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.

The hardware used in casing while drilling operations over the past 5 years has proved to be robust and reliable. Several directional wells have been drilled successfully with casing using positive displacement motors (PDMs), but the drilling efficiency was significantly degraded in most of them. This paper documents the drilling of the first two wells to utilize a rotary steerable system (RSS) to improve drilling efficiency when drilling with casing. Combining an RSS with casing while drilling operations seemed to be a natural way to eliminate the major weaknesses in motor bottomhole assembly (BHA) designs. Rotary steerable systems had not previously been used with casing while drilling because both are new technologies focused on different environments. Casing while drilling developed on land operations while rotary steerables have been popular for offshore projects. ConocoPhillips has drilled more than 94 wells in the Lobo trend of South Texas since 2001 using the new technique. As part of a technology demonstration project to accelerate the technology to offshore applications, two wells were drilled with an RSS. The first was an operational test conducted by drilling vertically with the RSS. The second was a full directional test with a build to 29° and then a drop to vertical, including a 100° directional turn. Introduction ConocoPhillips embarked on an active field development program in 1997 aimed at drilling hundreds of wells over the next few years in the Lobo trend of South Texas. Since that time, over 900 wells have been drilled through the Wilcox (Lobo) section ranging in depth from 7,500 to 13,000 ft. However, in 2001, after drilling about 600 wells, the drilling efficiency had stagnated. A program was undertaken to find ways to reduce drilling costs sufficiently to extend the development potential for several years. In the previous years, great strides had been made in increasing rate of penetration (ROP), drilling each hole section with a single bit, and in improving general rig operation efficiency. Any major reduction in drilling time had to address the flat time more than the "making hole" times. The most significant flat times were associated with keeping and protecting the hole, trouble time which averaged about 1.5 days per well, and casing running operations. Stuck pipe and lost circulation were the most consistent contributors to the trouble events and accounted for about 75% of the trouble time in 2000 and 2001. A casing while drilling system was chosen for a five-well pilot project to evaluate the impact that this technology might have on the particular problems encountered at Lobo. Sufficient progress was made in drilling these first wells to justify moving to a second phase of drilling. The second phase proved that casing while drilling could eliminate the formation-related trouble time experienced with conventional drilling. A major finding was the confirmation that lost circulation was almost completely eliminated, allowing the drilling of additional wells formerly considered uneconomical. The wells were not drilled trouble-free, but the trouble was associated with the mechanical equipment limitations, which were overcome on the phase-two wells. Fontenot1,2 provides more information about the Lobo field and the performance on these early wells. Full-scale implementation of the Lobo casing while drilling program was initiated by bringing in three new rigs in late 2002 and early 2003. These 15,000-ft rigs,3 built specifically for ConocoPhillips, were introduced to optimize the drilling process. They provided an increased hookload rating; much better mobility for intrafield moves, a reduced footprint, and a semi automated casing handling system. The overall drilling program at Lobo has been scaled down, with one casing while drilling rig still operating. More than 94 wells have been drilled with casing in the area. The incidents of lost circulation have been almost totally eliminated in the casing drilled wells. The difficulty of balancing lost circulation at the intermediate casing shoe while controlling a reservoir pressure of up to 15 ppg during the trip out to run production casing has been eliminated. In some cases, casing drilled wells are completed with three strings of casing when four would be required if the wells were drilled conventionally.

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Casing while drilling (CWD) is an emerging technology being introduced in different areas around the world This new configuration, where the casing is used as a drillstring, presents new challenges for primary casing cementing operations compared to the conventional cementing operations. A full understanding of the required changes of the cementing methodology from conventional drillpipe drilling operations can contribute to the success of any CWD campaign. CWD cementing differs from conventional cementing practices because it is impossible to use standard centralizers attached to the casing while drilling because of extended and faster casing rotation. When more than one bit is required to reach the next casing point, CWD requires full-bore casing access to pull and run bottomhole assemblies (BHA) through the casing. In these instances, conventional floating equipment cannot be used. Wireline logging is normally conducted in cased hole after the cementing job. The cement volumes are calculated with a cement excess factor instead of a caliper log. This paper describes the methodology developed to successfully cement surface, intermediate, and production casings in more than 125 wells in south Texas where CWD was used. These same techniques can be applied in CWD operations elsewhere. Introduction CWD differs from conventional cementing practices in several ways. Centralization for Casing while Drilling In CWD operations, standard bowspring or welded-body centralizers are not recommended. The casing string will be subjected to longer and faster rotation while drilling the entire openhole section, and standard centralizers are not suitable for these conditions. They may cause severe wear damage and may lose their original placement, decreasing pipe centralization. In addition, these standard centralizers attached to the casing can be lost in the hole, causing additional problems when drilling ahead. In CWD, there is no option to place any type of centralizers with an OD larger than the gauge hole size. Bow-type centralizers are desirable where washouts are expected because they provide restoring force to centralize the casing in the hole. The bows on this type of centralizer have lower resistance to casing rotation. A good mud system is essential to minimize the hole washouts. If washouts are unavoidable, the reduced pipe standoff should be compensated by enforcing other best cementing practices, such as providing good mud properties, pumping rates, spacer design, etc.1

TX 75083-3836, U.S.A., fax 1.972.952.9435. AbstractDrilling with casing is a method of drilling wells using casing as the drillstring. This method has been used in South Texas to reduce well costs. To help reduce well costs even further, underbalanced drilling seemed to have the potential of enabling the elimination of an intermediate casing string.By drilling underbalanced, it would be possible to drill with a mud weight low enough to prevent loss of circulation in the shallow weak zones while dealing with the gas from the pay zones below. Drilling with casing eliminates the necessity of trips, thus simplifying underbalanced operations. However, once total depth (TD) was reached and drilling complete, it would be necessary to balance the well to allow cementing of the production casing string.The challenge was to develop a way to kill the well so that the cement could be placed and allowed to set while at the same time not breaking down and losing circulation into the weak upper zones. This had to be done by having different equivalent circulating densities (ECD) and equivalent mud weights (EMW) up and down the hole throughout the circulating and cementing operation. In addition, this technique needed to be relatively simple and easily accomplished in the field. All this had to be done without shutting in the well or using chokes, since that would also cause breaking down the upper zone and result in an underground blowout. This paper describes the development of the required methodology and its successful field application in South Texas. This same technique can readily be applied elsewhere.