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Nodal Analysis - Gas Lift Optimizer

1. Overview

This is the part of nodal analysis used optimize gas lift designs for maximizing oil rates with a given surface/subsurface set-up and operating conditions.

Instead of having to save multiple VLPs from different gas injection rates (for each different combination of wellbore configurations/surface conditions) in the IPR/VLP section and collecting the operating points for each case to determine the best conditions for maximum rate uplift, this section bundles these steps into a simple workflow, making it easier to visualize the results and quantify the change in gas lift injection rate is needed to achieve the maximum rate as shown in the GIF below.

IPRVLPvsGLOpt

In this section you can set up inflow performance relationships at different times in the well history and run sensitivities on the wellhead pressures (Casing and tubing pressure), test alternate wellbore configurations and bottomhole correlations - each combination of these, set up as a different cases - and assess the impact of varying gas lift injection rates for each of them. You can compare all these cases to assess the impact of surface operating conditions and decide whether the well a good candidate for gas lift optimization. You can potentially plan wellbore configuration updates, injection rate changes and surface pressure settings in accordance with these results to maximize the return on injected gas for a given well.

2. Basics

For a chosen date in well history, an IPR is created in the IPR/VLP section.

The VLP depends on a combination of the following - surface pressures, bottomhole correlation, wellbore configuration and lastly, gas injection rate. You can create multiple VLPs for different gas injection rates. Every VLP that intersects the generated IPR represents one operating point (intersection of inflow and vertical lift performance curves - the expected production rate, given the constraints).

You can recalculate this VLP for different gas injection rates for each case and collect all the operating points or expected production rates as a function of gas injection rates to build the oil rate vs gas lift rate curve for each case.

This tyical gas lift optimization curve, where the maximum oil rates that can be produced, given a certain wellbore configuration and wellhead pressure settings, will keep increasing with an increase in gas injection rate until the frictional pressure drop of injected gas yields lower oil rates or diminishing returns on injected gas. The current oil rate given the used gas injection rate is compared against the maximum achievable oil rate and the increase in injection rate required, using this curve, for each case. This helps make a case for economic investment and lift gas allocation decisions for maximizing total production rate uplift from gas lift systems.

Here are the brief steps:

  1. Select a date on which the IPR is to be drawn, from the main menu (IPR/VLP tab).
  2. The IPR in the IPR/VLP tab is used as the first/default case, including the tubing head pressure and casing head pressure when loading the gas lift optimization feature for the first time.
  3. Add more sensitivity cases by clicking 'Modify Cases' button. This opens a dialog box containing the first default case and you can run sensitivities on the following by adding more cases -
    a. Wellhead pressures (casing and tubing),
    b. Correlation (BHP correlation to be used for computing the VLP)
    c. Configuration (add a new wellbore configuration to compare different gas lift designs)
  4. All these cases will run automatically once saved.
  5. This generates a set of gas lift optimization curves for each case, with varying gas injection rates and resulting oil rates for the same. On this curve, the current operating point as well as the maximum achievable rate (and the associated gas injection rate) is annotated. These values are also displayed as summaries for each case where the cases were defined.

3. Case comparisons

Let's go over some typical case comparisons that are possible using the gas lift optimization section:

3.1. Varying wellhead pressures

In this case, different cases of CHP and THP were compared. The default case is the current operating condition and all the subsequent cases are planned changes in wellhead pressures.

GLOptVaryingWellheadP

You can see that the base case oil rate can be increased by 3.7 STB/D by increasing the gas injection rate by ~1500 Mscf/D. You can also see that reducing the tubing head pressure to 60 or 30 psia (as in the last two cases) can also achieve the same oil rate without any increase in gas injection rate, if that is preferred.

Note that varying the casing head pressure did not have any impact on the gas lift optimization curve here because the well had only one orifice valve at the end of tubing. In case the well has multiple valves, the typical oil rate vs gas injection profile would be different for different casing head pressures due to the change in depth of gas injection as illustrated below.

GLOptVaryingWellheadP

3.2. Comparing correlations

Here, only the correlations used to compute the VLP are varied and you can see how VLPs from other correlations respond to the increasing gas injection rate. Considering the Beggs & Brill as the ideal correlation for this well, we see that the correlation performs better in terms of the match with IPR compared to others and shows that an increase in oil rate upto 31 STB/D can be achieved by injecting ~650 Mscf/D of more gas.

GLOptVaryingWellheadP

3.3. Comparing wellbore configurations

This strictly requires that the alternative wellbore configurations used should be some form of gas lift. You can add or remove valves from the tubing, adjust valve depths, opening and closing pressures, and compute the gas lift optimization curve for alternate gas lift designs.

GLOptVaryingConfigs

4. Tips -

  1. IPR selection - Finalize on the IPR to be used in the IPR/VLP section. Then in the Gas Lift Optimization tab, ensure that the right values are populated for creating the underlying IPR by clicking 'Fetch values from IPR'. Uncheck and check again if the values seem like they are from a previous IPR selection. Run the cases again by clicking the run button next to each case, every time the IPR is updated. If you do not run some cases, that is equivalent to comparing results from two different points in well history.

  2. Running a Case - Each time a case is updated, it will run automatically and update the gas lift optimization curve. No need to re-run when directly modifying a case input. Need to rerun manually if the IPR is updated.

  3. New wellbore configurations - You can also compare new wellbore configurations - just click on Add configuration button where the cases need to be defined and start by copying the current configuration, or building a whole new configuration from scratch. Be sure to give it a name and save it. Once you go back to the case inputs, use the dropdown under configuration to select this newly created configuration by its saved name. Note that configurations once saved cannot be altered as of now, just create a new one all over again to make modifications.