How to Calculate the MWHIP of a Disposal Well

Overview

Maximum wellhead injection pressure, or MWHIP, is one of the key operating limits for an injection or disposal well. It defines the maximum pressure that may be applied at surface during injection. For disposal wells, MWHIP must be technically supportable because excessive injection pressure can create reservoir, caprock, containment, and compliance concerns.

This guide summarizes how MWHIP is typically determined for a disposal well and how step-rate injectivity testing can support a site-specific pressure request.

When This Process Applies

This process applies when preparing a new disposal application, amending an existing approval, or reviewing whether an approved MWHIP remains technically appropriate. Under Directive 065, injection and disposal wells require an approved MWHIP. If site-specific technical data is not available, default values referenced in Appendix O may be used. If an operator is requesting a different pressure, the application should include supporting technical information.

Step-Rate Injectivity Testing

A step-rate injectivity test is commonly used to identify the pressure at which the formation begins to fracture or part. During the test, fluid is injected at progressively higher rates while pressure is recorded. When the stabilized rate and pressure data are plotted, the relationship may show a distinct change in slope. That change can indicate the transition from normal matrix injection to fracture initiation.

A useful test should include enough points below the interpreted fracture pressure to define the pre-fracture trend, as well as points around and above the interpreted break point. The well should be given sufficient time to stabilize at each injection rate so the data is meaningful. Poor stabilization, limited rate steps, or inconsistent pressure response can make the interpretation less reliable.

Calculating MWHIP

Once the fracture pressure has been interpreted, the proposed MWHIP should be calculated conservatively so routine injection remains below the pressure that could fracture the formation. The calculation may require converting between bottomhole pressure and wellhead pressure and accounting for injection fluid gradient, well depth, and operating conditions.

The key objective is to connect the proposed MWHIP to the supporting data. The application should clearly describe the test, the data source, the interpretation of fracture pressure, and the rationale for the requested operating limit. The supporting information should allow a reviewer to understand how the recommended MWHIP was calculated and why it is appropriate for the disposal interval.

Key Considerations

Operators should ensure the test is representative of the disposal interval and the expected injection fluid. The MWHIP request should be consistent with reservoir containment, offsetting wells, caprock integrity, and any existing approval conditions. Where the fracture point is uncertain, additional technical analysis may be required.

Good documentation is important. Include the plotted data, the interpreted trend lines, the fracture pressure interpretation, the calculation method, and any assumptions used to convert pressure values. A clear explanation can reduce follow-up questions and improve the defensibility of the application.

Need Assistance?

Benoit Regulatory supports operators with disposal well applications, Directive 065 submissions, MWHIP reviews, step-rate injectivity test interpretation, and related technical documentation. Our team can help prepare the technical support needed for a clear and complete submission.