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NTG model in petrel
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Generating a Net to Gross (NtG) property
NtG (Net to Gross) indicates the percentage of a particular interval which is potential reservoir. On a log scale it is a discrete property, at any one depth in a well the sequence is either reservoir or non reservoir. It is useful to generate this as a discrete log in Petrel so that it can be easily edited in the well correlation panel. However, after upscaling it should be a continuous log referring to the percentage of potential reservoir in each grid cell, e.g. 0.5 for interbedded sand and shale, 0.9 for sand with little shale.
How to generate a discrete or continuous NtG log
Interactively using Well Section:
Copy the General discrete template and rename it as NtG_discrete, go to the Colors tab and specify two codes non-reservoir (0) and reservoir (1)
Open a New well section widow, display the wells and select the Paint discrete log class icon to make enable the Create new discrete log icon
Click on Create new discrete log icon and select the template NtG_discrete. Go to the Global well logs folder and check on the NtG_discrete log to display it on the Well section
Then interpret the areas of each of the wells as either reservoir or non-reservoir by positioning on the empty track for the NtG_discrete log and select the class (code) to paint
Now to create a countinuous log from a discrete log: in the Global well logs for NtG_discrete log go to the Settings\Operations tab\Resample log points, select the option As continuous log and push the button Resample. This operation ensures that the sampling points in the discrete log will also make sense in the continuos log
Then using the Calculator from Global well logs to generate a new continuous log reservoir by choosing a Net/Gross template and type NtG_Continuous=If(NtG_discrete=1,1,0). The new log should look identical to the old log with values of 1 or 0 along its whole length
Upscale the new NtG_Continuous log using arithmetic as the averaging method. Upscaled cells will have a value between 1 and 0 depending on the amount of reservoir within the log in the grid cell
Display the property in the Well section window together with the original log (NtG_Continuous) and use the color fill
In the figure below is shown the resultant NtG log
Note: In Well section to show the upscaled cells just for the values with Net to Gross, make a property copy and use the Property calculator Copy of NtG_Continuous=If(NtG_continuous=0,U,NtG_Continuous) and display it. Use the Show cell boundaries for properties icon to show or hide the layers on the track.
Using the Calculator with continuous logs as variables:
The NtG log can be calculated by using the Vsh (shale volume)
Then using the Calculator from Global well logs to generate a new continuous NtG log reservoir by choosing a Net/Gross template and type NtG=1-Vsh
Copy the General discrete template from the Templates pane and rename it as NtG_cutoff, go to the Colors tab and specify two codes non-reservoir (0) and reservoir (1)
Open the Calculator from Global well logs to make a NtG_Cutoff (discrete log) assigning the new template and type NtG_Cutoff=If(NtG<0.8,0,1). Note: the cutoff value should be defined according to your reservoir
Display the logs (Vsh, NtG and NtG_Cutoff) in a Well Section to compare and quality check
NtG (Net to Gross) indicates the percentage of a particular interval which is potential reservoir. On a log scale it is a discrete property, at any one depth in a well the sequence is either reservoir or non reservoir. It is useful to generate this as a discrete log in Petrel so that it can be easily edited in the well correlation panel. However, after upscaling it should be a continuous log referring to the percentage of potential reservoir in each grid cell, e.g. 0.5 for interbedded sand and shale, 0.9 for sand with little shale.
How to generate a discrete or continuous NtG log
Interactively using Well Section:
Copy the General discrete template and rename it as NtG_discrete, go to the Colors tab and specify two codes non-reservoir (0) and reservoir (1)
Open a New well section widow, display the wells and select the Paint discrete log class icon to make enable the Create new discrete log icon
Click on Create new discrete log icon and select the template NtG_discrete. Go to the Global well logs folder and check on the NtG_discrete log to display it on the Well section
Then interpret the areas of each of the wells as either reservoir or non-reservoir by positioning on the empty track for the NtG_discrete log and select the class (code) to paint
Now to create a countinuous log from a discrete log: in the Global well logs for NtG_discrete log go to the Settings\Operations tab\Resample log points, select the option As continuous log and push the button Resample. This operation ensures that the sampling points in the discrete log will also make sense in the continuos log
Then using the Calculator from Global well logs to generate a new continuous log reservoir by choosing a Net/Gross template and type NtG_Continuous=If(NtG_discrete=1,1,0). The new log should look identical to the old log with values of 1 or 0 along its whole length
Upscale the new NtG_Continuous log using arithmetic as the averaging method. Upscaled cells will have a value between 1 and 0 depending on the amount of reservoir within the log in the grid cell
Display the property in the Well section window together with the original log (NtG_Continuous) and use the color fill
In the figure below is shown the resultant NtG log
Note: In Well section to show the upscaled cells just for the values with Net to Gross, make a property copy and use the Property calculator Copy of NtG_Continuous=If(NtG_continuous=0,U,NtG_Continuous) and display it. Use the Show cell boundaries for properties icon to show or hide the layers on the track.
Using the Calculator with continuous logs as variables:
The NtG log can be calculated by using the Vsh (shale volume)
Then using the Calculator from Global well logs to generate a new continuous NtG log reservoir by choosing a Net/Gross template and type NtG=1-Vsh
Copy the General discrete template from the Templates pane and rename it as NtG_cutoff, go to the Colors tab and specify two codes non-reservoir (0) and reservoir (1)
Open the Calculator from Global well logs to make a NtG_Cutoff (discrete log) assigning the new template and type NtG_Cutoff=If(NtG<0.8,0,1). Note: the cutoff value should be defined according to your reservoir
Display the logs (Vsh, NtG and NtG_Cutoff) in a Well Section to compare and quality check
