Abstract:
A method of planning a procedure for cleaning a wellbore by injecting a cleaning fluid from a reservoir into the wellbore includes detecting properties and conditions of fluids circulating between the reservoir and the wellbore; preparing a data set from the detected properties and conditions of the fluids circulating between the reservoir and the wellbore; simulating a cleaning operation model of injecting the cleaning fluid into the wellbore based on the data set; determining parameter settings of the simulated cleaning operation model that satisfy prescribed constraints; and producing the procedure for cleaning the wellbore based on the determined parameters. A system for conducting a procedure for cleaning a wellbore includes a sensor unit; a control unit; and a cleanup simulation system.

Description:
FIELD OF THE INVENTION 
       [0001]    Embodiments of the present invention relate to methods and systems of planning a procedure for cleaning a wellbore, and, in particular, methods and systems applying simulation models prior to designing a procedure. 
       BACKGROUND OF INVENTION 
       [0002]    During the drilling of a wellbore, various fluids are typically used in the well for a variety of functions. The fluids may be circulated through a drill pipe and drill bit into the wellbore and, then, may subsequently flow upward through wellbore to the surface. During this circulation, the drilling fluid may act to remove drill cuttings from the bottom of the hole to the surface, to suspend cuttings and weighting material when circulation is interrupted, to control subsurface pressures, to maintain the integrity of the wellbore until the well section is cased and cemented, to isolate the fluids from the formation by providing sufficient hydrostatic pressure to prevent the ingress of formation fluids into the wellbore, to cool and lubricate the drill string and bit, and/or to maximize penetration rate. 
         [0003]    One way of protecting the formation is by forming a filter cake on the surface of the subterranean formation. Filter cakes are formed when particles suspended in a wellbore fluid coat and plug the pores in the subterranean formation such that the filter cake prevents or reduces both the loss of fluids into the formation and the influx of fluids present in the formation. A number of ways of forming filter cakes are known in the art, including the use of bridging particles, cuttings created by the drilling process, polymeric additives, and precipitates. 
         [0004]    After drilling/completion of a well and before start of production, all the foreign fluids, such as drilling mud, are to be removed from the wellbore and the invaded zone around the wellbore. The cleaning operation is conducted in the very early period of production right after opening a well to remove all the contaminated fluids from the wellbore and the formation in vicinity of the wellbore. A common technique to clean a well is to start producing from the well until the percentage of contaminates in produced formation fluid is negligible. However, there are still some problems associated with these techniques. For example, there are various certain and uncertain factors that may influence the efficiency of the cleanup process, for example, physical and chemical properties and conditions of the fluids and the sidewall of the wellbore, and interaction therebetween. These properties and conditions include temperature, pressure, viscosity, pH of the fluid in the well, and the like. Furthermore, the degradation process of the filter cake is not easily controllable, particularly in situ. Because the internal and external factors are not always stable in the natural environment, precisely understanding and controlling the factors is frequently difficult. 
       SUMMARY OF INVENTION 
       [0005]    In one aspect, embodiments disclosed herein relate to a method of planning a procedure for cleaning a wellbore by injecting a cleaning fluid from a reservoir into the wellbore comprising: detecting properties and conditions of fluids circulating between the reservoir and the wellbore; preparing a data set from the detected properties and conditions of the fluids circulating between the reservoir and the wellbore; simulating a cleaning operation model of injecting the cleaning fluid into the wellbore based on the data set; determining parameter settings of the simulated cleaning operation model that satisfy prescribed constraints; and producing the procedure for cleaning the wellbore based on the determined parameters. 
         [0006]    In one aspect, embodiments disclosed herein relate a system for cleaning a wellbore by injecting a cleaning fluid from a reservoir into the wellbore such that the cleaning fluid circulates between the reservoir and the wellbore comprising: a sensor unit that detects properties and conditions of fluids circulating between the reservoir and the wellbore; a control unit that prepares a data set from the detected properties and conditions of the fluids circulating between the reservoir and the wellbore; and a cleanup simulation system that performs: simulating a cleaning operation model of injecting the cleaning fluid into the wellbore based on the data set; determining parameter settings of the simulated cleaning operation model that satisfy prescribed constraints; and producing the procedure for cleaning the wellbore based on the determined parameters. 
         [0007]    Other aspects and advantages of the invention will be apparent from the following description and the appended claims. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0008]      FIG. 1  is a schematic drawing of a typical drilling system. 
           [0009]      FIG. 2  is a block diagram of a cleanup simulator system and a control unit in accordance with one embodiment of the present invention. 
