Using Annular Casing Pressure to Monitor Well Integrity

Ensuring well integrity throughout a well’s lifecycle is essential for maintaining safe and efficient operations in the oil and gas industry. Effective annular casing pressure (ACP) management is key to preventing problems with well barrier elements (WBEs) such as casing leaks, sustained casing pressure (SCP), and subsea safety valve (SSV) malfunctions. Implementing a structured ACP monitoring and reporting system ensures transparency during operational handovers and provides an auditable record for forensic analysis if required. Without effective integrity management, operators risk unnecessary well shut-ins and costly interventions.

Annular Casing Pressure (ACP) 

The illustration below shows the standard designations for the “A”, “B”, and “C” annuli in a subsea well. The “A” annulus is the void between the production tubing and the smallest casing string while the “B” and “C” annuli are the voids between the successive outer casing strings.  

The sealed annuli contain trapped fluids that later become heated by the high flow rate of formation fluids during production. The hotter fluids expand, causing an increase in the annulus pressure on the well. Eliminating any pressure fluctuations due to thermal effects, a pressure change in the “A” annulus is an indication of a leak. The pressure measured at the wellhead in the “A” annulus, or in the other annuli that terminate at the wellhead, is called the annular casing pressure (ACP).  

Annular casing pressure (ACP) is a primary indicator to detect leaks within the barrier envelope. Deviations in pressure within the “A” annulus may indicate leaks in one of the barrier elements, excluding the Downhole Safety Valve (DHSV). Troubleshooting involves adjusting pressure in the adjacent “B” annulus and the production tubing to identify the failing barrier element. The rate of pressure change can help assess well integrity risk levels. 

A production packer anchors the tubing string and isolates the “A” annulus from both the formation and the tubing’s interior. Leaks in these seals or any connections can lead to SCP within the “A” annulus. For subsea wells, only the pressure in the “A” annulus can be monitored and bled down, while surface and land wells allow monitoring and bleeding at the wellhead. 

Annular Casing Pressure Management for Offshore Wells

API Recommended Practice (RP) 90-1 identifies three sources of annular casing pressure: 

• Thermally induced pressure – as the well temperature increases during initial production 

• Operator-imposed pressure – factors such as gas lift and injection 

• Sustained casing pressure (SCP) – due to degradation or failure of barrier elements such as leaks through tubing, casing, wellhead seals, or cement 

Of the three pressure sources, SCP is the only one that will rebuild once bled off. SCP may result from communication with a pressurized formation or from a barrier that inadvertently creates a flow path. The following potential communication paths can affect the “A” annulus and the “B” and “C” annuli. 

Communication Paths

“A” Annulus Flow Paths:

• Tubing connection leaks
• Holes or parting in tubing string
• Gas lift or chemical injection mandrel failures
• Packer seal or control line leaks
• Tree or wellhead connection failures

“A” Annulus Annular Paths:

• Casing or tubing hanger leaks
• Production casing string failures
• Cement seal failure combined with casing leaks
• Uncemented sections exposing production casing

“B” and “C” Annuli Risks:

• Cement seal failure
• Casing leaks
• Exposed formation sections
• Wellhead packoff/seal leaks

Occurrence of Sustained Casing Pressure 

The occurrence of SCP tends to increase throughout a well’s lifecycle, from drilling to abandonment. Continuous monitoring of ACP is therefore considered best practice. 

Methods and Frequency of Monitoring Annular Casing Pressure

API RP 90-1 Section 9 outlines recommended procedures for monitoring annular casing pressure (ACP) in offshore wells. It highlights the importance of equipping all accessible annuli with the ability to monitor pressure, perform bleed-offs, and inject fluids when necessary. Initial pressure testing should be conducted to establish baseline values and identify any existing ACP resulting from operational conditions.

Regular monitoring is essential, with operators responsible for setting a minimum frequency to ensure pressure remains within diagnostic limits. The section distinguishes between operator-imposed and thermally induced pressures and emphasizes the need for close observation following any operational changes.

When sustained casing pressure (SCP) is detected outside of acceptable thresholds, the frequency of monitoring should be increased. For inaccessible annuli, a formal risk assessment is required to determine the feasibility of restoring monitoring capabilities and to assess the implications of any pressure sources.

Overall, this section offers a comprehensive framework to support safe and effective ACP management and ensure long-term well integrity.

Conclusion 

Annular casing pressure management is a critical component of well integrity management. By understanding the causes of ACP fluctuations, implementing best practices, and leveraging digital solutions, operators can proactively prevent integrity failures, minimize costly interventions, and enhance operational safety. Standardized reporting, automated data collection, and routine diagnostics ensure regulatory compliance and long-term sustainability. Investing in a structured ACP management strategy ultimately protects assets, personnel, and the environment. For more information on how our digital solutions can enhance your well integrity management, contact IPT Global today.