What is Pressure Gradient?

It can be defined as pressure per unit length. If the pressure is in psi and length is in feet then the unit of pressure gradient is psi/ft.

Following terms are used in common in context to pressure gradient:

• Hydrostatic pressure gradient
• Formation pressure gradient

If the hydrostatic pressure exerted by a drilling fluid column of density 10 ppg in wellbore at a depth of 1000 feet is 520 psi, then the hydrostatic pressure gradient is defined as the hydrostatic pressure per unit length, which is the ratio of hydrostatic pressure and the depth at which the hydrostatic pressure is considered, ie. 520 psi/1000 ft = 0.52 psi/ft.

What is Hydrostatic Pressure?

While drilling a well, drilling fluid (mud) is pumped through drill string, it comes out through bit nozzles, enters the annulus, travels up the annulus and finally reaches the surface and comes back to mud pit through flow line. Figure below is illustrative of what i have stated.

Thus in the process of drilling the annulus is always kept full of mud.

OGP - Recommendation for Enhancement to Well Control Training, Examination and Certification

In response to Mocondo and Montara well control incidents, the International Association of Oil & Gas Producers (OGP) formed three Global Industry Response Groups (GIRGs) to control well control incident management through prevention, intervention and response, respectively. The Well Expert Committee (WEC) was established out of the Prevention GIRG to enhance well control incident prevention through better improvements in well engineering design and well operations across the oil & gas industry. 

The Report contains recommended improvement to current well control training, examination and certification process, and some of related philosophies that should be adopted throughout the industry to improve well control preparedness and performance for all type of operations conducted on all wells worldwide - ie construction, intervention/ work-over and production/ injection of both onshore and offshore wells.

Read the complete OGP report here.

Secondary Well Control

If due to any reason the hydrostatic pressure exerted by drilling fluid column falls below the formation fluid pressure, the formation fluid starts flowing into the wellbore. In such case we say that 'the primary control is lost' and the well has taken a kick. 

What next? 

Once the kick is detected, the Driller should immediately close the well using his company's well control policy. In such cases the well can be closed by closing Blow Our Preventer (BOP).

Thus, 'SECONDARY WELL CONTROL' is defined as the use of BOP and well control procedures to control the kick when the primary well control is lost.

Primary Well Control

During normal drilling operation, we always keep the wellbore full of drilling fluid (mud). The drilling fluid column in the wellbore exerts a pressure at the bottom of wellbore, called hydrostatic pressure.  Hydrostatic pressure is the pressure acting on wellbore when the mud pumps are off and there is no circulation of fluid in wellbore. 

Also, the compressed formation fluid contained in the formation has a trapped pressure called formation pressure. If the hydrostatic pressure of drilling fluid column is greater than or equal to the formation pressure, the formation fluid cannot enter the wellbore and we say that the well is under control.If the hydrostatic pressure of drilling fluid column falls below the formation pressure, the formation fluid starts flowing into the wellbore. In this situation we say that the wellbore has taken a kick.

What is Blow-Out?

In the previous blog we discussed about what is kick and its occurrence. Drillers play a major role in detecting and handling the kick. 

As discussed, if the early warning signs of kick is unnoticed by the driller, more and more kick enters the wellbore. As the kick is always lighter than the drilling mud, its presence in wellbore further reduces the bottom hole pressure (BHP) of the wellbore. The further reduction in BHP encourages further flow of kick in the wellbore but now with more strength than before! It rushes into the wellbore with maximum available strength, offloading the mud from the well. The offloading of mud by the kick can be observed on flow line. The volume of mud coming out of well through flow line increases. This increases the percentage of flow out through flow line. The flow-out percent can be read by the driller on his console.

What is Kick?

When the bottom hole pressure (BHP) in the wellbore becomes less than the formation pressure, the formation fluid starts flowing from the formation to the wellbore. Such flow of formation fluid from formation to wellbore is termed as kick.

Kick can be either of water, oil or gas. It is a measurable quantity and is expressed in terms of units of volume like litre or barrel.