Abstract:
A system for controlling gas in a subsea drilling operation is disclosed in one embodiment. The system includes a subsea blow-out preventer, riser coupled to the blow-out preventer, a gas sensor, a controller, and a signal pathway. The gas sensor is configured for placement below the riser and configured to contact wellbore fluids during normal drilling operation. The controller configured to automatically cause manipulation the blow-out preventer based upon information from the gas sensor. The signal pathway couples the gas sensor with the controller.

Description:
BACKGROUND 
       [0001]    This disclosure relates in general to drilling wellbores through earth formations and, but not by way of limitation, to controlling gas in the wellbore fluid. 
         [0002]    In deepwater drilling with a subsea blow-out preventer (BOP) there is risk of gas getting into the riser. Small amounts of gas may be undetected during the drilling process, particularly when drilling close to kick tolerance limits. Large expansion of gas in the drilling riser can occur to partially empty the riser. The volume of gas increases as it travels from the ocean floor toward the surface. Hydrostatic pressure can lead to riser collapse, the uncontrolled release of hydrocarbon at the surface when the diverter overloads or other problems. 
         [0003]    In one embodiment of the invention, early kick detection is a consideration for rig safety and efficiency. The lower kick tolerances associated with deep water operations can be addressed by kick detection systems that are more sensitive and reliable that those which are usually available for conventional drilling operations. For example, lower fracture gradients than similar land or shallow water situations reduce the kick tolerance margin. However, kick detection in deepwater operations can be difficult. Two early warning signs of kicks are an increase in flow rate and pit volume. These signs are difficult to detect when drilling from floating vessels due to the nature of the drilling vessel motion. Waves can cause fluctuations in the pits that can complicate volume estimates. Similar problems affect the outflow rate measurement. 
         [0004]    Failure to detect a gas influx lower in the wellbore in such an operation can lead to gas being circulated into a deepwater riser. This is even more likely when drilling with oil-based mud due to the solubility of the methane in the drilling fluid. Typically there is very limited pressure control at surface once the gas has been circulated past the BOP stack on the seafloor. The gas in the riser that is circulated during the drilling process can expand rapidly near surface and can lead to blow-out conditions. Furthermore, if the riser does become partially evacuated, there is also a risk of riser collapse. 
         [0005]    When a kick is taken while drilling with a marine riser, there is a possibility that the gas can migrate or be circulated above the subsea BOP (SSBOP) stack. When this occurs, the choke and mud-gas separator are no longer available to control the flowrates when the riser gas reaches the surface. Even if the gas influx is detected early and the annular preventer is closed, some of gas influx may already be above the annular preventer. Additionally, there may be some gas above the annular preventer because detection of the kick did not occur until the gas had been circulated above the SSBOP stack. 
         [0006]    An early flowcheck in the riser, immediately after shutting in the well, may show a flow indicating that the large bubbles are still rising. However, once all the small gas bubbles have been suspended in water based mud or dissolved in oil base mud, a flow check in the riser may falsely read negative even though there is gas in the riser. If a large amount of gas gets above the SSBOP stack, it can rise rapidly and carry a large volume of mud out of the riser at high rates. One way of managing gas in the riser is to avoid such situations. 
       SUMMARY 
       [0007]    Gas influx detection is a consideration for rig safety and efficiency. One embodiment of the invention describes the placement of a sensitive methane sensor in the subsea blow-out preventer (BOP) below the lowest BOP circulation path. The methane sensor is coupled, via an umbilical, to the surface rig control system to allow remote monitoring of methane. Other embodiments could monitor other gases in the BOP and report that information to surface. Detection of gas triggers an automated shut-in of the well that will minimize both the risk of human error during a highly stressful time and the volume of gas that could get in the riser. Quick detection of gas and remediation can keep the amount of gas released into the riser below an amount that can safely be handled by the diverter. 
         [0008]    In one embodiment, the present disclosure provides a system for controlling gas in a subsea drilling operation. The system includes a subsea blow-out preventer, riser coupled to the blow-out preventer, a gas sensor, a controller, and a signal pathway. The gas sensor is configured for placement below the riser and configured to contact wellbore fluids during normal drilling operation. The controller configured to automatically cause manipulation the subsea blow-out preventer based upon information from the gas sensor. The signal pathway couples the gas sensor to the controller. 
         [0009]    In another embodiment, the present disclosure provides a method for controlling gas in subsea drilling. In one step, gas in wellbore fluid is detected before it passes the subsea blow-out preventer. A signal indicative of gas in the wellbore fluid is produce, where the signal is performed on the wellbore fluid below the riser. Reaction to the signal is automatic can could include adjusting the subsea blow-out preventer. 
