The Direct XCD* drillable alloy casing bit isa PDC bit specially made for drilling verticalor tangential wells to TD in one run. The bit drills on standard casing that is rotated at the surface.
The cutting structure of the Direct XCD casingbit can be fitted with 13-, 16-, or 19-mm PDC cutters, which are available in standard orpremium grades. The bit’s sub is composed of durable oilfield grade steel, while its bodyis made of a copper bronze alloy. This uniquealloy allows it to be drilled out by any standard PDC bit after the Direct XCD bit has drilledto TD, and the casing has been cemented inplace. After drillout, the drillout PDC bit cancontinue drilling the next interval, eliminating the need for a dedicated drillout run.
During drilling, rotating casing and there sulting smaller annulus cause cuttings to besmeared into the borehole wall, strengthening the wellbore. This action has been termed the plastering effect. It seals pores in the formation to reduce fluid losses and improves cementing to help protect well integrity. Because cuttings are plastered into the borehole wall, fewer are returned to surface. As a result of being subjected to grinding,the cuttings that are returned to surface are smaller in size.
The plastering effect process has proven toreduce NPT resulting from borehole-related problems, such as
■sloughing shales
■tight holes
■borehole bridges
■lost circulation
■large-diameter surface hole resulting inhard-to-remove cuttings from the annulus
■damaged producing zones
■stuck pipe.
CASE STUDY1
An operator working in Fremont County, Colorado,was drilling a 9-5/8-in well section through a part ofthe Denver-Julesburg basin’s Niobrara formation, which is characterized by unstable Pierre shale.The planned well depth was 5,000 ft, at 3,800 ft the BHA became stuck and was lost. After drilling aside track to 3,410 ft, the drilling team attempted torun 95?8-in casing but was unable to get the casing string past 2,444 ft. The operator pulled out of the hole and made a cleanout run with drill pipe to3,095 ft. However, after cementing the operator could only reach a depth of 2,800 ft.
After 68 rig days the operator considered abandoning the well but decided to try reaching the planned TD using a Direct XCD drillable alloycasing bit.
Reaming the casing back to bottom enabled the operator to drill the remainder of the interval to TDusing an 8.-in Direct XCD bit on 7-in casing. Upon landing and cementing the casing, the bit wasdrilled out. The casing-while-drilling operation took approximately 10 rig days and saved the USD5.5 million investment in the well, after which the operator continued running the production string.
Once the well was completed, the operator skidded the rig 20 ft to drill a second well. Usingthe same drilling design from the first well, the operator ran a 12.-in Direct XCD bit on 9-5/8-in casing to drill from 1,600 ft to interval TD at 4,900 ft in seven days. Compared with the USD 5.5 millioncost of the first well, the second well was drilled for USD 2.4 million.
Case Study2
As part of an effort to access a reservoir inthe Permian basin, an operator was using athree-string well design in which the casing point was above the field’s high pressure CO2 zone. This approach was effective andit countered problems the operator had experienced using a two-string design: CO2 kicks, stuck pipe, wellbore collapse, poor cementing, and NPT. However, the high cost of a three-string well design made it anunsustainable option for the operator.
To maintain the economic viability of theproject, Schlumberger recommendedconstructing a two-string well using the Smith Bits 77?8-in Direct XCD drillable alloycasing bit. Multilobe torque (MLT) rings were added to the operator’s 5/1-2-in buttressthreadedcoupled casing to provide positive makeup and increase torque. In addition,hydro-formed centralizers were used to helps tabilize the drilling casing string and center itfor cementing.
Using the Direct XCD casing bit to construct a two-string well design in five wells enabledthe operator to come out of the conventionally set 8-5/8-in casing and drill below the highpressureCO2 zone to TD with 5-1/2-in production casing. The 5-1/2-in production casing wascemented, and the Direct XCD casing bit wasdrilled out with a standard PDC bit. No kicks,stuck pipe, borehole collapse, or extendedNPT were experienced during the operation.
As a result of the operation, the expenseof an additional string was eliminated infive wells, and cement bond logs confirmedthe cementing job was a record best forthe operator.
CASE STUDY3
To mitigate mud losses in a field in the Middle East, casing drilling services were contracted. Theoperator sought to drill with 9?-in casing to isolate water-sensitive shale and to shut offlost circulation zones within the troubled formation. This would entail drilling to approximately2,490-ft TVD.
Other objectives for implementing nonretrievable casing while drilling included
■drilling through the formation’s loss zone and performing second-stage cementing
■testing the use of the technology to drill with 13?-in casing and eliminate a casing string
■eliminating stuck pipe, wiper trips, and unnecessary BHA pipe handling
■removing drillpipe tripping, reaming, and casing running time, as well as risk associated with conventional casing running operations.
By using a 12-in Direct XCD bit and 9?-in casing as part of the project’strial initiative, the operator aimed to mitigate issues related to losses in the troubled formation. To ensure isolation, a diverter valve (DV) tool and packer were incorporated into the casing drilling string. In addition, a multistage inflation packer collar was used in a casing drilling application.
The 13-in casing was set as a surface casing conductor at 1,500 ft. Casing drilling operations were planned using the Direct XCD bit (7-bladed with 16-mm premium PDC cutters), which was recently designed based on proven SHARC* high-abrasion-resistance PDC drill bit from Smith Bits, a Schlumberger company. The seven blades were designed for higher cutter density, with optimized nozzle configuration considering potential lost circulation.
The casing drilling operation went as planned, with no safety incidents, no vibration or bit whirl, and no borehole instability. Losses encountered during conventional drilling to 1,994 ft slowly healed during casing drilling; at TD no mud losses occurred, and there were no losses while cementing.
Average ROP was 58 ft/h, with penetration breaks reaching 140 ft/h. The casing was successfully pressure tested to 2,000 psi above the DV tool, above the float collar, and 10 ft above the shoe. The Direct XCD bit drill shoe was drilled out in 45 min using an 8.-in PDC bit. The entire operation saved 7 h off of plan.
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