Determination of Drillstring Parameters and Associated Control

Abstract

A method for modelling a drillstring in a wellbore, the method comprising providing a model of the drillstring, the model representing the drillstring by a sequence of alternating springs and elements, where each element describes the mass and/or the moment of inertia of a corresponding part of the drillstring and each spring represents at least one of: axial, torsional and/or bending stiffnesses of one of the corresponding parts of the drillstring, wherein the model describes one or more forces on each element by one or more systems of ordinary equations of first or second order, where each equation comprise a linear part that comprises constant coefficients and a non-linear part that includes one or more of: one or more non-linear terms, one or more non-smooth terms, one or more time dependent terms and/or one or more coupled terms; and the method comprises recalculating the model for a plurality of time steps, wherein recalculating the model for the respective time step comprises calculating one, two or three dimensional positions, orientations and/or associated derivatives of all of the elements for the respective time step based on one, two or three dimensional positions, orientations and/or associated derivatives of all element at a previous time step, by describing the non-linear part by a form of expansion with respect to time and solving the system of equations either analytically or by the use of exponential integrators for the duration of the respective time step. Also described is a method of cleaning a wellbore using the model from the above method.

1 . A method for modelling a drillstring in a wellbore, the method comprising:
 providing a model of the drillstring, the model representing the drillstring by a sequence of alternating springs and elements, where each element describes the mass and/or the moment of inertia of a corresponding part of the drillstring and each spring represents at least one of: axial, torsional and/or bending stiffnesses of one of the corresponding parts of the drillstring, wherein the model describes one or more forces acting on each element by one or more systems of ordinary equations of first or second order, where each equation comprises a linear part with constant coefficients and a non-linear part that includes one or more of: one or more non-linear terms, one or more non-smooth terms, one or more time dependent terms and/or one or more coupled terms; and   the method comprises recalculating the model for a plurality of time steps, wherein recalculating the model for the respective time step comprises calculating one, two or three dimensional positions, orientations and/or derivatives thereof of all of the elements for the respective time step based on one, two or three dimensional positions, orientations and/or derivatives thereof of all element at a previous time step, by describing the non-linear part by a form of expansion with respect to time and solving the one or more systems of equations either analytically or by the use of exponential integrators for the duration of the respective time step.   
     
     
         2 . The method of  claim 1 , wherein the recalculating of the model comprises dynamically and repeatedly recalculating the model according to a set, selected or predefined recalculation rate. 
     
     
         3 . The method of  claim 2 , wherein the predefined, or dynamically adjusted, recalculation rate has a time step in the order of 10 1  to 10 6  milliseconds. 
     
     
         4 . The method of  claim 3 , wherein the constant coefficients of the linear term are not recalculated for every timestep, but are recalculated according to a recalculation condition. 
     
     
         5 . The method of  claim 4 , wherein the recalculation condition comprises a change in one or more of; drillstring length or dimension, detected activity code, expected friction forces, and/or the expected intrinsic energy of the rock. 
     
     
         6 . The method of  claim 1 , wherein using the model to determine one or more parameters of the drillstring, in use comprises analytically solving the model in order to determine the one or more parameters. 
     
     
         7 . The method of  claim 1 , wherein the model comprise lateral movement functionality for determining properties associated with lateral movements of the drill string. 
     
     
         8 . The method of  claim 1 , wherein the model is configured to:
 implement a buoyancy factor to adjust the model for buoyancy of the drillstring;   determine a factor for friction forces on the drillstring due to fluid in the wellbore;   implement a normal force model for modelling normal forces acting on each of the weights or segments so as to determine the friction between an inner wall of the wellbore and the outer surface of the drillstring;   determine forces and torques acting on every element of the drillstring including the bottom hole assembly and the drillbit.   determine axial and rotational movements with derivatives on every element of the drillstring including the bottom hole assembly and the drillbit;   determine hookload of a drilling system that comprises the drillstring, in use;   determine surface torque of a drilling system that comprises the drillstring, in use and/or   determine which segments of the drillstring are moving and/or which are stalled.   
     
