Prediction of surface gas concentrations in drilling fluid

Abstract

A method for estimating surface concentrations of gas in a drilling fluid in use in a drilling rig includes measuring gas-out or gas-in concentrations while drilling a wellbore and processing the gas-out measurements or the gas-in measurements with a calibrated model to estimate corresponding gas-in concentrations or gas-out concentrations.

What is claimed is: 
     
         1 . A method for estimating surface concentrations of gas in a drilling fluid in use in a drilling rig, the method comprising:
 measuring first concentrations of the gas in the drilling fluid as it exits a wellbore (gas-out) or second concentrations of the gas in the drilling fluid as it is pumped downhole (gas-in) while drilling a wellbore; and   estimating the gas-in concentrations or the gas-out concentrations by evaluating the gas-out measurements or the gas-in measurements with a calibrated model.   
     
     
         2 . The method of  claim 1 , wherein the calibrated model is obtained by:
 measuring gas-out concentrations and gas-in concentrations while drilling at least one other section of the wellbore or another wellbore; and   evaluating the measured gas-out concentrations and gas-in concentrations with a model to obtain the calibrated model.   
     
     
         3 . The method of  claim 2 , wherein the model is configured to account for degassing of the drilling fluid with time as the drilling fluid moves through surface equipment located on the drilling rig. 
     
     
         4 . The method of  claim 3 , wherein the surface equipment comprises at least one member of the group consisting of a mud pump, a degasser, a shale shaker, a desilter, a desander, and a mud pit in fluid communication with the drilling fluid. 
     
     
         5 . The method of  claim 3 , wherein the model comprises a delay first order ordinary differential equation. 
     
     
         6 . The method of  claim 5 , wherein the model equates a first derivative of the gas-in concentrations with respect to time to a difference between a first product and a second product, the first product being a product of the gas-out concentrations at a time offset by a surface transit time and a first model parameter, and the second product being a product of the gas-in concentrations and a second model parameter. 
     
     
         7 . The method of  claim 2 , wherein the evaluating comprises determining a set of model parameters that provide a fit between the gas-out measurements and modeled gas-out concentrations. 
     
     
         8 . The method of  claim 7 , further comprising:
 repeating the measuring and the evaluating at a plurality of distinct sets of drilling conditions to obtain a corresponding plurality of sets of the model parameters; and   using a learning algorithm to generate a parameter model that correlates the plurality of distinct sets of drilling conditions and the plurality of sets of the model parameters.   
     
     
         9 . The method of  claim 8 , wherein the evaluating comprises:
 obtaining a set of drilling conditions for the other section;   processing the set of drilling conditions with the parameter model to obtain a predicted set of model parameters and thereby obtain the calibrated model; and   processing the gas-out measurements from the second section with the calibrated model to estimate the corresponding gas-in concentrations.   
     
     
         10 . The method of  claim 1 , wherein the gas-out measurements and the gas-in measurements comprise measurements of alkane gas concentrations. 
     
     
         11 . A surface system configured for use on a drilling rig, the surface system comprising:
 a gas measurement module configured to measure first concentrations of a gas in drilling fluid as it exits a wellbore (gas-out) and second concentrations of the gas in the drilling fluid as it is pumped downhole (gas-in) while drilling; and   a processor configured to:
 receive gas-out and gas-in measurements made while drilling a first section of a subterranean wellbore; 
 evaluate the gas-out and gas-in measurements with a model to generate a calibrated model; 
 receive gas-out measurements or gas-in measurements made while drilling a second section of a subterranean wellbore; and 
 estimate the gas-in concentrations or the gas-out concentrations by evaluating the gas-out measurements or the gas-in measurements with the calibrated model. 
   
     
     
         12 . The surface system of  claim 11 , wherein the model is configured to account for degassing of the drilling fluid with time as the drilling fluid moves through surface equipment located on the drilling rig. 
     
     
         13 . The surface system of  claim 12 , wherein the surface equipment comprises at least one of a mud pump, a degasser, a shale shaker, a desilter, a desander, and a mud pit in fluid communication with the drilling fluid. 
     
     
         14 . The surface system of  claim 12 , wherein the model comprises a delay first order ordinary differential equation. 
     
     
         15 . The surface system of  claim 14 , wherein the model equates a first derivative of the gas-in concentration with respect to time to a difference between a first product and a second product, the first product being a product of the gas-out concentration at a time offset by a surface transit time and a first model parameter, and the second product being a product of the gas-in concentration and a second model parameter. 
     
     
         16 . A method for estimating surface concentrations of gas in drilling fluid while drilling a wellbore, the method comprising:
 drilling the wellbore at a set of drilling conditions;   evaluating the set of drilling conditions with a parameter model to obtain a calibrated degassing model including a set of calibrated degassing model parameters;   measuring first concentrations of a gas in the drilling fluid as it exits the wellbore (gas-out) or second concentrations of the gas in the drilling fluid as it is pumped downhole (gas-in) while drilling; and   estimating the gas-in concentrations or the gas-out concentrations by evaluating the gas-out measurements or the gas-in measurements with the calibrated model.   
     
     
         17 . The method of  claim 16 , wherein the parameter model is obtained via:
 drilling a plurality of other sections of the wellbore or other wellbores at a corresponding plurality of distinct sets of drilling conditions;   making a plurality of sets of gas-out and gas-in measurements at a plurality of sets of drilling conditions;   processing the plurality of sets of gas-out and gas-in measurements with a degassing model to obtain a corresponding plurality of sets of degassing model parameters; and   correlating the plurality of sets of drilling conditions with the corresponding plurality of sets of degassing model parameters to obtain the parameter model.   
     
     
         18 . The method of  claim 16 , wherein the calibrated degassing model is configured to account for degassing of the drilling fluid with time as the drilling fluid moves through surface equipment on a drilling rig used to drill the wellbore. 
     
     
         19 . The method of  claim 16 , wherein the calibrated degassing model comprises a delay first order ordinary differential equation. 
     
     
         20 . The method of  claim 19 , wherein the calibrated degassing model equates a first derivative of the gas-in concentration with respect to time to a difference between a first product and a second product, the first product being a product of the gas-out concentration at a time offset by a surface transit time and a first model parameter, and the second product being a product of the gas-in concentration and a second model parameter.