Determining formation gas composition during well drilling

Abstract

Some aspects determining formation gas composition during well drilling can be implemented as a computer-implemented method, a computer-readable medium, or a computer system. A theoretical diffusion coefficient for a drilling fluid that comprises gas from a formation through which the drilling fluid is flowed is determined. The theoretical diffusion coefficient is based on an extraction of all of the gas from the drilling fluid. An experimental diffusion coefficient for the drilling fluid based on well drilling parameters is determined. A concentration of the gas at the formation is determined based, at least in part, on a difference between the theoretical diffusion coefficient and the experimental diffusion coefficient. The determined concentration at the formation is provided.

1 . A computer-implemented method comprising:
 determining a theoretical diffusion coefficient for a drilling fluid comprising a gas from a formation, the theoretical diffusion coefficient determined based on an extraction of all of the gas from the drilling fluid;   determining an experimental diffusion coefficient for the drilling fluid based on well drilling parameters including a flow rate of the drilling fluid through the well;   determining a concentration of the gas at the formation based, at least in part, on a difference between the theoretical diffusion coefficient and the experimental diffusion coefficient; and   providing the determined concentration of the gas at the formation.   
     
     
         2 . The method of  claim 1 , wherein determining the theoretical diffusion coefficient comprises solving Fick's first law of diffusion or Fick's second law of diffusion for the theoretical diffusion coefficient. 
     
     
         3 . The method of  claim 1 , wherein determining the theoretical diffusion coefficient and the experimental diffusion coefficient comprises determining the theoretical diffusion coefficient and the experimental diffusion coefficient from sample data describing the drilling fluid carrying the gas near the formation. 
     
     
         4 . The method of  claim 1 , wherein determining the theoretical diffusion coefficient and the experimental diffusion coefficient comprises determining the theoretical diffusion coefficient and the experimental diffusion coefficient from sample data describing the drilling fluid carrying the gas at the surface. 
     
     
         5 . The method of  claim 4 , wherein determining the theoretical diffusion coefficient and the experimental diffusion coefficient from sample data comprises measuring the concentration of the gas in a sample of the drilling fluid using one or more of gas chromatography, liquid chromatography, and mass spectrometry. 
     
     
         6 . The method of  claim 1 , wherein determining the experimental diffusion coefficient comprises:
 identifying physical and chemical properties of the drilling fluid at an instant in time; and   determining the experimental diffusion coefficient for the drilling fluid having the identified physical and chemical properties at the instant in time.   
     
     
         7 . The method of  claim 6 , wherein determining the experimental diffusion coefficient comprises providing the identified physical and chemical properties at the instant in time to a mathematical model that determines the experimental coefficient based on the identified physical and chemical properties at the instant in time. 
     
     
         8 . The method of  claim 1 , further comprising determining a plurality of experimental diffusion coefficients of the gas at the formation at a respective plurality of sequential instances in time. 
     
     
         9 . The method of  claim 8 , further comprising periodically determining the concentration of the gas at the formation based, at least in part, on a plurality of differences between the theoretical diffusion coefficient and the plurality of experimental diffusion coefficients. 
     
     
         10 . The method of  claim 1 , further comprising:
 determining a plurality of concentrations of the gas at a corresponding plurality of formations, each concentration of gas determined based, at least in part, on a difference between the theoretical diffusion coefficient and a corresponding experimental diffusion coefficient determined for each formation;   comparing the plurality of concentrations with each other, determining that a first concentration at a first formation is similar to a second concentration at a second formation; and   determining that the first formation is similar to the second formation based on determining that the first concentration at the first formation is similar to the second concentration at the second formation.   
     
     
         11 . A non-transitory computer-readable medium storing instructions executable by one or more processors to perform operations comprising:
 determining a theoretical diffusion coefficient for a drilling fluid comprising a gas from a formation, the theoretical diffusion coefficient determined based on an extraction of all of the gas from the drilling fluid;   determining an experimental diffusion coefficient for the drilling fluid based on well drilling parameters including a flow rate of the drilling fluid through the well;   determining a concentration of the gas at the formation based, at least in part, on a difference between the theoretical diffusion coefficient and the experimental diffusion coefficient; and   providing the determined concentration of the gas at the formation.   
     
     
         12 . The medium of  claim 11 , wherein determining the theoretical diffusion coefficient comprises solving Fick's first law of diffusion or Fick's second law of diffusion for the theoretical diffusion coefficient. 
     
     
         13 . The medium of  claim 11 , wherein determining the theoretical diffusion coefficient and the experimental diffusion coefficient comprises determining the theoretical diffusion coefficient and the experimental diffusion coefficient from sample data describing the drilling fluid carrying the gas near the formation. 
     
     
         14 . The medium of  claim 11 , wherein determining the theoretical diffusion coefficient and the experimental diffusion coefficient comprises determining the theoretical diffusion coefficient and the experimental diffusion coefficient from sample data describing the drilling fluid carrying the gas at the surface. 
     
     
         15 . The medium of  claim 14 , wherein the sample data comprises the concentration of the gasmeasured using one or more of gas chromatography, liquid chromatography, or mass spectrometry. 
     
     
         16 . A system comprising:
 one or more processors; and   a computer-readable medium storing instructions executable by the one or more processors to perform operations comprising:
 determining a theoretical diffusion coefficient for a drilling fluid comprising a gas from a formation, the theoretical diffusion coefficient determined based on an extraction of all of the gas from the drilling fluid; 
 determining an experimental diffusion coefficient for the drilling fluid based on well drilling parameters including a flow rate of the drilling fluid through the well; 
 determining a concentration of the gas at the formation based, at least in part, on a difference between the theoretical diffusion coefficient and the experimental diffusion coefficient; and 
 providing the determined concentration of the gas at the formation. 
   
     
     
         17 . The system of  claim 16 , wherein determining the experimental diffusion coefficient comprises:
 identifying physical and chemical properties of the drilling fluid at an instant in time; and   determining the experimental diffusion coefficient for the drilling fluid having the identified physical and chemical properties at the instant in time.   
     
     
         18 . The system of  claim 17 , wherein determining the experimental diffusion coefficient comprises providing the identified physical and chemical properties at the instant in time to a mathematical model that determines the experimental coefficient based on the identified physical and chemical properties at the instant in time. 
     
     
         19 . The system of  claim 16 , the operations further comprising determining a plurality of experimental diffusion coefficients of the gas at the formation at a respective plurality of sequential instances in time. 
     
     
         20 . The system of  claim 19 , the operations further comprising periodically determining the concentration of the gas at the formation based, at least in part, on a plurality of differences between the theoretical diffusion coefficient and the plurality of experimental diffusion coefficients.