Well Test Certificate

Introduction

Complete the steps outlined below to become a Well Test certified whitson⁺ user. It includes performing two analyses - a chow pressure group (CPG) test and a diagnostic fracture injection test (DFIT). You can learn more about the respective tests in the manual where we cover most of the theoretical fundamentals -

1. CPG - Chow Pressure Group Test
2. DFIT - Diagnostic Fracture Injection Test
3. DQI - Devon Quantification of Interference

Those certified have the software skills necessary to complete most CPG and DFIT evaluation projects in tight unconventionals.

Need help?

Send an e-mail to support@whitson.com.

Before Starting

Make sure you have watched these three videos in the Getting Started part of the manual (click here).

• 1.1 Login (1 min)
• 1.2 Overview of important basics (3 min 30 sec)
• 1.3 Zoom Plots (3 min)

Create a Project

1. Go to the Project module in the navigation panel.
2. Click ADD PROJECT up to the right.
3. Name the project "your name - whitson certificate".
4. Click SAVE.
5. All steps are shown in the .gif above.

Upload Required Data

You can download the files required for each test individually and upload using the mass upload dialog box on the wells page, as shown in the sections below.

Alternatively, all the data required for this exercise has been bundled into a mass upload file, ready to be uploaded, in the Mass Upload -> Examples section.

CPG Workflow in whitson⁺

Here is an example dataset to try it for yourself! Download the CPG Example Dataset here
The data is already in the supported mass upload template. Upload the same in using the mass upload dialog box in your project, like so -

You can analyze the Chow Pressure Group for the monitoring well in whiston⁺ by navigating to the Chow Pressure Group feature under Well Testing.

On the Pressure Difference plot on the top-left:

1. Move the CPG fit line (yellow, dashed line) to match the pressure data prior to the production well POP date. You can adjust the slope by:

   • Manually moving the fit line on the plot (grab the line at the ends to adjust),
   • Changing the Constant and Exponent of the power-law fit line directly in the input, or
   • Lasso fit the pressure data prior to POP using the lasso tool to auto-adjust the fit line.

2. Adjust the dashed vertical line along the monitoring well data to the date and time when offset production well is POP.

3. The difference in these CPG fit and the data is plotted along with it's derivative as \(\Delta p, \Delta p'\) vs. time (bottom-right). Observe the changes in this plot as the Fit line is changed.
   

On the \(\Delta p, \Delta p'\) vs. time plot (bottom-right):

Note

Bourdet Derivative and Weighted Central Difference give you the additional option to modify the smoothing window as a log cycle fraction or a step size respectively. Both these methods are higher in accuracy compared to simply using the Central Difference with a fixed step size.

Notice how switching between the different methods and window sizes in the GIF below changes the derivative shape and hence, impacts the CPG values calculated.

Use the integral icon ('Plot Integral') in the plot options to recalculate the pressure as an integral and recompute the derivative based on the derivative function selected above. You can also use the LOWESS filter toggle with an appropriate smoothing window in log cycles to smooth noisy pressure data.

These methods inherently smooths the pressure response, and removes additional noise that may be a part of the dataset to create a much smoother derivative and CPG plot. Since the pressure integral is already smooth, a smaller window size can be used for derivative calculation.

Lastly, on the Chow Pressure Group plot, you can set the final CPG value in three ways -

• Move the horizontal dashed line to track the average of the CPG values after POP date.
• You can also adjust it by entering a value for CPG fit.
  
• You can also select the CPG based on the slope in \(\Delta p\).

This is the CPG value that indicates the level of interference between the production well and the monitoring well:

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DFIT Workflow in whitson⁺

Before getting started -

Data, data and more data:

DFIT datasets could be far higher frequency than needed.
Datasets can vary from second by second frequency to tens of seconds for the entirety of the test. This can be harder to handle, and make the numerical derivative calculation unweildy.

Instead of uploading such high resolution data, the recommended practice -
Smooth the dataset externally by resampling the data in terms of pressure increments of 5 to 10 psi. Note that there is a further reduction using pressure increments of 30 psi in the DFIT feature to improve speed for dynamic calculations too.

