Casing Drilling Technology Potential for Cost Reducing Operations in South Texas Lobo Trend

Casing Drilling* consists of downhole and surface components that provide the ability to use normal oil field casing as the drill string so that the well is simultaneously drilled and cased.7,8 Casing Drilling provides certain obvious improvements in operational efficiency, such as eliminating most of the pipe tripping that is required with the conventional drilling process. Putting the technology into application in south Texas
Conoco has had a sustained, multi-rig, development program in the Lobo field of South Texas since the mid-1990s. Drilling efficiency using conventional techniques has improved to the point where additional gains are difficult to achieve. Drilling with casing was implemented as a potential way to provide a step change to drilling performance in this mature field. The Casing Drilling process has proved to significantly reduce the in-hole trouble time below the low value that was already obtained and has demonstrated the ability to further reduce drilling costs.
The first two phases of the Lobo Casing Drilling program were conducted with a dual-purpose (Casing Drilling/conventional drilling) rig. After drilling 22 wells, the Lobo Casing Drilling program is being expanded by replacing this rig with three new Casing Drilling rigs designed to further optimize Casing Drilling performance. The first of these rigs is on location and the other two will begin operations by the end of January 2003.
Lobo cost reductions needed
ConocoPhillips produces about 520 MMCFD of natural gas from the Lobo trend in South Texas and has had a sustained, multi-rig, development program there since 1997. By 2001, ten rigs were being employed to drill about 160 wells per year, but the drilling efficiency was stagnated with an average 10,500 ft well taking about 19.2 days from spud to rig release. Drilling efficiency using conventional techniques had been improved to the point where additional gains were difficult to achieve.
Further cost reduction was dictated by the need to develop smaller and smaller reservoirs. It became clear that a new approach was needed to provide any chance of achieving significant well cost reduction. A specific goal was adopted to reduce drilling costs sufficiently to make reservoirs smaller than 1.0 BCF economical. The ability to develop reservoirs this size would extend the development potential for several years as the untapped reservoir size becomes smaller and smaller in the highly faulted Lobo field.
Stuck pipe and lost circulation are the most consistent contributors to the trouble events for Lobo wells. These two items accounted for about 75% of the trouble time in 2000 and 2001 (Fig. 1), while well control and a failure to successfully run the 7-in. casing were also significant in 2001 and 2000, respectfully.6
Drilling with casing was identified in early 2001 as a technology that could potentially solve these problems and provide a step change in drilling performance in the mature Lobo field. Tesco’s Casing Drilling system was selected to evaluate the potential impact of drilling with casing on Lobo drilling economics. Three-phase trial program A three-phase program to test the Casing Drilling program was undertaken in the Lobo trend:
Phase 1-Five pilot wells that had simple drilling conditions were drilled within the Lobo trend. Performance on these five wells steadily improved and matched that of conventional drilling by the time the fifth well was completed.
Phase 2-Additional wells drilled proved that Casing Drilling could eliminate the formation-related trouble time experienced with conventional rigs. This allowed additional wells to be drilled that would otherwise be uneconomical. The wells were not drilled trouble-free, but the trouble was associated with the mechanical equipment limitations shown in Fig. 2. These mechanical problems can be fixed, as opposed to the formation ‚ related problems that are encountered when drilling with conventional rigs.
Phase 3-The third phase Lobo Casing Drilling program has been initiated by bringing in the first of the three new rigs to begin full-scale implementation of Casing Drilling at Lobo. The remaining two rigs will be operational by the end of January 2003. These 15,000 ft. rigs are designed to optimize the Casing Drilling process, but can also drill with drillpipe. They have an increased hook load rating, much better mobility for intra-field moves, a reduced footprint, better fuel efficiency, and a semi-automated casing handling system.
Overcoming lost circulation
Conventional drilling is deemed uneconomical in areas because of the high probability of dealing with stuck pipe and lost circulation-either recovering from them or setting extra casing to prevent them. Lost circulation is the most severe problem that occurs in drilling Lobo wells and is a contributing factor to the next most serious problem-stuck pipe. One would normally expect lost circulation to be a potential problem with Casing Drilling because the smaller annular clearance between the casing and borehole wall increases the frictional pressure losses, thus increasing the ECD. In fact, what has been found is that Casing Drilling significantly reduces lost circulation. One of the main benefits of Casing Drilling that appeared in phase one of the program was that no lost circulation events occurred.
On the surface it might seem that Casing Drilling would not be a good option for these wells because the casing could get stuck before reaching casing point. Committing to use the Casing Drilling process and making optimum use of it requires some re-thinking of issues related to risk. The proven experience of reducing lost circulation and stuck pipe coupled with the fact that well control is much safer when the well can be circulated with pipe on bottom makes a compelling argument that Casing Drilling should be the first choice for drilling these difficult zones.
The case of well 14
The location for well 14 was specifically chosen to test the theory that Casing Drilling reduces lost circulation. This location was surrounded by three conventional wells drilled recently. Each of these wells had trouble related to lost circulation as shown in Figure 3.
All three of the conventionally drilled wells experienced severe lost circulation that could not be cured with LCM pills and required cementing off the loss zone. The first well lost full returns while drilling with 8.7-ppg mud and the losses could not be cured with conventional LCM pills. Eventually the well required two cement plugs before a decision was made to set casing at 6,917 ft. rather than try to drill ahead to the intended casing point of 8,000 ft. with 150-bbl/hr losses. Once the casing was set, a leak-off test indicated that the formation was not strong enough to reach TD without setting a liner.
A similar situation occurred with the second well, except the 7-in. casing reached only 5,145 ft before requiring a liner. In this case, losses again occurred while drilling below the liner.
The third well reached the normal casing point with the 7-in. casing, but not without difficulty. The well was drilled to casing point and the BHA became stuck on a wiper trip while fighting lost circulation. The continued lost circulation impeded the fishing operation and the well was sidetracked.
The Casing Drilling rig was moved to a location between these three wells. Provisions were made to drill ahead blind if lost circulation was encountered. The well was drilled with no particular changes to normal Casing Drilling operating practices. No significant lost circulation was encountered, even while drilling with 10.5-ppg mud. The BHA was retrieved and the 7-in. casing was cemented in place at the normal casing point of 8,103 ft. The Casing Drilled well required 10 days to drill to 7-in. casing point and cement the casing as compared to the first offset which required 19 days