           [0010]      FIG. 3  is a flow chart showing a cleanup procedure of a wellbore in accordance with one embodiment of the present invention. 
           [0011]      FIG. 4  is a flow chart showing an optimization procedure of a cleanup plan in accordance with one embodiment of the present invention. 
           [0012]      FIG. 5  is a flow chart showing an optimization procedure of a cleanup plan performed during a job execution in accordance with one embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0013]    Referring initially to  FIG. 1 , a schematic drawing of a typical drilling system is shown. A drilling system  10  is provided for drilling a wellbore into an earth formation  100  to exploit natural resources, such as oil. The drilling system  10  includes a derrick  20 , a drill string assembly  30 , a fluid circulation system  40 , a sensor unit  50 , a winch unit  70 , a control unit (data providing unit)  85 , and a cleanup simulation system  90 . The derrick  20  is built on a derrick floor  21  placed on the ground. Derrick  20  supports the drill string assembly  30 , which is inserted into a wellbore  101  and carries on a drilling operation. 
         [0014]    The drill string assembly  30  includes a drill pipe  31 , a bottom hole assembly  32 , and a drive system  33 . The bottom hole assembly  32  is provided with a drill bit  34 . The drill pipe  31 , which has a hollow cylindrical structure, extends from drive system  33  to the bottom hole assembly  32 . During an operation of drilling the wellbore  101 , the drill pipe  31  is rotated by the drive system  33 , and this rotation is transmitted through the bottom hole assembly  32  to the drill bit  34 . 
         [0015]    The fluid circulation system  40  includes a fluid pump  41 , a reservoir  42 , a supply line  43 , and a return line  44 . The fluid circulation system  40  circulates a drilling mud through the drill string assembly  30  and into the wellbore  101 . Specifically, the fluid pump  41  pumps drilling mud, which is contained in the reservoir  42 , out to the supply line  43  and, then, the drilling mud is injected into the drill pipe  31 . The drilling mud injected into drill pipe  31  is then discharged from the drill bit  34  to the bottom of the wellbore  101  and returns to the reservoir  42  through the return line  44 . An electrically-operated choke valve  83  adjusts the amount of the fluid flowing in the supply line  43 . 
         [0016]    After completion of the drilling operation, cleaning operations of the wellbore are performed to be remove contaminated fluids from the wellbore and the invaded zone around the wellbore. The cleaning operation is conducted in the very early period of production right after opening a well to remove all the contaminated fluids from the wellbore and the near wellbore region of the formation, by producing formation fluids, which pushes completion brine out of the wellbore and invasion zones of the formation. Thus, this cleaning stage may continue until the percentage of contaminants in the produced formation fluid is negligible. 
         [0017]    The sensor units  50 ,  51 ,  52  detect properties and conditions of the fluids in the wellbore  101  and the fluid circulation system  40 . Each sensor unit includes, for example, a thermometer, pressure sensor, a pH sensor, a redox (reduction/oxidation reaction) potential sensor, a viscosity sensor, a particle size sensor, a flow meter, and the like. The sensor unit  50  additionally includes a depth sensor. The sensor unit  50  is suspended in the wellbore  101  by a cable  71  to monitor the characteristics and conditions of the fluid in the wellbore  101 . The winch unit  70  lifts the cable  71  to adjust depth position of the sensor unit  50  in the wellbore  101 . The sensor unit  51  is placed in the reservoir  42  to monitor the characteristics and conditions of the fluid injected into the wellbore  101 . The sensor unit  52  is placed on the return line  44  to monitor the characteristics and conditions of the fluid returning to the reservoir  42 . 
         [0018]    The control unit  85  monitors properties and conditions of the fluids in the wellbore  101  and the fluid circulation system  40 . Also, the control unit  85  controls the operations of drilling and cleaning the wellbore  101  based on the monitored properties and conditions. The control unit  85  includes, for example, a central processing unit (CPU), a read only memory (ROM), a random access memory (RAM), input/output ports, memory, and the like. The control unit  85  is electrically connected to the drive system  33 , a downhole pump  35 , a gas lift valve  36 , a fluid pump  41 , the winch unit  70 , the sensor units  50 ,  51 ,  52 , and the cleanup simulation system  90 . The control unit  85  operates the drive system  33 , a fluid pump  41 , the winch unit  70 , and the choke valve  83  for drilling and cleaning the wellbore  101  according to preset programs and various detection signals from the sensor units  50 ,  51 ,  52 . The downhole pump  35  and the gas lift valve  36  may be adjusted by the control unit  85  during the cleanup operation. 
