For the past couple of months, national news has featured up-to-the-minute coverage of the oil spill in the Gulf of Mexico. Reports often referred to the use of "drillers mud" in the oil drilling process, and at one point the media described an attempt to seal the well with it in order to stop the flow of oil into the sea. Do you know about drillers mud? Concrete Construction didn't, so we decided to find out. We quickly discovered very few people in the concrete industry know anything about it. You won't find it in any ACI publications because it doesn't fall within the association's jurisdiction. It falls under the guidelines set forth by the American Petroleum Institute (API), and we discovered there isn't a standard mix because the requirements of each well application are unique.
There are two types of mud: drillers mud and cement slurry.
When an oil well is drilled contractors, place drill bits at the bottom of drill pipes, turning them to do the actual drilling. The drill bits create a much larger hole than the drill pipe, so workers install a large steel pipe casing to protect the hole as they go down to prevent the sides of the hole from caving in. As the drilling gets deeper, it's common to reduce the diameter of the outside casing, making the structure of a well more complicated.
As a drill cuts through dirt and rock, the tailings must be cleared away constantly and brought to the surface. Drillers mud is used for that purpose. It flows through a hole in the center of the drill to keep the bit free of tailings. This mud is mixed with either fresh or salt water to flush ground up material away from the bit and bring it to the surface between the drill pipe and the casing. This noncementitious material is formulated to meet the special requirements for each well.
Jim Jarl, the quality control manager of Class H oil-well cement for Texas Lehigh Cement, Buda, Texas, says the cementing operation for an oil well is very difficult. The challenge involves filling the space between the steel casing and the dirt or rock sides of a well with a mixture of cement and various other materials designed to best secure the formations and bond to the casing. The void is typically 1 to 2 inches, and must be completely filled to secure the casing and prevent water or anything else from corroding the steel casing or escaping from around the outside of the casing pipe. "The cement slurry must adhere to the pipe and completely fill the space with no voids," Jarl adds.
In the case of the leaking well in the Gulf, cementing the casing starts more than a mile below the surface where temperatures can start at freezing and soon exceed 300° F in the ground below. "At 300° F normal portland cement can set instantly," says Jarl. "So the mixture must be designed to meet the specific expectations of each well."
Drilling slurries can be very complicated, starting with the cement. Class H portland cement is only used by the oil drilling industry and there are few cement producers who make it, as the product is considered high risk. Manufacturers remove the calcium aluminates (C3A) from the cement in order to provide a longer setting time.
The fine aggregate used is entirely dependant on well conditions. Companies develop mixes based on the conditions of a well. Mixes that must be heavier use hematite aggregate (an iron compound), mixes that must be lighter use fine-graded silica or silica flour, and clay materials can be added to reduce shrinkage. Retarding and superplastizing admixtures also are common.
How drillers install cement slurry
Time is of the essence when installing cement slurry because the cost of drilling per hour is very high and drilling operations cease while slurry is placed around casings. The mixes created for an application must give the installer just the right amount of time: The slurry must not set before placement is complete but should ideally set shortly afterward so drilling operations can proceed. Companies that design and install these mixes often make their own propriety admixtures to more carefully manage these exotic mixes.
To place cement slurries, contractors carefully calculate how much material is needed to fill a space. Then they pump that amount down through the casing to the bottom of the well to fill the void from the bottom up. Next they insert a plug or wiper plug on top of the slurry and press it downward with noncementitious mud or water, pushing the slurry around the bottom lip of the casing and up into the void, completely filling the void space around the casing. This process continues until the slurry moves all the way upward to the surface or previous placement, leaving the casing open so the drilling may proceed. Sometimes drillers will perforate through the casing and cement to expose oil bearing formations to the well.
When a well is terminated, the same cement slurry is used to fill the inside of the casing, completely sealing it.
The worldwide search for oil has become much riskier now. Oil companies are searching for oil under conditions that are much more difficult. In the Gulf, for example, drilling starts at ocean depths of 5000 feet or more, making cement slurry operations very intense. In response to these new environments, the requirements for cement slurry products are changing too, becoming more sophisticated.