Well control situations can get tricky during completion or workover operations. Sometimes, the standard methods involving circulation just won’t work. This can happen due to:
- Lack of pumps or malfunctioning pumps on site
- A plugged workstring
- Kicks encountered while pulling out the drill string or when the tubing is far above the perforations
- No drill string in the well at all
These situations require special well control techniques. The most crucial step, as always, is to shut in the well using the blowout prevention equipment (BOP) immediately upon encountering a kick. Once the well is shut in, solutions often involve practical measures:
- Bringing in a new pump or fixing the existing one (pumps)
- Perforating or bullheading down the casing (plugged workstring)
- Stripping back to bottom or bullheading if the tubing is stuck (kicks with tubing off bottom)
- Running a bridge plug or wireline-set retainer, bullheading, or using snub tubing if there’s no pipe in the well
However, if logistics prevent these solutions and a gas kick is present, Volumetric Control comes into play.
Volumetric Control: Managing Pressure as Gas Migrates
Volumetric Control allows for managing bottomhole and surface wellbore pressures while the gas kick migrates up the wellbore. This is particularly useful since most workover and completion casings are designed to withstand the pressure of a migrating gas kick without expansion, assuming a Minimum Allowable Surface Pressure (MASP) is maintained.
As the gas bubble moves upwards, it increases both bottomhole and surface pressures. Eventually, the perforated interval will start taking fluid, preventing the surface pressure from ever reaching the bottomhole pressure. However, there are situations where Volumetric Control becomes essential to avoid further complications:
- Open hole completions or sidetracked wells
- Old perforations or a higher perforated interval that could take fluid and cause crossflow or an underground blowout
- Formations sensitive to large volumes of fluid that could be severely damaged
- Formation fluids trapped in a sealed wellbore (above a bridge plug or tubing plug)
The Science Behind Volumetric Control: Boyle’s Law
The success of Volumetric Control hinges on understanding gas behavior and Boyle’s Law (P₁V₁ = P₂V₂). This principle states that at constant temperature, the pressure and volume of a gas are inversely proportional. In simpler terms, as the volume of a gas increases, its pressure decreases.
Volumetric Control involves carefully bleeding off small amounts of fluid at a time. This keeps the bottomhole pressure slightly above the reservoir pressure, allowing the gas kick to migrate upwards without causing a pressure surge that could compromise the well’s integrity. Importantly, Volumetric Control should only be used when the surface pressure is rising and threatens the well’s integrity.
References
Cormack, D. (2007). An introduction to well control calculations for drilling operations. 1st ed. Texas: Springer.
Crumpton, H. (2010). Well Control for Completions and Interventions. 1st ed. Texas: Gulf Publishing.
Grace, R. (2003). Blowout and well control handbook [recurso electrónico]. 1st ed. Paises Bajos: Gulf Professional Pub.
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