What is a Blowout?
A blowout is the uncontrolled release of crude oil and/or natural gas from an oil well or gas well after pressure control systems have failed. Modern wells have blowout preventers (BOP Systems) intended to prevent such an occurrence. An accidental spark during a blowout can lead to a catastrophic oil or gas fire.
A blowout is one of the most tragic and most expensive technical accident that can happen in the upstream oil and gas industry. When a blowout occurs, it is an immediate emergency situation that endangers life, the environment and all related equipment. Unfortunately, this will affect future production from the lost well and sometimes the energy of the reservoir.
History is demonstrating that over 85% of blowouts is caused by human error, 10% by microscopic defect in materials related of the well’s barriers and 5% by the non-detected/expected pressure anomalies in explorations (wild -cat wells).
Luckily…there are plenty of ways to prevent blowouts before they occur.
Preventing a Blowout
The downhole fluid pressures are controlled in modern wells through the balancing of the hydrostatic pressure provided by the column. Should the balance of the drilling mud pressure be incorrect (i.e., the mud pressure gradient is less than the formation pore pressure gradient), then formation fluids (oil, natural gas and/or water) can begin to flow into the wellbore and up the annulus (the space between the outside of the drill string and the wall of the open hole or the inside of the casing), and/or inside the drill pipe. This is commonly called a kick.
Ideally, mechanical barriers such as preventers (BOPs) can be closed to isolate the well while the hydrostatic balance is regained through circulation of fluids in the well. But if the well is not shut in (common term for the closing of the blow-out preventer), a kick can quickly escalate into a blowout when the formation fluids reach the surface, especially when the influx contains gas that expands rapidly with the reduced pressure as it flows up the wellbore, further decreasing the effective weight of the fluid.
Although drilling, completion, stimulation, workover and well testing operations are strictly monitored, there are many unknown or high-risk factors which may induce a kick. The aim for any technical program is to use and apply with success all Well control Second Line equipment and worldwide recognized procedures, standards and practice in order to shut in, circulate the kick and kill the well. All these efforts are done to avoid a well blowout with the most drastic consequences.
Knowing the warning signs and recognizing a kick makes a world of a difference, as well as keeping your BOPs in good shape. Make sure to regularly clean them with for example a powerful BOP cleaning tool to remove any metal fragments. And clean out any mud to reduce the risk of a leak or escaping pressure.
Preparation is key to keeping your operations safe. As Benjamin Franklin once said: “By failing to prepare, you are preparing to fail.” Knowing how your equipment behaves in extreme conditions will help you to prepare to what’s coming. Personnel is trained to recognize kicks so they are able to take action to prevent a blowout from happing.
A prepared man is worth two men
To control a kick, specific steps should be taken:
- Detect the kick and measure it. The bigger the volume of the kick, the more difficult it is going to be for killing the well.
- Seal/shut in the well ASAP, by using the BOP System.
- Read and record the stabilised shut in pressures.
- For the existing well situation, apply the most adequate Second Line well control method in order to circulate the kick out of the well and kill the well.
What is a kick? And how to detect one
A kick is an unplanned flow of formation fluids into the wellbore during drilling/workover/logging operations. It is caused by the hydrostatic pressure in the wellbore being less than that of the formation pressure. This induces a flow from the strata into the well bore, which can be caused in two ways.
The mud weight (hydrostatic pressure) is too low, or if dynamic and transient fluid pressure affect, due to the motion of the drillstring or casing, lowering the hydrostatic pressure in the wellbore below of the formation pressure actual value.
The primary means of detecting a kick while drilling is a relative change in the circulation rate back up to the surface into the mud pits. The drilling crew or mud engineer keeps track of the level in the mud pits and/or closely monitors the rate of mud returns versus the rate that is being pumped down the drill pipe.
Upon encountering a zone of higher pressure than is being exerted by the hydrostatic head of the drilling mud (including the small additional frictional head while circulating) at the bit, an increase in mud return rate would be noticed as the formation fluid influx blends in with the circulating drilling mud.
Conversely, if the rate of returns is slower than expected, it means that a certain amount of the mud is being lost to a thief zone somewhere below the last casing shoe. This does not necessarily result in a kick (and may never become one); however, a drop in the mud level might allow influx of formation fluids from other zones if the hydrostatic head at is reduced to less than that of a full column of mud
Early warning signs of an impending well kick while drilling:
- Sudden change in penetration rate;
- Reduction in drillpipe weight;
- Change in pump pressure;
- Change in drilling fluid return rate.
Other warning signs during drilling operations:
- Returning mud “cut” by (i.e., contaminated by) gas, oil or water;
- Connection gasses, high background gas units, and high bottoms-up gas units detected in the mudlogging unit.
A clear warning sign of a kick is a sudden increase in the amount of drilling fluid. Oilfield operators are trained to recognise the warning signs and know the importance of balancing the hydrostatic pressure with the formation pressure in the wellbore.
Basically, there are two types of Blowout Preventers: Annular BOPs and RAM-type BOPs, each of them having their own major role in well control and preventing a Blowout. BOP systems are the last barrier against of the down hole formation pressure therefore those systems have to be properly sized, installed, tested for their working pressure and for the function abilities and the drilling crew have to be well trained in operating with reliability these systems.
Our Pressure Control experts can help you with the selection of equipment needed to keep your drilling operations safe, whether you are looking to rent or buy.