Blowouts

Blowouts are very rare in the modern oil and gas industry, but they can cost companies millions of dollars in lost time, fines, lawsuits, and environmental cleanup costs. On site, they can also cause fires, explosions, and destruction of equipment, not to mention the potentially tragic toll of injury or loss of human life.

In order to drill an oil or gas well both safely and effectively, drillers must prevent blowouts.

If you’ve ever seen an old movie about oil, you’ve probably seen footage of a “gusher”, a fountain of oil spewing out of the ground. These gushers are the result of a blowout, an uncontrolled release of pressure from the well.

Piping and well equipment can be thrown out of the borehole far away in the event of a blowout, and the shockwave alone can be enough to injure nearby workers. In addition, oil may reach an ignition point on the surface and catch on fire. Oil fires associated with blowouts are extremely difficult to put out, and often need to be “snuffed” by controlled explosions.

Blowouts typically occur during the drilling phase of operations because the well cannot be completely sealed off from the surface. In a blowout, formation fluids or gases enter the annulus and flow to the surface, where they can expand extremely rapidly under the reduced pressure. An uncontrolled flow into a borehole is called a “kick”.  An uncontrolled flow out of a borehole is called a “blowout”.  Note that blowouts can not only occur on the surface but also underground.  And an underground blowout that ruptures to the surface is called “cratering” and is a penultimate disaster. 

Fortunately, modern drilling operations have multiple pressure containment systems designed to stop blowouts and they have become quite rare.

Detecting and Containing Kicks

When high pressure fluids enter the wellbore, the wellbore experiences a kick. If the kick is detected early enough, a blowout can be prevented by immediately closing in the well using the BOPE, circulating the kick out in a controlled manner, and eventually increasing mud density to exert a higher borehole pressure than the kicking formation.

Kicks are usually identified by one or more of the following indicators:

Increase in mud return rate

The classic indicator of a kick is an increase in the flow of mud in the surface return line. If mud is leaving the well faster than it’s being pumped in, formation fluids are almost certainly entering the wellbore. If 300 gpm is going in and 350 gpm is coming out, clearly 50 gpm is coming from the well!

Increase in the mud pit volume

If mud is returning to the mud pit faster than it’s being pumped out, the fluid level within the pit will rise, indicating a kick. However, this is a slower manifestation than an increase in mud return rate. Many operators use a trip tank to monitor mud volume while tripping pipe. The trip tank is small enough that if excess mud is returning from the well while tripping, it shows up more quickly than by monitoring the entire volume of mud in the system. One thing about the pit level rise, given the geometry of the pit, one can determine the volume increase in the pit.  This is a direct indication of the volume of the kick.  And smaller kicks are much easier to handle than larger kicks.

Flow with pumps off

If fluids are flowing from the well when the pumps are turned off, it’s almost certainly due to the invasion of the wellbore by formation fluids.

Other indicators leading to a well blowout

Additional common indicators leading to a well blowout include an increase in the rate of penetration and a high percentage of gas in the returning mud.

What happens when a kick is detected?

When a kick is detected, workers immediately take action to control the well. Most importantly, the blowout preventer needs to be engaged to seal the annulus.

After the well is shut in, drillers determine the mud weight needed to counteract the formation pressure and control the kick by measuring the pressure inside the shut-in well.  From that, the required mud weight needed to counteract the formation pressure can be determined.  This is called the “kill mud weight” (KMW).

Drillers then attempt to circulate this KMW mud through the wellbore.

Once the heavy mud has been fully circulated, the well can be checked for any continued flow.  If that is detected, the whole process is repeated.  If no flow is detected, the well can be opened up. If the kick has been contained successfully, normal drilling operations can resume.

Risk Factors for a blowout

There are a couple specific situations in which the risk of a blowout needs to be monitored particularly closely.

Overpressure

Regions of overpressure occur where pore pressure is higher than expected for a given depth. If unchecked, overpressured fluids can overcome the pressure of the drilling mud and cause a blowout.

How does overpressure develop?

When sediments are deposited, their pore spaces contain liquids such as water. As the sediments are buried and compressed into rock, the water is usually forced out of the pores. However, if an impermeable layer of rock blocks the escape of fluids, they become trapped and are pressurized by the weight of the overlying rocks, which increases as more rocks are deposited at the surface.

Overpressured units have more pore space than would be expected for their depth. Since high pore space allows a rock to hold more oil and gas, regions where oil and gas are extracted are likely to have areas of overpressure.

Since overpressures are contained by impermeable rock layers, they can be encountered with little obvious warning when drilling. Careful study of the local conditions and examination of the formation evaluation records for nearby wells helps geologists predict where overpressure is likely to occur.

High Natural Pore Pressure

In regions of high pore pressure, there can be a relatively small margin between the pore fluid pressure and the fracture pressure. In such regions, great care needs to be taken to maintain a proper mud weight.

Fortunately, there are methods for predicting naturally high pore pressure and drillers can usually plan ahead to ensure safe operation.