         [0019]    Further, the control unit  85  bi-directionally communicates with the cleanup simulation system  90 . Specifically, the control unit  85  stores information relating to the properties and conditions of the fluids in the wellbore  101  and the fluid circulation system  40 , and provides the stored information to the cleanup simulation system  90 . The cleanup simulation system performs various types of simulations for cleaning the wellbore  101  using the information provided by the control unit  85 , and designs an optimized procedure of cleaning the wellbore  101 . According to the optimized cleaning procedure, the control unit  85  edits and/or modifies the preset programs for a cleaning operation of the wellbore  101  by controlling the drive system  33 , the fluid pump  41 , the winch unit  70 , the choke valve  83 , and the like. 
         [0020]    Referring now to  FIG. 2 , a block diagram of a cleanup simulation system  90  and control unit  85  is shown. The control unit  85  processes information received from the sensor units  50 ,  51 ,  52 , and converts the information into appropriate data sets for the simulations by the cleanup simulation system  90 . The cleanup simulation system  90  receives the data sets from the control unit  85 , and performs simulations relating to property changes of the fluid in the wellbore  101  during the cleaning operation. The simulation result is fed back to the control unit  85 . The control unit  85  conducts an optimized cleaning operation of the wellbore  101  according to the simulation result from the cleanup simulation system  90 . 
         [0021]    The data sets prepared in the control unit  85  are categorized into four groups comprising reservoir data, wellbore data, fluid loss data, and a fluid loss profile. The reservoir data represents parameters related to properties of the fluid existing in the reservoir at the initial stage of the cleaning operation, and properties of the reservoir itself. The reservoir data includes, for example, reservoir geometry, rock and fluid properties, initial state of the reservoir at the time of simulation, and the like. 
         [0022]    The wellbore data represents parameters related to the geometry and trajectory of the wellbore, and characteristics and locations of devices inside the wellbore, such as the drill string assembly  30 , and devices on the surface, such as the supply line  43  and the return line  44 . The type of completion and the way that the reservoir is exposed to the wellbore can also be used as the wellbore data. 
         [0023]    The fluid loss data mostly includes volumetric historical information collected during the drilling and completion period, which relates to the amount of fluid lost into the formation. This information is used to mark the depths and locations where there has been a considerable amount of fluid loss. This information is also used to estimate the total amount of foreign fluid expected to return to the surface from the formation. 
         [0024]    The fluid loss profile includes measurements of fluid loss along the wellbore. The fluid loss profile can be obtained by direct measurements inside the wellbore. These measurements can be conducted, for example, by obtaining resistivity logs, which express the radius of the invaded zone around the wellbore. 
         [0025]    The cleanup simulation system  90  includes a porous media multiphase flow simulator  91 , a wellbore multiphase simulator  92 , and a process component simulator  93 , and an integration section  94 . The simulator  91 ,  92 , or  93  stores feasible models for simulating dynamic states of fluids with simulation objects. Each model is a function, in which a dependent variable (the simulation object) is a fluid property (for example, the density of the cleaning fluid) and independent variables are various factors that influence dynamic states of the fluid during the cleaning operation. When constructing the feasible models, uncertainty parameters, which possibly influence the dynamic states of the fluid during the cleaning operation, may also be included. The uncertainty parameters can be updated after the simulations. 
         [0026]    For example, the porous media multiphase flow simulator  91  is designed to simulate a multiphase fluid flow in porous media. Specifically, the simulator  91  simulates the interaction between the foreign fluid (the fluid flowing into the reservoir through the return line  40 ) and the fluid in the reservoir  42 . The dependent variables of the function in the simulator  91  are dynamic states of the two fluids in the reservoir, and the independent variables are temperature, pressure, and the like in the reservoir  42 . Accordingly, the simulator  91  is able to model the fluid flow in different pressure and temperature conditions. Further, the simulator  91  is particularly able to model the multiphase fluid flow with high accuracy around the wellbore and contact regions of phases. 
         [0027]    The wellbore multiphase simulator  92  is designed to simulate a multiphase flow of the fluids in a wellbore. The wellbore and the porous media are modeled interactively to represent the fluid flow from the reservoir to the wellbore and from the wellbore into the reservoir at all times. 
         [0028]    The process component simulator  93  is designed to simulate how process components throughout the flowline influence the pressure and temperature in the fluid circulation system  40 . The process components include, for example, the supply line  43 , return line  44 , the choke valve  83 , the fluid pump  41 , and any other components of the fluid circulation system  40 , which may significantly influence the pressure and/or temperature thereof. 
