What Causes a Blow-Out When Drilling an Oil or Gas Well?
Thousands of oil and gas wells are drilled in the United States, offshore or overseas every year. Despite the fact that most are drilled into “high-pressure” zones of hydrocarbons, almost no blow-outs or explosions occur during drilling operations. What causes the blow-outs or explosions that do occur?
The first thing to consider when thinking about blow outs and the causes is this: the oil or gas which is being sought has been created and sealed in rock formations deep under the earth’s surface over the course of millions of years. The combination of the rock squeezing or compressing the fluids and the earth’s gravitational pressures means the majority of the fluid or gases being sought are contained within rock formations under pressure. The oil or gas that is contained within these pockets of rock didn’t start out as hydrocarbons but started out as a carbon based substance at the earth’s surface. The process of carbon based plant or animal material becoming hydrocarbons takes millions of years and is caused by the material being covered by sand or rock, sedimentary material that squeezes the water from the carbon-based material and eventually results in either oil, gas or coal forming. The formation of these three carbon based hydrocarbon products usually requires enormous pressure. So, when extracting oil or gas through drilling, the first obstacle that has to be overcome is the pressure the hydrocarbons are under while being extracted.
The drilling process has been developed to enable the safe extraction of hydrocarbons from pressurized formations. The first procedure that is used is cementing. Surface casing is cemented into the hole and serves as a type of lid, like that on a pressure cooker that holds back the down-hole pressure. This casing is cemented even before drilling begins.
Once the surface casing is cemented into place and the surface cement has been tested, drilling begins. As the well is drilled, an “oil-based drilling mud” is used to lubricate the drilling bit and circulate the cuttings out of the hole as drilling occurs. The oil-based mud also contains a product called “barite” which “weights” up the mud or increases the mud weight so that the weight of the mud in the hole, when the mud is weighed and the mud weight combined with the weight caused by gravity from the depth of the hole, is sufficient to counter the pressure from the hydrocarbons that want to “escape” to the surface. The weight of the mud column is used to hold down the pressure of the gas which wants to escape up the hole and out of the well.
If the mud weight is not sufficient to hold back the hydrocarbons, or if the well is being drilled “underbalanced” or with more down-hole pressure than mud weight, then the hydrocarbons have to be ‘circulated” out of the hole. Safe drilling practices require the use of a choke manifold to direct the hydrocarbons which are under pressure through a “gas-buster” and into a flare where the hydrocarbons should be safely burned off. This is somewhat of an oversimplification for purposes of this article but gives the essentials of the process.
In addition to the drilling mud, the rig will have a BOP (Blow Out Preventer) stack in place to help prevent unwanted escape of pressurized hydrocarbons. The BOP stack usually consists of two sets of Rams and one “annular” preventer. The BOP stack consists of “pipe rams” which are the main means of preventing hydrocarbons from escaping, the ‘blind” rams which are used when no drill pipe is in the hole and completely shuts in the well and “shear” rams which cut away the heavy drill pipe and completely shut in the well. BOP’s are supposed to be tested on a regular basis to ensure they hold pressure.
A major source of blow-outs is caused by the use of the wrong set of rams to shut in the well. The drilling crews train mainly with the use of the pipe rams, and they are the rams that the crew is most familiar with and use on most occasions. However, there are times the pipe rams are not effective to shut in the well. Many blow-outs, explosions and serious injuries have occurred when the BOP’s were closed and the well was supposed to be shut-in. BOP’s are not fail-proof, and even if they seal, the pressures they face may exceed their rated capacity. Most BOP’s are rated at either 5,000 or 10,000 pounds – meaning they are supposed to hold against well pressure of up to either 5,000 or 10,000 pounds. Unfortunately, down hole pressures can exceed the rated pressure of the BOPs. In addition, the BOPs have to be activated to shut-in the well. While most rigs are supposed to have enough “safe” BOP stations where the BOPs can or should be capable of safe operation, these do not account for every situation, and it may be that the crew is unable to safely operate the BOPs. Safe operators provide for remote-operated BOPs in the event of the crew not being able to operate the BOPs from their normal locations.
“Trip tanks” are used to measure the amount of drilling mud that is being returned to the surface. If mud volume is increasing at too fast of a rate, it is an indication that a blowout may be about to occur. The increased volume of mud means that gas or oil is migrating into the hole, and the well may be about to blow out. Constant monitoring of the trip tank is a primary means of staying on top of well conditions and anticipating blowouts.
When the well is being drilled and the crew is tripping in or out of the hole, mud volume is lost. If that mud is not replaced the down-hole pressure may overcome the mud weight, and a blowout can occur.
By far, the most common cause of blowouts is drilling “underbalanced” – taking the risks of controlling the hydrocarbons that enter the hole by circulating them out while drilling. Safe drilling practices require the mud to be “weighted” up to hold back the pressure, but operators get in a hurry to produce the well, and time is money, so they drill underbalanced and lose control of the well. If the well is drilled underbalanced, a choke manifold and sufficient gas buster should be used to flare off gas that migrates to the surface during the procedure.
Making sure the surface cement and any additional cement is used properly is the first thing required to drill safely. Afterwards, ensuring that the mud weight is sufficient to control the well is the next means of controlling the well. The BOPs need to be constantly tested, and BOP drills need to be conducted regularly. If an operator chooses to drill underbalanced, then a choke manifold and proper equipment to flare off the gas that will naturally migrate to the surface needs to be in place.
Adequate safety measures need to be constantly in mind while drilling because a pressurized zone can be encountered at any time. Unfortunately, this can result in sudden increases in pressure and a blowout.