         [0010]    In yet another embodiment, the present disclosure provides a method for remotely controlling gas in subsea drilling. In one step, a first signal indicative of gas in wellbore fluid is detected before the wellbore fluid passes a subsea blow-out preventer. It is determined that the first signal indicates an level of gas above a predetermined threshold. A second signal is produced to command a subsea blow-out preventer to perform one or more adjustments. 
         [0011]    Further areas of applicability of the present disclosure will become apparent from the detailed description provided hereinafter. It should be understood that the detailed description and specific examples, while indicating various embodiments, are intended for purposes of illustration only and are not intended to necessarily limit the scope of the disclosure. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0012]    The present disclosure is described in conjunction with the appended figures: 
           [0013]      FIG. 1  depicts a diagram of an embodiment of subsea drilling equipment; 
           [0014]      FIG. 2  depicts a block diagram of an embodiment of a drilling system; and 
           [0015]      FIG. 3  illustrates a flowchart of an embodiment of a process for controlling gas in subsea drilling. 
       
    
    
       [0016]    In the appended figures, similar components and/or features may have the same reference label. Further, various components of the same type may be distinguished by following the reference label by a dash and a second label that distinguishes among the similar components. If only the first reference label is used in the specification, the description is applicable to any one of the similar components having the same first reference label irrespective of the second reference label. 
       DETAILED DESCRIPTION 
       [0017]    The ensuing description provides preferred exemplary embodiment(s) only, and is not intended to limit the scope, applicability or configuration of the disclosure. Rather, the ensuing description of the preferred exemplary embodiment(s) will provide those skilled in the art with an enabling description for implementing a preferred exemplary embodiment. It being understood that various changes may be made in the function and arrangement of elements without departing from the spirit and scope as set forth in the appended claims. 
         [0018]    Referring first to  FIG. 1 , a diagram of an embodiment of subsea drilling equipment  100  is shown. A drill string  104  extends through a riser  108  and into the wellbore. The wellbore passes down from the seabed  116 . The beginning of the wellbore is reinforced by a casing head  112 . An umbilical (not shown) is used to pass electrical signaling between the platform (not shown) and the a blow-out preventer (BOP)  106 . Additionally, kill and choke lines  154 ,  130  pass along the riser  108  to the surface. 
         [0019]    Drilling fluid passes down the drill string  104  and returns to the surface through the riser  108 . There are various components in a BOP  106  to control this process. An annular preventer  124  seals the annular space and can be remotely controlled as denoted by the arrow. Pipe and/or shear ram(s)  148  are respectively used to either hold the drill string in place, provide additional blow-out prevention or cut through the drill string  104 . Some embodiments could have multiple BOPs  106 , called a BOP stack. 
         [0020]    This embodiment has two kill lines  154  and two choke lines  130  in the BOP  106 . The kill lines  154  each have an electrically controlled valve  150 . Similarly, the choke lines  130  each have a choke valve  128  that is controllable remotely. The choke and kill lines  130 ,  154  can be manipulated to control the circulation of wellbore fluids under pressure in the event of a well control incident. 
         [0021]    In-situ real-time detection of methane can be achieved using immobilization onto an electrode surface a metal oxide compound, mimicking the catalytic center of the enzyme methane monooxygenase (MMO), which catalyses the partial oxidative conversion of methane into methanol. This methane gas sensor  132  produces a current from the reaction rate or turnover of the methane conversion that corresponds to the concentration of the target molecule(s) and can be recorded remotely. Such a device, could be based on either electrochemical or optical principles. 
         [0022]    The methane gas sensor  132  could be placed anywhere in the BOP or the wellbore to detect gas in the drilling fluid as it returns to the surface. In the depicted embodiment, the methane gas sensor  132  is placed below the lowest kill line in the subsea BOP. The methane gas or other light hydrocarbon molecules get into the drilling fluid from the formation during a kick situation. The kick is physically caused by the pressure in the wellbore being less than that of the formation fluids. 
         [0023]    When controlling gas in the subsea drilling equipment  100 , other sensors may be used. This embodiment includes a drillpipe pressure sensor  140  to measure pressure in the drilling fluid as it passes through the drill string  104 . On the return of the drilling fluid and cuttings in the riser  108  an annulus pressure sensor  144  is used. The flow in the annulus of the riser  108  is measured with a riser flow meter  136 . 
