Rainer van den Bosch
Mobil Erdgas-Erdol GmbH
Celle, Germany
Coiled tubing offered neither economic nor operational advantages over drill pipe for conveying logging tools in open hole shallow horizontal wells in Germany.
In the past 2 years, Mobil Erdgas Erdol GmbH (MEEG) participated in completing eight shallow horizontal wells. These were medium to short radius wells at measured depths of between 850 and 2,000 m (2,789 6,662 ft). The average horizontal section was 350 m (1,148 ft). The logging tools were conveyed by coiled tubing or drill pipe.
MEEG attempted to log five wells with coiled tubing-conveyed tools, four with 1 in. tubing. Total depth was reached reliably in only one well, the shallowest and with the shortest horizontal section.
Simulation programs were unreliable for calculating the downhole forces of the coil/tool combination or predicting possible helical lockups.
In wells with drill pipe-conveyed logs, the tool combination could always be pushed to total depth, an the operations were generally faster and cost less than logging with coiled tubing. Also, drill pipe allowed longer and heavier tool strings.
For reliable operations, coiled tubing needs to be more rigid, rig up/rig down times need to be improved, and the simulation programs must be more reliable for predicting downhole lockup.
LOGGING HORIZONTAL WELLS
To log highly deviated and horizontal sections, the options available are:
- Drill pipe conveyed (DPC)
- Coiled tubing conveyed (CTC)
- Logging while drilling (LWD).
LWD tools currently are available only for a limited range of hole sizes and measurements. Mobil's wells were drilled with 5 7/8 or 6-in. bits, and the logging program required imaging tools that have a very high data transmission rate. These conditions eliminated LWD tools.
The choices were, therefore, limited to either drill pipe or coded tubing conveyed logs. References 1 3 describe logging with coiled tubing. MEEG chose coiled tubing primarily for safety reasons, particularly while drilling in a gas storage reservoir.
Blowout preventers are an integral part of coiled tubing equipment; therefore, positive well control can always be maintained. With drill pipe operations, on the other hand, a potential safety hazard exists. Once the side entry sub is below the wellhead, a leak could develop between the drill pipe and logging cable whenever the Hydril BOP needs to be closed.
Furthermore, MEEG wanted to gain firsthand experience with coil tubing, a relatively new technique for open hole logging.
WELLS STUDIED
Figs. 1a 1h show profiles of the eight wells studied. Reitbrook wells 302 to 306 have a short radius, and the horizontal section is in chalk. Wells A, B, and C are medium radius oil wells drilled in sandstone reservoirs (Table 1).
Reitbrook, a depleted oil field, is currently used for gas storage. Deilmann Erdol Erdgas GmbH (DEE) operates the field in which MEEG has 50% interest.
Recently, five wells were drilled by MEEG as part of a gas storage expansion. These wells provide a reliable data base because all are in the same formation, a chalk rock, and have the same logging suite.
Each well had two runs scheduled. The first included a formation micro scanner (FMS), gamma ray (GR), and auxiliary measurement sonde (AMS). The AMS is a compression/tension tool. The combined FMS GRAMS was 18.5 m (61 ft) long, and weighed 550 kg (1,213 lb).
In the second run a dual induction tool (DIT) was included with the GR and AMS. The DIT GR AMS was 19.5 m (64 ft) long and weighed 450 kg (995 lb).
COILED TUBING RUNS
Reitbrook 303 (Fig. lb) was the first well logged with coiled tubing. Prior to logging, a dummy. tool was run on a coil with no cable.
The dummy, a steel tube whose length and diameter are identical with the longest rigid section in the actual logging string, contained nozzles to provide circulation in the event of differential sticking. With the dummy, no problem occurred in reaching total depth.
It should be emphasized that the dummy tool is both shorter and lighter than the complete logging assembly.
Considerable time was lost because of electrical connection problems in the crossover from coil to logging head. With the logging tool connected, helical lockup developed after 170 m in the horizontal section of Well 303.
Helical lockup occurs at a point where the pushing force at the surface can no longer overcome the friction along the well bore and the tubing buckles into a helix. References 4 and 5 describe the theory in detail.
A wiper trip after the first unsuccessful run did not improve the situation. Two partial logs were obtained.
Reitbrook 302 (Fig. la) is the shallowest well and has the shortest horizontal section (128 m). A dummy tool and a coil without cable were run prior to the logging run. No operational problems occurred and total depth was reached with both the dummy tool and logging tools.
To reduce rig time, no dummy was run is Reitbrook 304 (Fig. 1c). This proved to be the most unsuccessful operation for coiled tubing. Only 60 m of open hole could be penetrated before helical lockup occurred.
