Abstract:
A method of monitoring a well for unwanted formation fluid influx is disclosed. Measurements of well outflow are acquired during a period in which drilling operations are performed for the well. Occurrences of stagnant flow events during the period are determined. An outflow signature is generated from the well outflow measurements for each stagnant flow event. The outflow signatures are displayed sequentially in time of occurrence. Each outflow signature is analyzed for an anomaly.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims priority to U.S. patent application Ser. No. 61/569,636, which was filed Dec. 12, 2011. This priority application is hereby incorporated by reference in its entirety into the present application, to the extent that it is not inconsistent with the present application. 
    
    
     BACKGROUND 
     The invention relates generally to drilling of wells in subsurface formations. More specifically, the invention relates to monitoring and detecting kicks in a well. 
     During drilling of a well, drilling fluid (or “drilling mud”) is pumped from a mud pit into a drill string that is suspended in the well. The drilling mud flows down the drill string, exiting through a bit at the end of the drill string into the bottom of the well. The drilling mud then returns to the surface, carrying with it formation cuttings made by the bit. At the surface, the drilling mud flows through a mud return line into a mud treatment system, which cleans the drilling mud. The clean drilling mud is returned to the mud pit, from where the drilling mud is again pumped into the drill string. This circulation of the drilling mud continues while the bit is cutting the formation. The drilling mud performs a variety of functions, including carrying formation cuttings to the surface, cooling the bit, and controlling the hydrostatic pressure in the well such that the well does not take a kick. A well is said to take a kick whenever there is unwanted influx of formation fluids into the well. 
     The hydrostatic pressure in the well is controlled through the weight of the drilling mud. In spite of careful control of drilling mud weight, a well may take a kick unexpectedly. Thus the normal practice is to monitor the well for kicks so that as soon as a kick is detected measures can be put into place to circulate the kick out of the well and stabilize the well. If a kick is not detected early enough and controlled, it may result in blowout of the well. Strategies for detecting kicks generally include (i) monitoring increases in the difference between the volume of fluid pumped into the well and the volume of fluid returning from the well, (ii) monitoring increases in the difference between the rate at which fluid is pumped out of the well and the rate at which fluid is pumped into the well, (iii) monitoring fluctuations in drill pipe pressure, and (iv) monitoring increases in gas content of fluid returns from the well. It is common to use a combination of these strategies to effectively detect kicks during drilling operations. 
     The flow rate monitoring strategy is often used when drilling mud is not being pumped into the drill string, such as when making connections between drill pipes. The principle here is that if the well inflow is zero and the well is stable, the well outflow should also be zero. The current practice when using this strategy is to physically inspect the mud return line to confirm that flow stopped when the mud pump(s) stopped pumping drilling mud into the drill string. Rig personnel can look inside the bell nipple, which is a large diameter pipe at the top of the well to which the mud return line is attached, or further down the mud return line, such as at the shale shakers, to visually observe any signs of flow. However, when the rig crew first shuts the mud pump down, it generally takes some period of time for well outflow to drop to zero. To detect a kick early, the rig personnel inspecting the mud return line with the naked eye would need to be able to quickly distinguish between residual flow and anomalous flow that may be indicative of a kick. 
     SUMMARY 
     In one aspect, a method of monitoring a well for unwanted formation fluid influx includes acquiring measurements of well outflow during a period in which drilling operations are performed for the well. The method includes determining occurrences of stagnant flow events during the period and generating an outflow signature from the well outflow measurements for each stagnant flow event. The method includes displaying at least a portion of the outflow signatures sequentially in time of occurrence. The method includes analyzing each outflow signature for an anomaly. 
     It is to be understood that both the foregoing general description and the following detailed description are exemplary of the invention and are intended to provide an overview or framework for understanding the nature and character of the invention as it is claimed. The accompanying drawings are included to provide a further understanding of the invention and are incorporated in and constitute a part of this specification. The drawings illustrate various embodiments of the invention and together with the description serve to explain the principles and operation of the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The following is a description of the figures in the accompanying drawings. The figures are not necessarily to scale, and certain features and certain views of the figures may be shown exaggerated in scale or in schematic in the interest of clarity and conciseness. 
         FIG. 1  shows a system for drilling a well. 
         FIG. 2  shows a block diagram of a system for monitoring a well. 
         FIG. 3A  shows a partial outflow signature on a display device. 
         FIG. 3B  shows a complete outflow signature on a display device. 
         FIG. 4  shows a sequence of outflow signatures on a display device. 