Using Mass Upload sheets to get your DFIT data into whitson⁺ -

Add a well name to the Well Data sheet, you do not need to enter anything else in this sheet.
Use the same well name in the Production Data sheet to add DFIT data to the well -

Key information required: Pressure (bottomhole) vs time

1. Time can be in - Days or DateTime like 'YYYY-MM-DD hh:mm:ss.000'. Smallest resolution is 1 millisecond.

2. Pressure (in psia) vs time data - If only wellhead pressure is available, use the hydrostatic head of water at TVD to correct to bottomhole depth. Enter these in the p_wf or Gauge Pressure column.

3. Injection rate (in STB/d) vs time data (if available) Enter these in the q_w or Water Rate column on the mass upload Production Data sheet.

Additional required information - that cannot be uploaded via mass upload template.
Young's modulus, Poisson's ratio, reservoir fluid viscosity and compressibility.
We also need the estimated fracture height for the assumption of PKN fracture geometry.

You can enter all this information in the DFIT feature under 'Physical Assumptions'.

DFIT dataset example

Here's an example DFIT dataset formatted to be uploaded via the standard mass upload template -
Example DFIT Dataset
Courtesy: MCclure et. al, Resfrac

Choices/Assumptions that need to be made -

1. Choose the preclosure method of choice (G-function and H-function with Radial and PKN fractures) - recommended to go with H-function method with radial fracture geometry.
2. Choose the postclosure method of choice based on late time impulse flow signatures (linear/radial flow) - Note that the preclosure assumption on fracture geometry also auto-applies to the post closure analysis.

Some notes on automated selections in the software

These preliminary steps are done automatically, you may still need to review these selections -

1. Shut in is detected automatically - from zero rate - Instantaneous ISIP is identified as the pressure right after the well is shut in.
2. Initial pressure, P_w,init is chosen as the first point in the pressure data.
3. Cumulative injection volume, and maximum sustained injection rate upto the shut in time (characteristic rate), is calculated from the plot. Injection duration is autocomputed from the two above.
4. Slope in the pressure vs cumulative injection (Wellbore storage plot) is detected to calculate the wellbore storage. If rate data is unavailable, just enter the values cumulative injected volume, maximum sustained rate, and wellbore storage.
5. Pore pressure is calculated automatically by extrapolating the post closure linear transient (default) on the inverse square root time plot.
   

Key Steps to perform an analysis of a DFIT -

1. Upload the downloaded template with the DFIT dataset in the mass upload section of the project.

   
   

2. Navigate to the DFIT feature under the Well Testing section.
   

   
   

3. Review the automatic selection of parameters from the plots, i.e. Literal ISIP, initial pressure, injected volume, characteristic rate, wellbore storage coefficient, minimum dP/dG. Enter the additional parameters relevant for the calculation, i.e. Young's modulus, Poisson's ratio, reservoir fluid viscosity and compressibility.

   
   

4. Dropdowns for preclosure method and post closure methods allow you to switch between the different methods as well as fracture geometry assumption. Selection of frac geometry (Radial/PKN) in preclosure methods enforces the same geometry assumption in postclosure methods. Note that PKN fractures will need fracture height as an additional input.

   
   

5. G function plot picks - Identify point of dP/dG minimum. Identify point of G*dP/dG maximum. This automatically computes the effective ISIP, Fracture Contact Pressure and Minimum Principal Stress and are used in subsequent calculations for permeability. The resolved values could be overwritten.

   
   
   

6. Identify if the late time shut in data has post closure transients - Linear flow will have a half-slope signature and radial flow (rare, beware of false radial signatures in gas wells) will have a unit slope signature. Add the interpretation lines if needed and switch the plot (to the right) to impulse radial flow plot by clicking the 'plot radial' button. Align the slope to the pressure points near x=0 and let that extrapolate to calculate the pore pressure.

