ConocoPhillips was experiencing downhole connection fatigue failures while drilling with 7 in. casing in the Lobo Trend in South Texas. A 7-5/8 in. casing drilling drill collar, a heavy wall tubular similar to traditional drill collars, was developed, analyzed and successfully used to eliminate the failures. Grant Prideco suggested a different string design and developed a higher fatigue resistant connection to be threaded on 7-5/8 in. 47.1 lb/ft (0.625 in. wall) casing. This heavy weight pipe and connection would be used on the bottom of the string as drilling with casing drill collars. Introduction Casing drilling is utilized to develop several fields in the Lobo Trend in South Texas for ConocoPhillips. Three areas are being developed: Northern, Central, and Southern. While all of the wells to date in the Northern area had been completed without problems, about one out of every three wells in the Central and Southern areas were experiencing casing fatigue failures in the 7 in. 23 lb/ft P-110 casing strings. The type of connections used in these casing strings were American Petroleum Institute (API) Buttress threaded connections with a metal insertable torque ring and then Grant Prideco Drilling With Casing/Coupled (DWC/C?) threaded connections, both of which were designed for casing drilling. ConocoPhillips approached Grant Prideco to find a solution to the problem. Original String Design A typical string design used by ConocoPhillips in the Central and Southern areas was 9-5/8 in. 36.00 lb/ft J-55 surface casing drilled in and set at around 1,500 ft. An intermediate string of 7 in. 23.00 lb/ft P-110 was drilled in and set at around 8,000 ft. The production casing was 4-1/2 in. 11.60 lb/ft P- 110 and 4-1/2 in. 13.50 lb/ft P-110 drilled in and set at around 10,800 ft. See figure 1. A wireline retrievable bottom hole assembly (BHA), which consists of a drill lock assembly, tandem stabilizer, under reamer, and Poly-crystalline Diamond Compact (PDC) bit, as well as a motor assembly or rotary assembly, was installed on the bottom of each casing drill string. Figure 2 illustrates the 7 in. Casing Drilling BHA. Because of the multiple components, the BHA was very stiff compared to the casing string directly above the profile nipple. The 7 in. 23.00 lb/ft P-110 connection that was originally used was an API Buttress connection with an insertable ring to add torque resistance when the pin ends shouldered against the ring during make up. The use of this connection in the Northern area proved to be satisfactory. Some failures near the BHA were noted when this connection was used in the Central and Southern area. These connections were later changed to the DWC/C connection because of these failures. Grant Prideco developed the DWC/C connection specifically for Casing Drilling applications. This threaded and coupled connection uses the API Buttress thread form in combination with a special patented coupling design. The coupling has a stress relief groove in the center that allows the pin noses to butt together for more torsional resistance and increases the fatigue life of the connection.1. See figure 3. However, this connection also experienced failures near the BHA in the Central and Southern areas.

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