         [0024]    With reference to  FIG. 2 , a block diagram of an embodiment of a drilling system  200  is shown. The blocks associated with the subsea drilling equipment  100  are shown with the dashed rectangle. The subsea drilling equipment  100  includes functional blocks for the annular preventer(s)  124 , the choke lines valve(s)  128 , the methane gas sensor  132 , the riser flow meter  136 , the drillpipe pressure sensor  140 , the annulus pressure sensor  144 , and the pipe and/or shear ram(s)  148 . 
         [0025]    One embodiment of the invention uses an integrated control and information service (ICIS)  204 . For example, a Varco™ V-ICIS system that controls the subsea drilling equipment  100 , pumps, drillstring compensation  216 , block position, and drillstring rotation speed could be used. The drillstring rotation control  212  could be a rotary table or a top drive in various embodiments that is controlled by the ICIS  204 . The Varco™ V-ICIS is one of the commercially available platforms for rig floor integration control and automation. It is designed for both offshore and land rig operations, and allows rig floor operators to focus on strategic drilling operations, rather than manual equipment operation. 
         [0026]    Through various controls and measurements, the V-ICIS can automatically perform many tasks. V-ICIS integrates the control of the following drilling systems using joysticks and touch screens for operator interface: automated drilling equipment, top drives, pipe handling equipment, iron roughnecks, pressure control, annular preventer  124 , pipe/shear ram(s)  148 , kill lines  154 , choke lines and valves  130 ,  128 , diverters, automated mud systems, automated fluid transfer systems, automated mud chemical dosing systems, shaker load control systems, drawworks  208 , SCR controls, drillstring compensator  216 , drilling information systems, bulk tank control systems, and/or customer defined controls and interfaces. The V-ICIS also gathers information to aid in decision-making, for example, a drillpipe pressure sensor  140 , an annulus pressure sensor  144 , a riser flow meter  136 , and/or a methane gas sensor  132  could be used in various embodiments. 
         [0027]    Such a drilling system  200  can be tailored to piece together in an automated manner the sequence of events to safely stop circulation and shut the well in once gas has been detected in the riser when combined with the novel methane gas sensor  132 . The sequence is tailored for the total number of BOPs in the stack and configuration of each BOP  126 . Further, the drilling system  200  can mitigate the gas before it damages the riser or platform. The ICIS  204  can be implemented with a computing device with software and/or hardware. 
         [0028]    Referring next to  FIG. 3 , a flowchart of an embodiment of a process  300  for controlling gas in subsea drilling is illustrated. Once the methane gas has been detected by the sensor  132 , via an umbilical connection to the V-ICIS system  204 , the following sequence of events can be automated while drilling. Similar procedures can be followed while tripping, while out of hole, etc. The ICIS  204  controls the process, but allows manual disable. The depicted portion of the process  300  begins in step  304  where gas level information is read from the methane sensor  132 . These readings could happen continuously or at a predetermined interval. Other embodiments only report gas levels above a threshold as an alarm. In any event, gas level information is relayed to the ICIS  204  in step  308 . 
         [0029]    It is determined in step  312  if a kick condition exists by measurement of the gas in the drilling fluid. The driller may be flagged that gas has been or is about to be circulated into the riser  108  so that he or she is aware that control of the rig equipment is being taken over by the ICIS  204  (there is a manual override if necessary). In step  316 , the ICIS  204  sends a command to the rotary table or top drive to stop rotation of the drillstring  104 . The ICIS  204  sends a command to the drawworks control  208  to raise the drillstring  104  to the hang-off position in step  320 . A command to close annular preventer or top preventer and open choke line failsafe valves  128  in steps  324  and  328 . 
         [0030]    The ICIS  204  is aware the pipe locations so it can then check the space out and close the hang-off pipe rams  148  at the appropriate location in step  332 . The ICIS  204  sends a command to hang-off, use the drillstring compensator  212  in step  338  and close the pipe ram locks in step  342 . The pressure in the BOP  106  can then be bled off between the pipe rams  148  and the annular preventer  124  in a controlled manner by the ICIS  204 . Once the pressure is bled-off, the annular preventer  124  is opened in step  350 . 
         [0031]    The annulus and drillpipe pressures are read from the pressure sensors  144 ,  140  and the pit volume change is determined in step  354 . The riser flow meter  136  is read in step  358 . If there is no drillstring in the hole and/or the flow in the riser  108  is fast as determined in step  362 , blind and/or shear rams  148  may be used by the ICIS  204  in step  366  before the stabilized casing pressure is noted. After stabilization, the riser  108  is then monitored for flow again in step  358 . 