     
    
    
     DETAILED DESCRIPTION 
     It is to be understood that the following disclosure describes several exemplary embodiments for implementing different features, structures, or functions of the invention. Exemplary embodiments of components, arrangements, and configurations are described below to simplify the present disclosure; however, these exemplary embodiments are provided merely as examples and are not intended to limit the scope of the invention. Additionally, the present disclosure may repeat reference numerals and/or letters in the various exemplary embodiments and across the Figures provided herein. This repetition is for the purpose of simplicity and clarity and does not in itself dictate a relationship between the various exemplary embodiments and/or configurations discussed in the various Figures. Moreover, the formation of a first feature over or on a second feature in the description that follows may include embodiments in which the first and second features are formed in direct contact, and may also include embodiments in which additional features may be formed interposing the first and second features, such that the first and second features may not be in direct contact. Finally, the exemplary embodiments presented below may be combined in any combination of ways, i.e., any element from one exemplary embodiment may be used in any other exemplary embodiment, without departing from the scope of the disclosure. 
     Additionally, certain terms are used throughout the following description and claims to refer to particular components. As one skilled in the art will appreciate, various entities may refer to the same component by different names, and as such, the naming convention for the elements described herein is not intended to limit the scope of the invention, unless otherwise specifically defined herein. Further, the naming convention used herein is not intended to distinguish between components that differ in name but not function. Additionally, 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.” All numerical values in this disclosure may be exact or approximate values unless otherwise specifically stated. Accordingly, various embodiments of the disclosure may deviate from the numbers, values, and ranges disclosed herein without departing from the intended scope. Furthermore, as it is used in the claims or specification, the term “or” is intended to encompass both exclusive and inclusive cases, i.e., “A or B” is intended to be synonymous with “at least one of A and B,” unless otherwise expressly specified herein. For the purposes of this application, the term “real-time” means without significant delay. 
       FIG. 1  shows an example of a system  100  for drilling a well  102  in subsurface formation  103 . A drill string  104  extends through a rotary table  106 , a bell nipple  108 , blowout preventers  110 ,  112 , and a wellhead  114  into the well  102 . The rotary table  106  is mounted in a drill floor  116 , and the bell nipple  108 , blowout preventers  110 ,  112 , and wellhead  114  are below the rotary table  106 . (In marine drilling, the blowout preventers and wellhead would be located at or near the seafloor and a marine riser may extend between the seafloor and the drill floor.) The drill string  104  is coupled at the top to a top drive  118 , which is coupled to a swivel  120 . A traveling block  122  coupled to the swivel  120  hangs down from a crown block  124  at the top of a derrick  126 . The traveling block  122  travels up and down the derrick  126  via a pulley system. The bell nipple  108  is in hydraulic communication with a mud pit  128  via a return flow line  130 . A shale shaker  132  filters debris out of the drilling mud flowing into the mud pit  128 . A mud pump  134  is in hydraulic communication with the mud pit  128  via a suction flow line  136 . The mud pump  134  is also in hydraulic communication with the swivel  120  via a discharge flow line  138 . The swivel  120  is in hydraulic communication with the top drive  118 , which is in hydraulic communication with the drill string  104 . 
     The mud circulation system starts at the mud pit  128  containing the drilling mud. The mud pump  134  pumps drilling mud from the mud pit  128  into the swivel  120 . From the swivel  120 , the drilling mud flows into the top drive  118  and then into the drill string  104 . The drilling mud flows down the drill string  104  and exits into the bottom of the well  102  through a bit  140 . At the bottom of the well  102 , the drilling mud commingles with formation cuttings made by the bit  140 . The drilling mud with the formation cuttings is then forced up a return annulus  142  defined between the well  102  and the drill string  104  into the bell nipple  108 . From the bell nipple  108 , the drilling mud flows through the shale shaker  132  into the mud pit  128 . The shale shaker  132  removes debris from the drilling mud. Additional conditioning of the drilling mud may occur inside the mud pit  128  before the drilling mud is again circulated through the system. (For a dual-bore drill string, the return annulus would be defined inside the drill string. Also, a device other than a bell nipple may be used to divert the drilling mud from the return annulus to the mud pit.) 