The first operation with a 1 3/4 in. coil was in Reitbrook 306 (Fig. 1e). Both MEEG and the coil operator did detailed simulations before the logging job. The simulations suggested a possible lockup near total depth with 1 in. coil but no lockup with 1 3/4-in. coil.
A recent evaluation with different simulation programs yielded similar results.6 The problem of unreliable simulations seems to be related to the input parameters rather than with the programs.
The larger 1 3/4 in. coil and injector head required a longer rig up/rig down time and special equipment to transport the coil on land. No dummy, was run.
Although the well profile of Reitbrook 306 shows a favorable horizontal section (deviation 870), 290 m was the farthest either tool combination penetrated before helical lockup. A wiper trip did not improve the situation.
Figs. 2a and 2b show the forces experienced by the coiled tubing while running in the hole. At the helical lockup depth of 1,068 m, no obstruction is stopping the logging tool because the compression force is not increasing. Rather, the friction along the well bore prevents the coil from pushing the tool farther.
In Well A (Fig. 1f), the logging program included a run with a DIT GR AMS tool (19.50 m long and weighing 450 kg ) and a second run with an EDAC LDT CNT-GR tool (18.70 m long and weighing 500 kg). EDAC-LDT CNT is a combination of an eccentralized dual axis caliper, a lithology density tool, and a compensated neutron tool. No dummy was run.
With the 1 in. coil, both runs failed to reach total depth. In the horizontal section, Run 1 locked up after 385 m and Run 2 after 162 m.
To obtain a minimum amount of data, a third descent (EDAC GR) was attempted. This 13.3 m long and 140 kg toolstring locked up after 270 m in the horizontal section.
DRILL PIPE RUNS
The first horizontal well to be logged in Germany with drill pipe conveyed tools was Well C (Fig. 1h). The tools consisted of a DIT-LDT GR AMS, 22.7 m long and weighing 550 kg. No problems occurred in reaching total depth; however, the logging required the longest relative logging time for drill pipe runs of 7.3 min/m (2.2 min/ft). This was caused by the slow tripping time to reach the short, 161-m horizontal section.
After the aborted coiled tubing operation, Reitbrook 304 was successfully logged with drill pipe without an additional wiper trip. No abnormal pushing/dragging forces were observed during the operation.
After the partially successful coiled tubing operation, Reitbrook 306 was successfully logged with drill pipe. As with Well 304, no abnormal pushing/dragging forces were observed during the operation. But one additional trip was required because of logging tool failure.
Reitbrook 305 (Fig. 1d) was logged only with drill pipe conveyed tools. An additional trip was necessary because of an incorrect crossover, in the drillstring, that prevented the downhole wet connect to pass.
Well B (Fig. 1g) was logged with an FMS AIT-LDT GR tool, 39.9 m long and weighing 880 kg. The AIT is an array induction imager tool.
Because of the long, 715 m horizontal interval, the simulator indicated problems in reaching total depth with coiled tubing. Also, coiled tubing would require two logging runs. With drill pipe, the interval could be logged in only one trip.
In Well B, the tool did fail while logging upward but no additional run was made because the data acquired while logging downward were sufficient.
PROGRAM EVALUATION
The bars in Fig. 3a denote the percentage of the horizontal section that could be logged in each well. A value of 100% indicates that the toolstring was pushed to total depth and data were recorded either while logging downward or upward over the entire horizontal section.
The 100% success rate in all wells with drill pipe conveyed logs is evident, while the average success rate of coiled tubing conveyed logs is only about 62%.
Normalized cost (Fig. 3b) is the sum of the cost of the rig, coiled tubing operator (where applicable), and logging operator divided by the length of the obtained log. Because the same logging program was run in each well, direct cost comparison is only possible for the Reitbrook wells. Wells A, B, and C had different logging programs; however, these wells are included for future reference.
The cost of the coiled tubing operation in the Reitbrook 304 well is a theoretical value. No actual charges were made because the job was unsuccessful and was completed with drill pipe.
In the Reitbrook wells, the average logging cost with coiled tubing, excluding Reitbrook 304, is $304/m. With drill pipe, the cost is only about one third and averages $105/m. This difference is a direct result of the higher charges for coiled tubing operations and the increased time needed for each job.
Total time was counted from rig up to rig down. This includes the time to log the horizontal interval once or twice as well as the additional time caused by tool failure. In the case of coiled tubing operations, the time includes wiper trips for conditioning the well. For drill pipe conveyed logging, the time includes trips caused by mechanical or electrical problems or by unsuccessful cable latching.
For comparison, total time is normalized to min/m of log (Fig. 3c). The average 3.6 min/m needed for drill pipe-conveyed logs is less than half of the average coiled tubing logging time of 7.8 min/m.