     
         9 . The method of  claim 1 , wherein the method comprises using the model to determine whether one or more segments of the drillstring are in an upper side or lower side of the part of the wellbore in which the respective segment is located when viewed in a lateral cross section. 
     
     
         10 . The method of  claim 7 , comprising determining whether the one or more segments of the drillstring lie on an upper side of the interior of the wellbore or on a lower side of the interior of the wellbore based on the sign of the normal forces and/or lateral accelerations on the section(s) of the drillstring. 
     
     
         11 . The method of  claim 1 , comprising determining an alarm or alert condition and raising an alert or alarm based on the determination that the alarm or alert condition has been met based on an output of the model and/or automatically controlling a drilling operation and/or a issuing a control command to control a drilling operation based on the output of the model. 
     
     
         12 . The method of  claim 11 , wherein the alarm or alert condition is indicative of one or more of: overpulls, tookweights, maxed out torque, top drive stallouts and/or erratic torque. 
     
     
         13 . The method of  claim 1 , comprising:
 automatically detecting when one or more joints of the drillstring is added or removed;   determining a new drillstring length accounting for the addition or removal of the joints of the drillstring; and   automatically updating the model with the new drillstring length.   
     
     
         14 . The method of  claim 1 , wherein the model comprises a bit rock model that describes the interaction between the drill bit of the drillstring and rock currently being drilled and determining one or more of: a weight on bit, a torque on bit, a rate of penetration of the bit and/or a hole depth or the wellbore, using the model comprising the bit-rock model. 
     
     
         15 . The method of  claim 14 , wherein the method comprises discounting effects due to axial vibrations in the bitrock model. 
     
     
         16 . The method of  claim 14 , comprising:
 determining the interaction between the drill bit and the rock currently being drilled using the bit rock model based on an intrinsic specific energy associated with the rock currently being drilled;   determining values of one or more of: the weight on bit, the torque on bit, the rate of penetration of the bit, the hole depth and the intrinsic specific energy by minimizing the difference between the measured and calculated values of one or more of: the hookload, the top side torque, or any other surface and/or downhole measurements.   
     
     
         17 . The method of  claim 1 , wherein the model comprises a friction model component for determining friction forces between the drill string and the wellbore wall based on a method that comprises updating the friction coefficients by minimizing the difference between measured and calculated values of one or more of: the hookload, the top side torque, or any other surface and/or downhole measurements. 
     
     
         18 . The method of  claim 1 , wherein the model is used to store wear characteristics of every joint of the drillstring, every position in the casing and in the open hole. 
     
     
         19 . The method of  claim 1 , comprising using values calculated using the model to detect erroneous measurements, such as one or more of: hookload, weight on bit, rate of penetration, block height, bit depth, hole depth, rotary speed, torque, downhole weight on bit, downhole rotation speed, downhole torque, circulation rate, stand pipe pressure and ECD. 
     
     
         20 . The method of  claim 1 , comprising using the model to control how activities are executed, such as one or more of: starting and stopping rotation, starting and stopping tripping in, starting and stopping tripping out, starting and stopping reaming in, starting and stopping reaming out and starting and stopping to drill. 
     
     
         21 . The method of  claim 20 , where measurements of the heave motions of the rig is taken into account in order to reduce axial vibrations. 
     
     
         22 . The method of  claim 1 , comprising using the model to detect resonant frequencies while drilling under various combinations of one or more of the following: rotary speed, weight on bit and rate of penetration, estimated intrinsic specific energy. 
     
     
         23 . The method of  claim 1 , comprising:
 storing states of the model for a plurality of time instances as snapshots, the states comprising historic information that the module requires to restart for a certain time instance; and   providing a functionality to restart calculation of the drillstring form any of these stored snapshots.   
     
