Before many of the new chemical and nanoparticle technologies for wellbore strengthening arrived to the marketplace, casing while drilling (CWD) was used for more than a decade to mechanically achieve the same end. Weatherford International and other service companies have developed multiple technologies for CWD, as well as the similar method of liner drilling (LD). With CWD, the casing is the drillstring, and with LD, the liner is the drillstring.
CWD and LD technologies are building up a track record as operators encounter more difficult-to-drill frontier fields. Weatherford said it was contracted by a North Sea operator to use its LD technology in a field where three previous attempts resulted in two sidetracks and well suspension. The company said that with LD, the liner was set at the desired depth interval.
The primary applications of CWD and LD have so far been mainly offshore where operators are drilling through depleted zones to reach a deeper reservoir, which is becoming more common in the North Sea and US Gulf of Mexico, and in wells where operators are seeking to reduce time-consuming trips in and out of the well.
The technology is also being applied more in high-pressure/high-temperature wells where drilling windows are very narrow. “There is not much room between pore pressure and fracture gradient, so you need to get creative,” said Moji Karimi, Weatherford product line champion for CWD and LD technologies.
When using these two technologies, the primary mechanism of wellbore strengthening involves the interaction of casing with the wellbore wall as it drills deeper into the Earth. “That creates a more slick and impermeable (filter) cake that adds more integrity to the wellbore, in terms of the pounds per gallon that it can take,” said Karimi, referring to what is technically called the equivalent circulating density.
The filter cake created is then compacted between the casing and the wellbore, adding stability. Karimi explained that this mechanical interaction can also heal induced fractures in the wellbore. The friction created by steel casing also raises the temperature around the wellbore, “which we know is one of the mechanisms of increasing the pressure containment of the wellbore.”
There are two types of CWD and LD systems, nonretrievable and retrievable. Nonretrievable systems have a drillable bit at the bottom of the casing and, as the name implies, “whatever goes down, stays down,” Karimi said. Once this casing is at desired depth, it is cemented in place and then a conventional drilling string is lowered and drills through the bit just cemented in place.
Retrievable systems can be equipped with a bottomhole assembly (BHA) unit that is locked into the first joint of the casing string. This allows operators to attach specialized equipment to the BHA, including logging while drilling, measurement while drilling, or a mud motor.
In the case of retrievable systems, once the casing and BHA have reached total depth, “you can go back in through the casing with wireline, or drillpipe, latch onto the top of the BHA and retrieve it,” Karimi said. “And the whole time, the casing stays on the bottom.”
“Now you can actually have tools and drill directional sections in multiple runs, but with the nonretrievable system, you are limited to the capability of the bit,” he said.
And, because drillable bits are designed to be drilled out with a conventional bit, they can drill only through a certain amount of rock before they begin to dull and some rocks are too hard for drillable bits to be used at all. Weatherford said its drillable bits are made of an aluminum alloy, while other service companies use copper, bronze, and steel alloys.