Coiled tubing rig up and rig down time was about 4 hr, time that can be effectively used during drill pipe operations to run in the hole. Also, the dummy runs to verify that the tool could be pushed through the build up radius and wiper trips account for the higher overall coiled tubing operating time. This time cannot be offset by the faster tripping, and the considerable rig time reductions reported in literature could not be confirmed.17
DATA QUALITY
Because tool movement during coiled tubing conveyed logging is comparable with normal wire line logging, data quality has been described as superior to that obtained from drill pipe conveyed logging.8
Data quality, was compared in the Reitbrook 306 well. In this well, a 280 m interval was logged both with coiled tubing and drill pipe. A formation micro scanner (FMS) was run in both wells for fracture identification. The FMS log provides a good quality measure because the data are sampled even, 0.1 in., and the interpretation is dependent on a smooth and consistent tool movement.
After processing the FMS data, no problems were encountered in interpreting the data over sections where the drill pipe was pulled steadily. However, FMS data were detrimentally affected in the limited intervals where the tool was stopped to remove a drill pipe stand.
This minimal data loss is acceptable because all of the drill pipe conveyed logs reached total depth. Success of coiled tubing conveyed logs was about 40% lower.
EVALUATION RESULTS
The evaluation of directly comparable data shows that for open hole logging in shallow horizontal wells, drill pipe conveyed logging is more successful, twice as fast, and only one third as expensive as coiled tubing-conveyed logging. Coiled tubing conveyed logging suffers from the following inherent problems that must be solved before it can be used routinely:
The 1 in. and 1 3/4 in. coils are not rigid enough to push an open hole toolstring combination over an extended horizontal section. From MEEG's experience, the maximum horizontal length that could be logged reliably is about 290 m with a 1 3/4 in. diameter coil and a 18.5 m long and 550 kg toolstring. However, logging intervals of over 500 m have been reported with smaller and lighter toolstrings.8
- A successful test run with dummy tools is no guarantee that a coil will successfully push a logging tool to total depth.
- The casing configuration must be considered as an additional constraint, A typical tapered well of 9 5/8 in. casing, 7 in. liner, and 5 7/8-in. open hole does little to mechanically support or guide the coil.
- The time to rig up/rig-down the coil and injector head needs to be reduced because it cannot be offset by faster tripping time.
- The currently available simulation programs are inadequate. It has been observed that the friction coefficients input for a logging tool in a horizontal open hole of 0.3 to 0.4 (for steel on rock) do not reflect the actual parameters and hence the result is not a reliable indication of whether or not a coiled tubing operation will be successful.
A note of caution must be made. The experiences described here were obtained with open hole logging and the conclusions are valid for this type of operation only. The conclusions should not be generalized for other coiled tubing operations.
Service companies are aware of MEEG's experiences and are addressing the problems. MEEG feels that in the future, with more information and improved technology," open hole logging with coiled tubing will be an attractive alterative to drill pipe conveyed logging.
REFERENCES
- Fertl, W.H., and Nice, S.B., "Well Logging and Completion Technology, for Horizontal Wellbores," Transactions Gulf Coast Association of Geological Societies, Volume XL, 1990, pp. 217 26.
- Nice, S.B., and Fertl, W.H., "New Logging, Completion Techniques Boost Horizontal Well Productivity" Petroleum Engineer International, November 1990.
- Blount, C., and Walker, E.J., "Coiled Tubing .... operations and services, Part 6 Tubing assisted logging and perforating, World Oil, May 1992, pp. 90 96.
- van Adrichem. (W., and Newman, K.R., "Validation of Coiled Tubing Penetration Predictions in Horizontal Wells," Paper No. SPE 24765, 67th SPE Annual Technical Conference and Exhibition, Washington D. C., October 1992.
- He, X., and Kyllingstad, A., "Helical Buckling and Lockup Conditions for Coiled Tubing in Curved Wells," Paper No. SPE 25370, SPE Asia Pacific Oil & Gas Conference & Exhibition, Singapore, February 1993.
- Kostrzewa, M. "Scope and Limitations of Applying CT Technology, in Deep Horizontal Wells," Master's Thesis, ITE University Claustal, Claustal Zellerfeld, Germany, November 1993.
- Fertl, W.H., and Hotz, R.F., "Efficiently log and perforate 600 + wells with coiled tubing," World Oil, July 1987.
- Campell, A., and Fraser, I., "Innovative Coiled Tubing Techniques in Horizontal Wells," 22nd Petroleum Itinerary Congress and Exhibition, Tihany, Hungary, Oct. 6 9, 1993.
- Kilgore, M.D., "New Muscle for Coiled Tubing," Paper No. OTC 7034, 24th Annual OTC, Houston, May 1992.