   
   

7. Calculation should dynamically run when any of the inputs are changed to compute permeability. If you have reliable post closure transients, use the permeability estimates from the postclosure analysis. Correct preclosure permeability estimates by about 1.5 since they statistically tend to overestimate the permeability.

   
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DQI Workflow in whitson⁺

DQI dataset example

Here's an example DQI dataset - formatted in the whitson template, the same as the one used in the original paper, formatted to be uploaded via the standard mass upload template -
Example DQI Dataset
Courtesy: Devon, Resfrac.

Step 1 - Load the dataset

Simply drop this dataset downloaded from above in the mass upload dialog box. You can also switch to the examples tab and click Upload on the Well Test Certificate

You can also upload your own pressure interference dataset if you'd like. Ensure that the pressure measurements are in the p_wf or gauge pressure column and the same Well Name exists on the Well Data sheet.

Step 2 - Initialize PVT

You need PVT initialization to gather fluid properties like fluid compressibility, viscosity, etc. from the black oil tables. You want to ensure that the PVT initialization matches the fluid that can be expected between these wells.

For a DQI test shortly after the initial stimulation in the offset well prior to the well being produced, initialize PVT with 100% water saturation - this is what we'll do here in this example dataset, as shown in the GIF above.

For tests where the well pair has been producing hydrocarbons, initialize the PVT with the GOR associated with the reservoir fluid in place.

Step 3 - Align the POP line

Set the vertical dashed line at the POP of the offset well. Taking a quick look at the pressures in the production data, we can set the offset well POP to 1.48 days in this example. You can enter it in or move the vertical dashed line graphically to the point in time when the offset well is POP'ed.

Step 4 - Align the line to Pressure trend before POP

Fit the prior pressure trend by moving and adjusting the slope line to align with the stabilized pressure trend prior to POP. You can do this manually or you can just lasso fit the stabilized pressure data.

Notice how the pressure and pressure derivative vs time after POP plot is dynamically recalculated as you make adjustments to the prior pressure trend.

Step 5 - Click Autofit to calculate the DPI

Once the prior pressure trend and POP is picked, we have a plot of . Click the AUTOFIT button (off to the top-left of the page) - This fits the analytical model to the pressure and pressure derivative data to determine model parameters -

1. α (controls hydraulic diffusivity) - impacts the timing of the onset of pressure interference.
2. κ (controls fracture conductivity) - impacts the shape of the curve after the interference.

Assessing fit quality: The priority is to ensure that the fit matches the initial response - the first 10s or 100s of psi. The analytical solution is not expected to match the transient after the initial response.

Resolving these two parameters allows you to calculate the fracture conductivity 'C' and maximum possible drainage length along the fracture, 'L', given W (fracture width or aperture) and dW/dP (change in fracture aperture with change in pressure) and other parameters.

Then the Ld is calculated, given the well spacing, 'y'.

This is plotted in the empirical correlation developed to calculate DPI, given the Ld.

The resulting DPI should be close to 95%.

Note down the α and C, both of which are indicators of good or bad well-pair connectivity, apart from the DPI. This will help you rank the well-pair connectivity on large scale tests with multiple well pairs

DO-IT-YOURSELF (optional)

Did you know that the CPG and DQI datasets are both interference tests between different well pairs? This means that the CPG dataset can be used to calculate the DPI for that well pair and the DQI dataset can be used to calculate the CPG value for the other well pair.

Do you want to give this a try? Use the wells in your project and assess the interference datasets with an alternate techniques.

Report the answers in your submission below for some extra kudos!

Want to learn more?

Schedule a Well Test session with one of our engineers. Contact support@whitson.com.

Done?

When you are done analyzing your Well Tests please:

1. Send an e-mail to certification@whitson.com
2. Make the subject: "whitson⁺ Well Test certificate: [YOUR NAME HERE]".
3. Include the link to your project.
4. If you have any notes, comments, or observations related to the well, feel free to share them with us. Also feedback on the user friendliness of the software is always appreciated.

After that we'll provide some feedback on your evaluation and issue your whitson⁺ certificate if all looks good.