         [0029]    Integration section  94  integrates simulation results from all of the simulation models by the simulator  91 ,  92 ,  93 . The integration result by the section  94  may include a time profile in concentration of the cleaning fluid inside the wellbore during the cleaning operation with the most preferable parameter settings from the perspective of cost efficiency, time efficiency, and the like. The set parameters may include a pumping rate of the fluid pump  41 , opening degree of the choke valve  83  during the cleaning operation, concentration of the cleaning fluid reserved in the reservoir  42  at the initial stage of the cleaning operation, and the like. Further, the integration section  94  constructs a cleaning operation program based on the integration result. 
         [0030]    The cleanup simulation system  90  displays the simulation results from the simulators  91 ,  92 ,  93 , and the integration result from the integration section  94  for job monitoring purposes, and prints the results as documents to be used for scenario selection and optimization algorithms. 
         [0031]    Referring now to  FIG. 3 , a wellbore cleaning procedure is shown in a flow chart. In the cleaning procedure, the fluid and the filter cake in the wellbore  101  are cleaned by the cleaning fluid supplied from the reservoir  42 . 
         [0032]    At Step  101  of the process, the control unit  85  prepares data sets based on the information from sensor units  50 ,  51 ,  52 , and inputs the data sets into the cleanup simulation system  90 . 
         [0033]    At Step  102 , using the data sets received from the control unit  85 , the cleanup simulation system  90  performs the simulations to find the most preferable parameter settings for the cleaning operation from the perspective of cost efficiency, time efficiency, and the like. 
         [0034]    At Step  103 , the cleanup simulation system  90  evaluates whether the parameter settings obtained at Step  102  satisfy prescribed constraints. If the evaluation result is positive, at Step  104 , the cleanup simulation system  90  constructs a cleaning operation program based on the simulation model with the most preferable parameter settings, which were obtained at Step  102 . If the evaluation result is negative, by returning Step  102 , the cleanup simulation system  90  re-performs the simulation model with alternative parameter settings so that the simulation result satisfies the constraints. The constraints may be categorized into completion constraints and production constraints. Regarding the completion constraints, the maximum drawdown needed to produce from the formation will be known. The maximum drawdown is governed by the maximum drawdown that the downhole completion can handle. For example, in the case of a gravel-packed completion, there is a maximum drawdown that one can apply in the bottom hole. That is, there is a maximum drawdown that can be applied on the formation to prevent collapsing the well. Other production issues, such as sand production for unconsolidated formations, can be considered. 
         [0035]    Regarding the production constraints, the minimum bottom hole pressure to lift the cushion fluid to the surface will be known. This leads to whether there is a need for any artificial lift systems or whether the well will flow naturally. The minimum flow rates that can prevent the cushion fluids from slipping back down to the bottom hole are calculated during the simulation. This is a very important factor in ensuring that all the foreign fluids have been removed from the wellbore. Water coning and gas coning can be studied before performing the real operation. For each scenario, the possibility of water/gas coning is analyzed to prevent major damage to the productivity of the well. 
         [0036]    Finally, at Step  105 , the control unit  85  conducts the cleaning operation of the wellbore  101  by running the cleaning operation program. 
         [0037]    Based on the above procedure, the control unit  85  and the cleanup simulation system in accordance with one or more embodiments of the present invention cooperate with each other so as to design the most preferable plan for a cleaning operation of the wellbore  101  after completion of the drilling operation. For example, the maximum flow rate that needs to be handled on the surface or downhole for any particular cleanup plan will be known. This will be used to select the appropriate cleanup facilities for any particular cleanup scenario. Further, duration of cleaning operation, which is some of the most important information that one can achieve by simulation of cleanup process for any particular well, can be precisely estimated. 
         [0038]    The cleanup simulation system  90  in accordance with one or more embodiments can be used to select the optimum cleaning procedure to decrease the duration of operation. Minimization of cleanup duration is done with an optimization algorithm to assure the quality of the job. As noted before, the quality can be reflected by the amount of fluid abandoned inside the formation and also the final predictability of the wellbore right after the cleaning operation. 
         [0039]    Further, the cleanup simulations can be operated independent from the drilling system described in the above embodiments. For example, the cleanup simulations referring to the porous media multiphase flow model, the wellbore multiphase flow model, the process components model, or any combination thereof, can be performed based on data sets including the reservoir data, the wellbore data, the fluid loss data, the fluid loss profile, or any combination thereof. An appropriate cleanup plan for a wellbore can be advantageously scheduled and/or modified according to the simulation results. 
