Autodriller system

Abstract

An autodriller provides an automatic self calibration system to predict both axial and rotational friction coefficients from the surface measurements such as hookload and surface torque while a drill bit is off bottom. The friction coefficients are used to calculate the friction forces between drillstring and the wellbore while the bit is on bottom during drilling operation to estimate downhole weight on the bit and bit torque, and use the estimates to alter the drilling process.

1 . A method of drilling a formation, the method comprising:
 drilling the formation with a drilling system that includes a drillstring and a bit;   estimating a friction force on the drillstring;   estimating a downhole weight on bit using a surface measurement and the estimated friction force; and   taking an action to change drilling of the formation based on estimated downhole weight on bit.   
     
     
         2 . The method of  claim 1  in which estimating a friction force on the drillstring comprises applying a friction model. 
     
     
         3 . The method of  claim 1  in which the action comprises modifying drilling by modifying a drilling parameter. 
     
     
         4 . The method of  claim 1  in which the friction force is estimated according to the friction model and according to a first surface measurement conducted while the bit is off bottom. 
     
     
         5 . The method of  claim 4  in which the first surface measurement is a hook load measurement. 
     
     
         6 . The method of  claim 5  in which the surface measurement is a hook load measurement. 
     
     
         7 . The method of  claim 1  in which the drilling parameter is one or more of surface torque, drillstring rotation rate, hook load and mud weight. 
     
     
         8 . The method of  claim 1  in which estimating the downhole weight on the bit comprises:
 (a) determining the static weight of drillstring; 
 (b) determining an axial friction coefficient including pipe rotation effect; 
 (c) determining the effect of downhole weight on the bit on value of axial friction force during drilling; and 
 (d) determining downhole weight on the bit using axial friction coefficient and surface hookload measurements. 
 
     
     
         9 . The method of  claim 1  incorporated in an autodrilling system in which the autodrilling system automatically adjusts surface WOB utilized by the autodrilling system. 
     
     
         10 . The method of  claim 1  in which taking an action comprises the autodrilling system displaying surface WOB from hook load measurements and estimated downhole weight on bit. 
     
     
         11 . The method of  claim 1  further comprising identifying a founder point where estimated weight on bit is non-zero and rate of penetration is not increasing. 
     
     
         12 . The method of  claim 1  further comprising calibrating the axial friction coefficient and the rotational friction coefficient from the surface measurement and identifying a drilling problem. 
     
     
         13 . The method of  claim 12  in which the drilling problem is one or more of string sticking and insufficient hole cleaning. 
     
     
         14 . The method of  claim 8  wherein the static weight of drillstring is calculated using survey data, drillstring specification and local buoyancy factor at any bit depth which are given by mud logging unit on the rig site. 
     
     
         15 . The method of  claim 8  wherein the axial friction coefficient including the drillstring rotation effect is estimated using surface measured hookload determined when the bit is off bottom. 
     
     
         16 . The method of  claim 15  wherein the surface measured hookload is determined while the bit is off bottom and has a drillstring rotation, and the bit is sufficiently close to bottom that the drillstring rotation is the same as the expected drillstring rotation in the formation to be drilled. 
     
     
         17 . The method of  claim 1  in which, during drilling, different equations are employed to determine the downhole weight on bit depending on whether a part of the drillstring in a curved section is in compression or tension. 
     
     
         18 . The method of  claim 1  further comprising:
 calculating hookload by using axial friction coefficient and estimating the weight on the bit; 
 comparing calculated hookload with measured hookload value; and 
 if the difference between calculated hookload and measured hookload value is negligible, the estimated downhole weight on the bit is taken as downhole weight on the bit, and if the difference is not negligible, another value will be estimated for downhole weight on the bit and this procedure is repeated to determine the downhole weight on the bit. 
 
     
     
         19 . The method of  claim 8  further comprising estimating downhole torque on bit comprising the steps of:
 (a) determining the rotational friction force while drilling bit is off bottom; 
 (b) determining the rotational friction coefficient including axial pipe movement effect from surface torque measurements while bit is off bottom; 
 (c) determining the effect of downhole weight on the bit on value of rotational friction force during drilling; and 
 (d) determining downhole bit torque by using rotational friction coefficient and estimated downhole weight. 
 
     
     
         20 . The method of  claim 19  in which taking an action comprises determining when to pull the bit off the bottom and pulling the bit off bottom. 
     
     
         21 . The method of  claim 19  wherein the rotational friction coefficient including the drillstring axial movement effect is estimated while the bit is off bottom and there is no torque at the bit. 
     
     
         22 . The method of  claim 21  wherein the measured surface torque is measured while the bit is moving downwardly and sufficiently close to bottom that the drillstring rotation at the point of measurement is the same as drillstring rotation in the formation to be drilled. 
     
     
         23 . The method of  claim 19  in which, during drilling, different equations are employed to determine the downhole torque on bit depending on whether a part of the drillstring in a curved section is in compression or tension. 
     
     
         24 . The method of  claim 19  wherein rotational friction force while bit is on bottom is estimated by using rotational friction coefficient and downhole weight on the bit. 
     
     
         25 . The method of  claim 1  used in sliding drilling using a mud driven motor. 
     
     
         26 . The method of  claim 25  further comprising finding K, where K is the estimated downhole weight on bit divided by pressure differential across the mud driven motor. 
     
     
         27 . The method of  claim 26  in which K is used to improve drilling performance. 
     
     
         28 . Computer readable media containing, in non-transient form, instructions for instructing an autodriller system to carry out the method of  claim 1 . 
     
     
         29 . A drilling system comprising a rig, a drill string connected downwardly into a borehole, and an autodriller system, the autodriller system being configured to carry out the method of  claim 1 .