Freeze services are essential in the oil and gas industry for creating a temporary pressure barrier when conventional barrier options are not feasible. This method involves the transformation of fluids inside wellbore components into ice through the application of extremely low temperatures.
The freeze process commonly places a temporary pressure barrier in well casings, annular spaces, tubing, drill pipes, spools, valves, and other pressure-containing components. This technique provides operators with a reliable and efficient solution when conventional barrier options, such as mechanical plugs or packers, are not possible.
The process of creating a freeze involves reducing the temperature of the wellhead component until the existing fluid within the wellbore component transforms into ice. This ice is then used as a testable pressure barrier, which can withstand significant pressure levels during well testing or maintenance activities.
Before initiating a freeze operation, several crucial steps must be undertaken to ensure a safe and effective outcome. Understanding the current condition of the well, reviewing the surface equipment (well casing, wellhead, and tree), and determining the current fluid in each annulus of the well are essential steps in this process.
It is crucial to assess the surface components and verify their ability to withstand the extreme cryogenic temperatures that will occur during the freeze operation. The specific surface equipment of the well will guide the most effective and safe method to produce the freeze plug. Equally important is selecting the best freezable medium to achieve the highest quality ice plug in the well.
Liquid N2, combined with integral temperature monitors, is commonly used to ensure a competent freeze plug can be safely and efficiently set. Once the freeze plug is established, a positive and negative pressure test is performed to confirm the temporary barrier is in place, allowing surface operations to commence.
When applied within specific parameters, cryogenic freezes can be set across multiple strings of casing, resulting in an effective temporary barrier in the innermost tubular. Careful planning and execution of the freeze operation are critical to ensure a successful outcome and the safety of all personnel involved.
To establish a freeze barrier, it is crucial to have a freezable medium such as brine, water, or water-based mud present within the ID of the component. In the event of any flow or the presence of void, gas, or hydrocarbon-based fluid within the freeze area, the injection of a freezable medium into the ID of the component is required. This can be accomplished through existing wellhead/tree valves or a Hot Tap operation, if necessary.
In cases where liquid hydrocarbons are present, a water-based fluid can be injected into the freeze zone, remaining suspended in the freeze zone above the hydrocarbon-based fluid. This technique allows for the implementation of a freeze when it is not feasible to entirely remove the hydrocarbon-based fluids in the well. In addition, cryogenic Freezing has been proven to be a secure method to establish a temporary barrier in the wellbore or component, facilitating the completion of remedial work at the surface.