       FIG. 2  shows a system  200  for monitoring the well  102  (in  FIG. 1 ). The system  200  has a measurement module  202 , a processing module  204 , and a display device  206 . The measurement module  202  includes one or more sensors, such as sensors  208 ,  210 , and  212 , for measuring one or more parameters related to well monitoring. In one embodiment, the sensor  208  measures well outflow, which is the rate at which drilling mud flows out of a well. In one embodiment, the sensor  210  measures well inflow, which is the rate at which drilling mud is pumped into a drill string in a well. In one embodiment, the sensor  212  measures movement, such as axial movement or rotation, of a drill string performing operations in a well. In  FIG. 1 , the sensor  208  is arranged in the return flow line  130  to measure well outflow. The sensor  208  may be any flowmeter that can work with particulate fluid. The flowmeter may make relative or absolute measurements. In one embodiment, the sensor  208  is a paddle-type flowmeter. The sensor  210  is arranged in the discharge flow line  138  to measure well inflow. The sensor  212  is arranged on the top drive  118  to measure movement of the drill string  104 . It is possible to arrange the sensors  208 ,  210 ,  212  at locations other than indicated in  FIG. 1  as long as the desired parameters can be measured at the other locations. 
     In  FIG. 2 , the processing module  204  receives the measurements made by the sensors  208 ,  210 ,  212  via an input interface  214 . Transmission of measurements from the sensors  208 ,  210 ,  212  to the processing module  204  may be direct or indirect. In the latter case, for example, measurement signals from the sensors  208 ,  210 ,  212  may be preprocessed or stored elsewhere before being transmitted to the processing module  204 . The processing module  204  includes a memory or storage device  216  for holding the measurements as well as other data and programs, such as a well monitoring program. The memory or storage device  216  may take the form of one or more floppy disks, a CD-ROM or other optical disk, a magnetic tape, a read-only memory chip (ROM), and other forms of memory or storage device well known in the art or subsequently developed. The processing module  204  has a processor device  218 , which can read programs and data on the memory or storage device  216 . The processing module  208  executes programs and controls operations of the processing module  204 . The programs executed by the processor device  218  may be in binary form or object code, in source code, or in some intermediate form such as partially complied code. The processing module  204  communicates with the display device  206  via an output interface  220 . The communication interfaces  214 ,  216  of the processing module  204  may include wired or wireless links that allow the system to operate in a substantially real-time manner. 
     The processing module  204  generates outflow signatures for stagnant flow events. A stagnant flow event occurs when drilling mud is not being pumped into the drill string  104  (in  FIG. 1 ) and when the drill string  104  is not moving. Typically, stagnant flow events occur when making up or breaking out pipe connections. The outflow signatures are generated from the well outflow measurements. However, to generate the outflow signatures for the stagnant flow events, knowledge of the starting time and ending time of each stagnant flow event is needed. In one embodiment, this knowledge may be gleaned from the well inflow measurements made by the sensor  210  and from the drill string movement measurements made by the sensor  212 . It should be noted that other records besides well inflow measurements and drill string movement measurements may be used to determine the starting times and ending times of stagnant flow events. For example, the controller of the mud pump  134  (in  FIG. 1 ) may send messages to the processing module  204  whenever there is a switch in the state of the mud pump  134 , and the controller of the top drive  118  (in  FIG. 1 ) may send messages to the processing module  204  whenever there is a switch in the state of the top drive  118 . The processing module  204  can make note of the time of the messages and use it to determine the starting and ending times of stagnant flow events. 
     The processing device  214  executes the well monitoring program while drilling operations are being carried out on the well  102  (in  FIG. 1 ). Drilling operations encompass all operations related to drilling of the well  102 . The well monitoring program, when executed, causes the processing module  204  to listen for stagnant flow events. Listening may involve requesting for and receiving data that would indicate whether drilling fluid is being pumped into the drill string  104  or not and whether the drill string  104  is moving or not. When the well monitoring program detects a stagnant flow event, the well monitoring program causes the processing device  218  to generate an outflow signature for the stagnant flow event and causes the display device  206  to render the outflow signature as the stagnant flow event is unfolding. The well monitoring program causes the processing device  218  to analyze the outflow signature to determine whether the outflow signature includes an anomaly that may indicate a kick or other well control event. The well monitoring program may cause the processing device  218  to generate an alarm if it is determined that the outflow signature includes an anomaly or otherwise indicates a potential kick or well control event. How the well monitoring program works will be further explained by way of an example. 
