Predictions of gas concentrations in a subterranean formation

A method, comprising:

• making gas concentration measurements in drilling fluid as the drilling fluid exits a wellbore (gas-out measurements) while drilling;
• using a surface model to predict gas concentrations in the drilling fluid before the drilling fluid enters the wellbore (predicted gas-in concentrations) from the gas-out measurements by estimating gas transport and degassing of the drilling fluid in surface equipment attributed to a decrease of gasses in the drilling fluid due to pressure differences between the drilling fluid and air at a surface location adjacent the wellbore as well as mechanical interaction of the drilling fluid with the surface equipment and other components of a circulation system;
• using a subsurface model to predict gas-out concentrations from the predicted gas-in concentrations by estimating formation gas mixing with the drilling fluid as the formation gas is released from a formation during drilling at location in the wellbore at or near a drill bit utilized in the drilling, wherein the predicted gas-out concentrations are related to an estimated formation gas concentration in the subsurface model;
• iteratively comparing the predicted gas-out concentrations with the gas-out measurements while adjusting an estimation of the formation gas concentration in the subsurface model to generate the resultant estimated formation gas concentration while drilling;
• and
• outputting the resultant estimated formation gas concentration to a mud logger as indicative of fluid degassing rates and which types of gases are present in a formation undergoing the drilling, wherein the resultant estimated formation gas concentration minimizes a difference between the predicted gas-out concentrations and the gas-out measurements.

A method, comprising:

• making first gas concentration measurements in drilling fluid as the drilling fluid exits a wellbore (gas-out measurements) or second gas concentration measurements in drilling fluid before the drilling fluid is pumped into the wellbore (gas-in measurements) while drilling the wellbore;
• selectively estimating a formation gas concentration by evaluating the gas-out measurements or the gas-in measurements with a model, wherein, in response to the gas-in measurements being made, the selectively estimating comprises:
• using a subsurface model to predict gas-out concentrations from the gas-in measurements by estimating formation gas mixing with the drilling fluid as the formation gas is released from a formation during drilling at location in the wellbore at or near a drill bit utilized in the drilling, wherein the predicted gas-out concentrations are related to the estimated formation gas concentration in the subsurface model;
• using a surface model to predict gas-in concentrations from the predicted gas-out concentrations by estimating gas transport and degassing of the drilling fluid in surface equipment attributed to a decrease of gasses in the drilling fluid due to pressure differences between the drilling fluid and air at a surface location adjacent the wellbore as well as mechanical interaction of the drilling fluid with the surface equipment and other components of a circulation system;
• and
• iteratively comparing the predicted gas-in concentrations with the gas-in measurements while adjusting an estimation of the formation gas concentration in the subsurface model to generate a first resultant estimated formation gas concentration while drilling;
• wherein, in response to the gas-out measurements being made, the selectively estimating comprises:
• using a subsurface model to predict gas-in concentrations from the gas-out measurements by estimating formation gas mixing with the drilling fluid as the formation gas is released from a formation during drilling at location in the wellbore at or near a drill bit utilized in the drilling, wherein the predicted gas-in concentrations are related to the estimated formation gas concentration in the subsurface model;
• using a surface model to predict gas-out concentrations from the predicted gas-in concentrations by estimating gas transport and degassing of the drilling fluid in surface equipment attributed to a decrease of gasses in the drilling fluid due to pressure differences between the drilling fluid and air at a surface location adjacent the wellbore as well as mechanical interaction of the drilling fluid with the surface equipment and other components of a circulation system;
• and
• iteratively comparing the predicted gas-out concentrations with the gas-out measurements while adjusting an estimation of the formation gas concentration in the subsurface model to generate a second resultant estimated formation gas concentration while drilling;
• and
• outputting the first resultant estimated formation gas concentration in response to the gas-in measurements being made to a mud logger as indicative of fluid degassing rates and which types of gases are present in a formation undergoing the drilling, wherein the first resultant estimated formation gas concentration minimizes a difference between the predicted gas-out concentrations and the gas-out measurements or outputting the second resultant estimated formation gas concentration in response to the gas-out measurements being made to a mud logger as indicative of fluid degassing rates and which types of gases are present in the formation undergoing the drilling, wherein the second resultant estimated formation gas concentration minimizes a difference between the predicted gas-out concentrations and the gas-out measurements.