         [0032]    If the volume of gas above the BOP  106  or BOP stack is kept small by detection equipment and shut-in, the gas can be safely handled at surface by allowing the gas bubbles to disperse and/or controlling the rate at which gas is brought to the surface. The controlled rate of gas could flow through the riser boost line if the annular preventer is closed during a well control event in the main borehole. Small amounts of gas in the riser  108  can be mitigated with a riser gas handler below the slip joint and/or with a diverter at surface, which can give sufficient back pressure to control the flowrate. Should the gas surface, it may do so rapidly and at a high rate with little warning without early detection of the gas. 
         [0033]    If there is gas in the riser  108  and a significant amount of gas in the main wellbore, simultaneous riser and well killing is performed in one embodiment. This is a complex procedure and can split the attention of the operations personnel leading to oversight or error when done manually. Automation of the riser gas handling reduces such a risk, by focusing attention on well-established primary well control techniques for the main wellbore in a process controlled by the ICIS  204 . International Association of Drilling Contractors (IADC) well control procedures for deep water recommend that personnel be minimized on the rig floor when there is gas in the riser due to the severity of the risk. Methane gas detection and rig automation is another way of ensuring minimum risk of exposure of rig personnel to hazardous situations. 
         [0034]    A number of variations and modifications of the disclosed embodiments can also be used. For example, the above embodiments show a single gas sensor, but other embodiments could have a plurality of gas sensors. The multiple gas sensors could be located in various locations in the BOP or within the casing. 
         [0035]    Specific details are given in the above description to provide a thorough understanding of the embodiments. However, it is understood that the embodiments may be practiced without these specific details. For example, circuits may be shown in block diagrams in order not to obscure the embodiments in unnecessary detail. In other instances, well-known circuits, processes, algorithms, structures, and techniques may be shown without unnecessary detail in order to avoid obscuring the embodiments. 
         [0036]    Implementation of the techniques, blocks, steps and means described above may be done in various ways. For example, these techniques, blocks, steps and means may be implemented in hardware, software, or a combination thereof. For a hardware implementation, the processing units may be implemented within one or more application specific integrated circuits (ASICs), digital signal processors (DSPs), digital signal processing devices (DSPDs), programmable logic devices (PLDs), field programmable gate arrays (FPGAs), processors, controllers, micro-controllers, microprocessors, other electronic units designed to perform the functions described above, and/or a combination thereof. 
         [0037]    Also, it is noted that the embodiments may be described as a process which is depicted as a flowchart, a flow diagram, a data flow diagram, a structure diagram, or a block diagram. Although a flowchart may describe the operations as a sequential process, many of the operations can be performed in parallel or concurrently. In addition, the order of the operations may be re-arranged. A process is terminated when its operations are completed, but could have additional steps not included in the figure. A process may correspond to a method, a function, a procedure, a subroutine, a subprogram, etc. When a process corresponds to a function, its termination corresponds to a return of the function to the calling function or the main function. 
         [0038]    Furthermore, embodiments may be implemented by hardware, software, scripting languages, firmware, middleware, microcode, hardware description languages, and/or any combination thereof. When implemented in software, firmware, middleware, scripting language, and/or microcode, the program code or code segments to perform the necessary tasks may be stored in a machine readable medium such as a storage medium. A code segment or machine-executable instruction may represent a procedure, a function, a subprogram, a program, a routine, a subroutine, a module, a software package, a script, a class, or any combination of instructions, data structures, and/or program statements. A code segment may be coupled to another code segment or a hardware circuit by passing and/or receiving information, data, arguments, parameters, and/or memory contents. Information, arguments, parameters, data, etc. may be passed, forwarded, or transmitted via any suitable means including memory sharing, message passing, token passing, network transmission, etc. 
         [0039]    Moreover, as disclosed herein, the term “storage medium” may represent one or more memories for storing data, including read only memory (ROM), random access memory (RAM), magnetic RAM, core memory, magnetic disk storage mediums, optical storage mediums, flash memory devices and/or other machine readable mediums for storing information. The term “machine-readable medium” includes, but is not limited to portable or fixed storage devices, optical storage devices, wireless channels, and/or various other storage mediums capable of storing that contain or carry instruction(s) and/or data. 
         [0040]    While the principles of the disclosure have been described above in connection with specific apparatuses and methods, it is to be clearly understood that this description is made only by way of example and not as limitation on the scope of the disclosure.