     For illustration purposes, at some t 1     —     start , a stagnant flow event S 1  starts. The processing module  204  detects that the stagnant flow event S 1  has started and sends a signal to the display device  206  to display a start marker corresponding to t 1     —     start . Then, the processing module  204  starts processing the well outflow measurements made from time t 1     —     start  to generate an outflow signature that is representative of the well outflow from time t 1     —     start . The processing module  204  sends signals to the display device  206  as the outflow signature is generated, and the display device  206  renders the outflow signature relative to the start marker.  FIG. 3A  shows outflow signature s 1  on display device  206  as the outflow signature is being generated. An outflow signature is an impression of a well outflow and shows simply how the well outflow is trending with time, i.e., whether the well outflow is increasing or decreasing or not changing and whether any increase or decrease in well outflow is fast or slow or small or large. The processing module  204  continues processing the well outflow measurements and updating the outflow signature until the time t 1     —     end  when the stagnant flow event S 1  ends. The processing module  204  sends a signal to the display device  206  to display an end marker corresponding to time t 1     —     end .  FIG. 3B  shows the complete outflow signature s 1  on the display device  206 . 
     After the stagnant flow event S 1  has ended, the processing module  204  goes back to listening for the next stagnant flow event. For each new stagnant flow event detected, the processing module  204  will generate a new outflow signature and send signals to the display device  206  to render the new outflow signature along with a few or all of the previous outflow signatures. On the display device  206 , the outflow signatures are displayed sequentially in time of occurrence. Also, the outflow signatures are separated spatially so that one outflow signature can be told apart from another outflow signature visually.  FIG. 4  shows an example of a sequence of outflow signatures on the display device  206 . The sequence includes completed outflow signatures s 1 , s 2  and outflow signature s 3  that is being generated. Drilling personnel can visually observe the outflow signatures on the display device  206  as they are being generated by the processing device  218  (in  FIG. 2 ). If rig personnel see an outflow signature that is anomalous, the rig personnel may assume that the well has taken a kick and raise an alarm. Alternatively, the processing module  204  can analyze the outflow signatures and trigger an alarm if an anomalous outflow signature is detected. The alarm may be acoustic or visual. In the latter case, the alarm may graphical or textual. The alarm could be sent to a control room to enable the rig crew to take appropriate actions. 
     During a stagnant flow event, the well inflow will be zero. If the well has not taken a kick, the well outflow should also be zero during a stagnant flow event. Initially, the well outflow will not be zero due to residual flow. However, the well outflow should drop to zero and stay at zero if the well is stable. Therefore, an outflow signature may be considered anomalous if the outflow signature does not show a generally decreasing flow. In other words, an outflow signature is anomalous if it indicates non-residual well outflow. In such a case, the processing module  204  of  FIG. 2  may trigger an alarm. Another way that the processing module  204  may determine when an outflow signature is anomalous is by pattern recognition. For example, the processing module  204  may compare each outflow signature to a set of test outflow patterns known to be indicative of a kick in a well, i.e., a set of anomalous flow patterns. If the pattern of the outflow signature matches any of the test outflow patterns, the processing module  204  may raise an alarm. In certain embodiments, the processing module  204  may compare an outflow signature to a previously generated outflow signature to determine whether the outflow signature includes an anomaly that may indicate a kick or other well control event. 
     In addition to monitoring well outflow during stagnant flow events, other kick indicators may be monitored. For example, the volume of the mud pit  128  (in  FIG. 1 ) may be monitored during tripping events. Abnormal gains in the volume of the mud pit  128  may indicate that a kick is underway in the well  102  (in  FIG. 1 ). A suitable sensor, such as an infrared camera, may be used to monitor the fluid level in the mud pit  128 , which can be translated to the volume of fluid in the mud pit  128 . The measurements may be transmitted to the processing module  204 , which can process the measurements and cause the display  206  to render a volume history of the mud pit. The volume history can be in the form of volume signatures, where each volume signature would correspond to a tripping event. The volume signature would simply show how the volume of the mud pit is trending. The volume history will typically be shown separately from the outflow signatures. However, the time frame of the volume history displayed may correspond to that of the outflow signatures displayed. This would allow both the volume history and outflow signature to be used to detect a kick in the well. If the processing module  204  finds an anomalous outflow signature during analysis of outflow signatures, as described above, the processing module  204  can check the volume history to see if there has been an abnormal gain in the volume of the mud pit  128  before triggering an alarm that the well may have taken a kick. 
     While the invention has been described with respect to a limited number of embodiments, those skilled in the art, having benefit of this disclosure, will appreciate that other embodiments can be devised which do not depart from the scope of the invention as disclosed herein. Accordingly, the scope of the invention should be limited only by the attached claims.