Using Numerical Models
Subtopics:
Running Numerical Models Created in WellTest 2013v2 and Earlier
Black Oil Numerical Models for Liquid-Rich Gas Wells
Related Topic:
Setting Up the Model
After the model has been created, you need to populate all the required parameters. To set up the model:
1. Some of the model parameters (i.e., initial pressure, fluid saturations, net pay etc.) are already specified in the Properties tab. To copy parameters from the Properties tab to the model, click the Defaults icon (
) on the toolbar, and select Reservoir from the drop-down list.
2. If you have an analytical model, copy parameters from the analytical to numerical model by clicking the Defaults icon () and select the analytical model from the drop-down list.
3. In the Numerical Model pane, select your calculation method . You can use the rates of any of the fluids presented in the model to calculate rates for the remaining fluids, and the sandface flowing pressure. Alternatively, you can use the sandface flowing pressure to calculate rates for all the fluids presented in the model.
After you set reservoir and completion dimensions, WellTest creates an appropriate grid for the model to get accurate results with minimal calculation time. However, you can adjust gridding options by clicking the Options tab in the Numerical Model pane.
Using the Model
After the model is set up, you can use the model:
| Note: | Before the model starts a simulation, it performs multiple consistency checks on the input properties. |
1. Run calculations by clicking the Synthesize icon (
) on the toolbar.
While the model is calculating, a rotating gear is displayed on the header of the tab. To stop the simulation, click the Stop Simulation (
) icon.
While the model is calculating, you can see how the spatial distribution on pressure changes with time. Both top and cross-section views are available. The position of the cross-section is defined by the blue triangular sliders on the schematic plot.
2. You can replace one or more plots on the dashboard by clicking the Add an available view icons.
3. After the calculation is finished, you can see how pressures or saturations change through time by clicking the following icons on the toolbar of any Color Shading plot.
4. To display grid lines on the top view, click the Toggle Grid Lines icon (
).
5. To display the legend, click the Toggle Color Legend icon (
).
6. To history match the model, change the parameters manually. Most of the history matching parameters are located on the History Model pane. However, you may also want to change some parameters located in the Properties tab (e.g., relative permeability curves, Rs curve, Rv curve, etc.).
After the model has been populated and calibrated to the production and pressure data, production forecasts can be created and compared under a variety of different constraints. The procedure for forecasting using a numerical model is similar to Forecasting Using Analytical Models.
| Note: | Wellhead forecasting is not currently available for the numerical models. |
Running Numerical Models Created in WellTest 2013 v2 (v 7.7.0) and Earlier
Black Oil Numerical Models for Liquid-Rich Gas Wells
Numerical calculations for liquid-rich gas wells are different between WellTest 2013 v2 (v 7.7.0) and earlier versions.
In WellTest 2013 v2 (v 7.7.0) and earlier: it is possible to create a (black oil) numerical model for a liquid-rich gas well. However, liquid drop-out is not modeled directly in such models. Instead, gas and condensate rates measured at surface conditions are combined into a richer single-phase gas using recombination; then rates and properties of the recombined gas are used in numerical modeling.
This approach is appropriate if condensation takes place mostly at the separator and in the wellbore. In cases where there is a significant liquid drop-out in the reservoir, the model may be inaccurate, because the liquid drop-out in the reservoir and its influence on the flow hindrance are not taken into account. Another disadvantage of numerical modeling for liquid-rich gas wells in previous versions of WellTest, is that only the recombined gas rate is reported, rather than reporting a separate dry-gas rate and condensate rate.
In WellTest 2014 v1 (v 7.8.0) and later: liquid-rich gas wells are modeled using Gas Condensate numerical models. In Gas Condensate numerical models, liquid drop-out in the reservoir is modeled directly. Both phases (i.e., gas and liquid) are taken into account in the model; therefore flow hindrance due to interaction between phases is also honoured.
If you have a (black oil) numerical model created in WellTest 2013 v2 (v 7.7.0) or earlier for a liquid-rich gas well, this model will not run in WellTest 2014 v1 (v 7.8.0) or later. This happens because starting from WellTest 2014 v1 (v 7.8.0), liquid-rich gas wells are modeled using Gas Condensate numerical models. You have two options for converting such older models:
Option 1: If you want to consider liquid drop-out in the reservoir, you have to create a new Gas Condensate numerical model. After the model is created, copy all parameters from the existing black oil numerical model, using the Defaults Manager. The gas condensate model then runs in WellTest 2014 v1 (v 7.8.0).
The advantage of this option is that it takes into account liquid drop-out in the reservoir and honours flow hindrance due to interaction between phases. However, because the nature of calculation has changed compared to the earlier version, you have to re-match the model.
Option 2: If you believe that liquid drop-out in the reservoir is negligible, you can model the well as a wet-gas well:
1. On the Production Editor tab, copy the Recombined Gas Rate column to the Gas Rate column.
2. In Properties tab, navigate to the Gas Properties group and copy the value for Recombined Gas Gravity (Gr).
3. In the Properties tab, change the Gas Type to Wet Gas.
4. In the Properties tab, paste the copied value for Gr into the Gas Gravity (G) field.
Now the conventional numerical model created in the earlier version of WellTest runs.
The advantage of Option 2 is that the calculation essentially follows the one performed in earlier versions of WellTest: instead of modeling liquid drop-out directly, we use recombination, and then use recombined gas rates and properties in numerical modeling. Thus, the calculation results do not change, so you do not have to re-match the model. However, it is important to remember that this model is only applicable if liquid drop-out in the reservoir is negligible.