After completing a well, drillers must test their well’s mechanical integrity. Mechanical integrity tests take a variety of different forms, and drillers and regulators sometimes combine these tests for a more holistic image of the well’s integrity.
Pressure Test for Integrity
One common form of mechanical integrity test is the pressure test. Both positive and negative pressure tests are used to assess well integrity.
Positive pressure test
During a positive pressure test, the well is sealed off by closing the blowout preventer or other surface equipment then the inside of the wellbore is pressurized. Any significant loss in pressure during a standard 30-minute monitoring period suggests that the well has poor mechanical integrity and may require a workover or a new cement job. A workover is an often expensive process of performing major maintenance or remedial treatments on an oil or gas well using invasive techniques.
Negative pressure test
The negative pressure test is the opposite process. The pressure on the inside of a sealed well is brought down and then monitored. In this case an increase in pressure provides evidence of flow into the well, which again requires attention.
Continuous Monitoring of Well Integrity
A mechanical integrity test must occur before the well enters production and again periodically during its lifetime as regulations mandate. Operators must shut down a well to perform a pressure test. However, there are some emerging technologies for monitoring well integrity that allow for continuous monitoring. For example, fiber optic sensors may be distributed through a well to measure temperature, pressure or acoustic changes in the wellbore. Also, emerging satellite imaging technology is improving real-time monitoring of both well and subsurface integrity.
One of our experts explains the importance of mechanical integrity testing. She also discusses how new technologies (e.g., real time monitoring with fiber optics, ground deformation monitoring with InSAR) are improving our ability to monitor well integrity.
Transcript
Well Integrity Testing and Monitoring – Azra N. Tutuncu – Colorado School of Mines
One of the most important aspects of oil and gas production is well integrity. This is critical throughout the life cycle of the well. A leaking or ruptured well can cause anything from a minor delay to a major disaster.
Testing and monitoring help operators prevent any potential problems. If an incident does occur, testing and monitoring provide an early warning, which allows for successful mitigation.
The most common causes of well integrity issues are poor casing or cementing. Selection of high quality cement, and proper well cleaning prior to the cement job support the important role of the casing in preventing potential migration of fluids.
We employ a variety of testing methods, to evaluate the integrity of an injection well. A mechanical integrity test or MIT is required before a well is put into service, and at specified time intervals thereafter.
The testing frequency depends on federal and state regulatory requirements. The purpose of this test is to assure that the mechanical component of the well remain sufficiently functional to protect the environment and human health.
An MIT consists of evaluating the internal and external integrity of the well. A well has internal mechanical integrity, if there are no significant leaks in the well, and the mechanical components of the well function properly. This is typically tested by pressurizing the well under controlled conditions. Most states require that the test be witnessed by regulatory agency staff.
External mechanical integrity means there is no significant fluid movement through vertical channels adjacent to the injection well bore. If a well has lost mechanical integrity and been repaired, an MIT is required before the well is put into service. MITs are also conducted on wells prior to being plugged and abandoned.
In the past, a well had to be shut down in order to prepare for most tests. This meant that tests might only be conducted every five years. Today, we can use fiber optics to monitor well integrity in real time. Different types of fiber optic sensors are distributed through a well to measure temperature, pressure, strain and acoustic changes in the wells. The fiber optic data, along with the pressure data gathered, are used to detect leaks, intrusion and ground movement in and around a well.
While these techniques are expensive to implement, one advantage of fiber optic monitoring, is real time monitoring of the integrity status of the well, and early detection of any issues for quick mitigation. We can also monitor well integrity from a distance, a very great distance in fact. InSAR monitoring, or interferometric synthetic aperture radar monitoring, is a technique for mapping ground deformation using images of the earth’s surface that are collected from orbiting satellites. The imaging is so precise it can measure ground level changes of one millimeter.
This data are used to assess risk associated with drilling, hydraulic fracturing production and enhanced soil recovery operations, for example subsidence.
Both active and plugged wells can be monitored by the InSAR method. When a well can no longer produce, mechanical integrity remains important. Operators are required to fill the well bore in a process known as plugging. Plugging consists of four steps.
The first step is, the removal of all surface equipment. If any equipment is lost or stuck, and cannot be removed, the well plugging plan must be revised and approved by authorities. Next, the casing is removed or cut, then the bore hole is filled with cement, sealing off the bore hole and preventing seepage. Finally, any remaining opening from the bore hole to the surface is covered with uncontaminated soil. A plugged well with high mechanical integrity will serve as impermeable barrier, that prohibits migration of fluids between subsurface formations. This prevents nearby aquifers from being contaminated by reservoir fluids.
As technology improves, we will likely have even more precise methods of testing and monitoring well integrity. By detecting problems when they are small, operators minimize downtime and public safety is increased. This is a win-win for operators and all stakeholders involved in or affected by oil and gas production.
Images: “Cement Bond Log” by Top Energy Training