent upon the data being collected. The raw data are sampled at 10 kHz continuously at more than 6000 locations (referred as “channels” in this study) with 1 m spatial separation along the fiber. The gauge length is set to 5 m. When recording, the measured optical phase is differentiated in time, so the raw DAS data is linearly associated with strain rate along the fiber. 
         [0057]      FIG. 3  shows a Low-Frequency Cross-well signal. This data is obtained from an observation or production well that is located near or adjacent to the target well. A single stage (A-C) can be seen where pumping starts (A), the ball seats (B), and then pumping stops (C). Initially between A and B, there is an extension at about 13,750 feet (previous fracture site) until the ball seats (B). Then compression occurs at the previous fracture site, and extension begins at the new fracture site between 13,500 and 13,750 feet. Extension continues until pumping stops (C). Compression slowly occurs between the stopping of the current stage (C) until starting the next stage (E). During the compression, the movement of the perforation gun (D) can be observed. Once the next stage starts (E) similar extension and compression patterns can be observed. The process is repeated multiple times until the length of the well is fractured. 
         [0058]    This application can be used to detect the hydraulic fracture intersections on the monitor wells and estimate the fracture widths. 
       EXAMPLE 3 
     In-Well Signal During Production 
       [0059]    The production well with optical fiber installed is shut in for a period of time. During the shut-in period, small differences in formation temperature permeate into the well bore through thermal conduction. When production resumes, the fluid flow within the well causes a measurable wave of temperature fluctuations to move in the direction of the flow. Flow rates along the well can be inferred from these temperature changes, and variations in rates provides an indication of the relative productivity of the well intervals. 
       EXAMPLE 4 
     Cross-Well Signal During Production 
       [0060]    Two nearby production wells with optical fiber installed may be separated by 50-1000 ft can both be shut in for a varying periods of time. When one of the wells is brought back to production, the low-frequency DAS signal in the other well shows the low amplitude response at several individual zones along the wellbore, illustrating the fracture connectivity between these two wells. 
       EXAMPLE 5 
     Vertical Monitor Well Response 
       [0061]    A vertical well with optical fiber installed may be placed near a horizontal well that is being stimulated. When the vertical dipping hydraulic fractures hit a vertical well with fiber installed, the low-frequency DAS signal can be used to map the time and location of the fracture propagation front. The height (vertical extension) of the fracture can also be estimated from the signal. 
       EXAMPLE 6 
     Quick Assay 
       [0062]    Several venders provide solutions including placing a temporary fiber optic cable in a well and take DAS and DTS measurements. In this case the low-frequency signal is only controlled by the small temperature variation of the wellbore. Example 1 and 3 are still applicable under this configuration. Quick installation of a fiber optic cable may be just the fiber optic cable or bundle of cables, but the fiber optic cable may also be installed via coiled tubing or slick-line if required. 
       EXAMPLE 7 
     Heavy Oil Horizontal Wells 
       [0063]    fiber optic signals including DAS and DTS signals are effective tools for steam chamber monitoring in SAGD production for heavy oil reservoirs. The temperature variation along the borehole has been traditionally monitored through thermocouples. The low frequency DAS has the potential to provide very detailed distributed in-well and cross-well temperature and strain signal during steam injection and oil production. This can be used for steam chamber growth efficiency monitoring between multiple well pairs and monitoring production rates to ensure breakthrough, if it occurs, is halted quickly. The application can be extended to additional thermal EOR process monitoring such as ESAGD, downhole heating, cyclic steam injection and the like. 
       EXAMPLE 8 
     Plug &amp; Abandonment 
       [0064]    Low Frequency DAS can also have applications for passive monitoring in a well plug and abandonment scenario where the primary signal will be relating to strain changes during a pressure drawdown phase. This application will require longer time measurements or periodic measurements. In one embodiment measurements are taken daily for a 15 minute period. These results are transmitted to a central location for analysis. Flow may cause a strain or temperature change that can be observed if the integrity of the plugged well is compromised. 
         [0065]    In closing, it should be noted that the discussion of any reference is not an admission that it is prior art to the present invention, especially any reference that may have a publication date after the priority date of this application. At the same time, each and every claim below is hereby incorporated into this detailed description or specification as a additional embodiments of the present invention. 
         [0066]    Although the systems and processes described herein have been described in detail, it should be understood that various changes, substitutions, and alterations can be made without departing from the spirit and scope of the invention as defined by the following claims. Those skilled in the art may be able to study the preferred embodiments and identify other ways to practice the invention that are not exactly as described herein. It is the intent of the inventors that variations and equivalents of the invention are within the scope of the claims while the description, abstract and drawings are not to be used to limit the scope of the invention. The invention is specifically intended to be as broad as the claims below and their equivalents. 
       REFERENCES 
       [0067]    All of the references cited herein are expressly incorporated by reference. The discussion of any reference is not an admission that it is prior art to the present invention, especially any reference that may have a publication data after the priority date of this application. Incorporated references are listed again here for convenience:
   1. U.S. Pat. No. 6,778,720, “Dual slope fiber optic array interrogator,” (2004).   2. US8950482, “Fracture monitoring,” (2009).   3. US20060272809, “Wellbores utilizing fiber optic-based sensors and operating devices,” (2006).   4. US20090114386, “Systems and methods for distributed interferometric acoustic monitoring,” (2009).   5. US20130298635, “Techniques for Distributed Acoustic Sensing,” (2013).   6. US20130298665, “System and method for monitoring strain &amp; pressure,” (2013).   7. US20130233537, “Fracture Characterisation,” (2013).   8. US20140202240, “Flow Velocity and Acoustic Velocity Measurement with Distributed Acoustic Sensing,” (2014).   9. US20140216151, “Flow Monitoring,” (2014).   10. US20140260588, “Flow Sensing Fiber Optic Cable and System,” (2014).   11. US20140358444, “Method of Hydraulic Fracture Identification Using Temperature,” (2014).   12. US20160003032, “Matrix Temperature Production Logging Tool,” (2016).   13. Boman, K, “DAS technology expands fiber optic applications for oil, gas industry,” Rigzone, May 4, 2015 issue.   14. Webster, P., et al., “Developments in Diagnostic Tools for Hydraulic Fracture Geometry Analysis,” Unconventional Resources Technology Conference (URTeC), Denver, Colo., 12-14 Aug. 2013.   15. Optasense, “Pipeline Integrity Management : Leak Detection,” www.optasense.com (2013).