         [0040]    Furthermore, the simulation results can be used in many different ways during cleanup. For example, the simulation results can be used to ensure that the planned cleanup is successful in removing the contaminations from the wellbore vicinity. That is, assuming that a satisfactory estimation of the initial profile of the fluid lost into the formation was obtained, by modeling the cleanup process, it is possible to have a good estimation of the fluid loss profile around the wellbore after the cleaning operation has finished. Accordingly, the simulation results can be used as quality control for the cleaning operation. This is particularly important as fluid fractions on the surface are not always a good indication of a successful cleanup job. Even after achieving a negligible value of basic sediment and water, it is possible to leave a large amount of foreign fluids inside the formation. 
         [0041]    The cleanup simulation system in accordance with one or more embodiments of the present invention is capable of determining a cleanup plan having the highest cleanup efficiency during the cleaning operation. Based on the type of formation and the type of completion, different scenarios are proposed to perform the cleaning operation. In another example, based on the reservoir properties of each section of well, a schedule for start of production from each interval can be determined. The amount of fluid lost in each interval is also a major factor during the selective cleanup planning. 
         [0042]    Further, the cleanup plan created based on, for example, the above process may be optimized by an additional optimization procedure. Referring to  FIG. 4 , a flow chart is shown of an optimization procedure of a cleanup plan. At Step  201  of the process, a base case of job design is created by the clean up simulation system  90 . At Step  202  of the process, sensitivities of the base case to job execution parameters are determined. The sensitivities may include, for example, sensitivity of job duration or cleanup efficiency to choke size, choke sequence, and choke duration, etc. At Step  203  of the process, the above sensitivity information is input to the optimization system. At Step  204  of the process, an allowed range for each execution parameter is input to the optimization system. The ranges are specified as, for example, the minimum and maximum values of choke sizes, and wellhead pressures, etc. At Step  205  of the process, the key metrics of the optimization are specified. The key metrics may be related to technical, operational, and financial issues, such as technical and operational efficiencies, and cost minimization, respectively. At Step  206  of the process, the results from the optimization may be validated by the cleanup simulation system  90 , for example, to ensure that the results are not biased for local minima. At the end of the process, an optimized plan for a job execution (actual cleanup operation) is obtained. 
         [0043]    Further, all of the studies that have been done during the planning of the operation may be validated in real time during the cleaning operation. This validation is done in two ways. First, different scenarios can be chosen based on the uncertainty of the parameters that have been used during the simulation and plan study. Second, based on the feedback from the measurements during the cleaning operation, the behavior of the wellbore can be checked against the simulation results and the closest scenario is used for the remainder of the process. 
         [0044]    For example, referring now to  FIG. 5 , a flow chart is shown of an optimization procedure of a cleanup plan performed during a job execution. At Step  301  of the process, a job (actual cleanup operation) is started with the cleanup plan that was obtained previously. At Step  302  of the process, during the operation, observation data are acquired, for example, based on downhole and surface measurements of various parameters, such as rates, cuts, pressures, and Pit Volume Totalizer (PVT), etc. At operationally feasible time intervals, the measurement results are used to update the input parameters used in the present job design (Step  303 ). For example, if the observed value of fluid density is different from the value applied to the previous simulation, the observed value will be applied to the next simulation as an updated value for the parameter. At Step  304  of the process, updated values for the input parameters are applied to a new job design. At Steps  305 - 306  of the process, incremental adjustments (preset value change in one adjustment step) may be made to move the execution from the previous values to new values for the input parameters. After the incremental change at Step  305 , the changed value is validated at Step  306 . Such an incremental change process continues until no further adjustments are needed (for example, until the measurement values are consistent with the input value). The above optimization procedure is repeated until job completion criteria are satisfied. Such criteria may be defined as allowable values for recovery % of non-reservoir fluid loss in drilling and completion, the concentration of the non-reservoir fluids in well effluent, job duration, and the like. 
         [0045]    Advantages of one or more embodiments of the present invention may include one or more of the following. One or more embodiments provide a method for planning a procedure for cleaning the wellbore based on results from various types of simulation models, which refer to factors that influence the efficiency of the cleaning process. One or more embodiments allow cleanup procedure cost and time savings to be realized. One or more embodiments involve not only designing a cleaning operation procedure using a preset facility, but also, involve designing a cleaning procedure for a wellbore that includes actually re-designing the facilities, such as a choke valve, a supply line, a return line, and the like, of the drilling system. 
         [0046]    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.