The density of drilling fluid plays a critical role in well control during both drilling and completion operations. This article aims to explore the various factors that can result in low-density drilling fluid, potentially leading to well control challenges.
Accidental Dilution and Fluid Addition
Maintaining the hydrostatic pressure necessary to balance or slightly exceed formation pressure requires constant monitoring and adjustment of drilling fluid density. Accidental dilution of drilling fluid with makeup water in surface pits or the addition of low-density formation fluids into the mud column can reduce fluid density, triggering a potential kick. Rigorous vigilance in monitoring mud pits is essential to ensure the required fluid density is consistently maintained.
Large volumes of gas in the returns can cause a drop in the average density and hydrostatic pressure of the drilling fluid. Notably, gas cutting often occurs in an overbalanced condition downhole. If a formation containing gas is drilled, the gas within drilled cuttings can expand as it moves up the annulus, leading to gas cutting at the surface. Detecting this is crucial, as a flowing well indicates a kick, necessitating immediate well shut-in and initiation of the proper kill procedure.
Oil or Saltwater Cutting
Invasions of oil or saltwater from drilled cuttings or swabbing can reduce the average mud column density, causing a drop in mud hydrostatic pressure. While the effect of these liquids on average density is less pronounced than gas, the impact on bottomhole pressure can be substantial. Liquids, being less compressible, result in uniform density reduction throughout the mud column.
Settling of Mud Weighting Materials
The settling of desirable solids or drilled cuttings in a mud can significantly reduce mud density, affecting hydrostatic pressure. Barite sag, more prevalent in highly deviated wells, requires a combination of sound mud design and operational practices for management.
Loss of Equivalent Circulating Density (ECD)
Shutting down pumps during drilling connection can lead to a reduction in dynamic bottomhole pressure, causing the loss of ECD. This loss can allow formation fluids to enter the wellbore, known as “connection gas.” Observation of connection gas is an indication that static mud overbalance is lost, necessitating a potential increase in mud weight.
Improper cement mixing, lost circulation, or casing float equipment failure can compromise cement density and reduce hydrostatic pressure, leading to well control issues.
Cement Slurry Transition
As cement transitions from a slurry to a solid state, there’s a temporary reduction in hydrostatic pressure due to self-supporting cement solids before the structure becomes impermeable. This can potentially lead to an influx.
Closely monitoring the well throughout all phases of drilling, completion, and cementing operations is imperative for preventing and mitigating well control events. Nurturing a proactive approach ensures the integrity and safety of the wellbore.
To prevent well control events caused by low drilling fluid density, it’s essential to:
- Maintain strict pit discipline and monitor fluid properties regularly.
- Use appropriate mud additives to prevent gas cutting and control fluid rheology.
- Monitor for oil or saltwater invasions and address them promptly.
- Implement proper mud design and operational practices to minimize barite sag.
- Maintain pumps running during pipe connections to avoid ECD loss.
- Exercise caution during cementing operations and closely monitor pressure changes.
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.