     
         24 . The method of  claim 1 , comprising:
 determining one or more parameters of the wellbore using a rollback functionality and/or by providing the snapshots of the drillstring, wherein the determining of the one or more parameters of the wellbore comprises trying parameters or parameter curves by:
 determining a difference between a measured or calculated property and the corresponding values of the property determined based on the parameter curve; and 
 rolling back or reverting to a snapshot and trying a different parameter or parameter curve when the difference between the measured or calculated property and the corresponding values of the property determined based on the parameter curve is greater than a threshold. 
   
     
     
         25 . The method of  claim 1 , wherein the system of equations for the second order differential equations is converted to a larger system of first order differential equations that are solved by exponential integrators. 
     
     
         26 . A method for cleaning a wellbore using a drillstring located within the wellbore and movable by a drillstring handling system, the method comprising:
 operating the drillstring handling system to pull and/or lower the drillstring, with or without rotation, so as to selectively induce lateral motions of the drillstring in the wellbore in a direction between an upper and lower part of the wellbore; wherein   the operation of the drillstring handling system to selectively induce the lateral motions is responsive to one or more outputs of a model of the drillstring, the model representing the drillstring by a sequence of alternating springs and elements, where each element describes the mass and/or the moment of inertia of a corresponding part of the drillstring and each spring represents at least one of: axial, torsional and/or bending stiffnesses of one of the corresponding parts of the drillstring.   
     
     
         27 . The method of  claim 26 , comprising:
 selectively inducing lateral motions in selected segments of the drillstring depending at least in part on a determination using the model of whether the respective segments are located in a lower side of the wellbore in cross sectional view through the wellbore or in an upper side of the wellbore in cross sectional view through the wellbore.   
     
     
         28 . The method of  claim 26  comprising:
 selectively inducing lateral motions in selected segments of the drillstring depending at least in part on a determination using the model of whether the respective segments are axially rotating or stalled and/or whether the drillstring is sticking or slipping in the wellbore. 
 
     
     
         29 . The method according to  claim 26 , comprising cleaning the wellbore by selectively inducing lateral motions in the drillstring in sections of the wellbore that are obliquely oriented with respect to vertical such that they lie within a threshold range of angles from vertical. 
     
     
         30 . The method according to  claim 26 , wherein the inducing of the lateral motions in the drillstring in the wellbore comprises switching between two or more of:
 (i) inducing a plurality of lateral motions in the drillstring in which the bottom hole assembly attached to the drillstring is either static or moves less than a threshold amount;   (ii) inducing a plurality of lateral motions in the drillstring in which the bottom hole assembly attached to the drillstring moves more than a threshold amount; and   (iii) inducing harmonic axial movements in the drillstring that are at a harmonic or resonant frequency of the drillstring.   
     
     
         31 . The method of  claim 30 , wherein the switching is responsive to parameters calculated using the model and/or based on the geometry of the wellbore and/or based on a determination that an associated segment of the drillstring is located on a lower side of the wellbore in cross sectional view through the wellbore or in an upper side of the wellbore in cross sectional view through the wellbore. 
     
     
         32 . A system comprising at least one processing device and a computer readable data store storing a computer program, the computer program comprising instructions that, when the program is implemented by the at least one processing device, cause the processing device to carry out the method of  claim 1 . 
     
     
         33 . The system of  claim 32  configured to communicate with a controller for controlling a drillstring handling system, the drillstring handling system comprising a hoist for hoisting the drillstring and a rotation mechanism for rotating the drillstring, the controller comprising at least one processor and a computer readable data store storing a computer program, the computer program comprising instructions that, when the program is implemented by the at least one processor, cause the processor to carry out the method of  claim 1  to control the drillstring handling system. 
     
     
         34 . A computer program product comprising instructions that, when the program is implemented by the at least one processor, cause a processor or controller for a drillstring handling system to carry out the method of  claim 1 .