Diesel eliminates sticking while drilling sandstone laterally
A series of development wells with lateral sections in Egypt have been drilled successfully with diesel fuel after differential sticking virtually halted operations. The wells were being drilled by Gulf of Suez Petroleum Company in the Sidki Field.
The sections were depleted Nubia sandstones with pore pressures of 5.5 ppg. The initial wells on the horizontal sections were drilled with invert oil based muds with densities as low as 7.7 ppg in order to minimize the differential between hydrostatic and pore pressures.
The effort failed and differential sticking became a constant problem. Gilsonite and calcium carbonate, added to improve the filter cake, were insufficient to solve the problem. Water based fluids were considered and discarded because of temperature and density considerations. The static bottom hole temperature was 295° F and salt added to water in order to counter high temperatures would raise the fluid density to undesirable levels.
The diesel fluid eliminated the filter cake completely, allowing deep fluid penetration of the sandstones surrounding the lateral borehole. The penetration raised the reservoir pressure and reduced the hydrostatic differential. Drilling fluid losses as high as 200 b/d occurred on the wells, but the loss was expected and the cost was relatively low ($18/bbl).
Fluid turbulence in the lateral sections provided sufficient cuttings transport but not so in the vertical sections. The problem of vertical transport was solved in two steps. Sweeps, using diesel/water/gel combinations to raise the fluid density were pumped downhole after every connection. Also, conventional back-reaming was undertaken.
Reference:Holt, C. A. "GUPCO Uses Unique Horizontal Drilling Fluid: Diesel," The Brief, Vol.1, Issue 8, No. 45, September, 1995.
- This newly developed hydraulic workover rig can conduct slimhole, multi-lateral, and under-balanced drilling well operations with a 22-component modular package that can be rigged up in 24 hours. The unit was fabricated for Transocean Petroleum Technology of Tananger, Norway by Stewart & Stevenson of Houston.
Taut leg mooring providing timely deepwater solution
The only questions remaining on the use of polyester ropes for taut mooring of semisubmersible production vessels in deepwater are durability, aging effects, shark bite phenomena, the possibility of heat buildup during high cycling periods, and suitable end terminations.
Deepstar, the multi-operator research consortium, installed a taut polyester line on the Mars Field in the US Gulf in July for test purposes. Petrobras will conduct a similar taut line test in the Bijupira-Salema Field in early 1996. Norsk Hydro is also examining the use of taut lines in deepwater off Norway and has elected to await the results of the Deepstar test. Both tests should resolve the remaining questions.
Unless the tests turn up negative results, it appears the taut system using polyester ropes will be the system of choice for deepwater mooring. The taut rope system has important benefits for mooring deepwater production vessels, and as an alternative for dynamic positioning during drilling operations. Preliminary results have indicated the following advantages of taut lines over conventional catenary mooring systems:
- 50% reduction in cost
- 40% reduction in the seabed footprint
- 30-40% reduction in line length
- 70% reduction in dry weight
- 40% reduction in vessel offset or excursion
- 50% reduction in mooring pre-tension
- 50% reduction in vertical mooring force on platform.
In addition, Petrobras has calculated that the use of taut lines in place of dynamic positioning on drilling units can save $10 million per unit. The Brazilian operator considers mooring a strategic issue in the development of the Marlin and Albacora Fields because of the "shortage of dynamically positioned drilling units, a congested seafloor, and the possibility of optimizing flexible lines layout."
Floating production systems are becoming increasingly popular because of the growing environmental requirements, residual liability, and costs surrounding removal of fixed structures, especially the larger units in deepwater. Taut line mooring further reduces the cost of floating production deployment.
But taut line mooring is not a trouble-free choice.
- Natural frequencies in the axial direction must be accounted for, unlike catenary mooring systems. Norsk Hydro's study of taut lines determined that axial vibrations in high levels of pre-tension should be avoided and sufficient pre-tension be applied to windward in order to prevent undesirable compressive loads on the leeward polyester terminations.
- Piles are required for taut line mooring, which have a higher vertical component in the tension regime than catenary mooring. Catenary mooring makes use of embedment anchors, to reduce costs. Norsk Hydro suggests a less expensive alternative to piles, since they are a costly component in taut mooring.
- Duplex weld subsea: Stolt Comex Seaway says it has completed the first duplex hyperbaric weld in subsea construction. Stolt's Thor Welding System was used to weld and 18-in. pipe at 170 meters depth off The Congo.
- Well departure: BP has drill a well at Wytch Farm that has a departure distance of 26,361 ft with a reach/true vertical depth ratio of 5.0. The bottomhole assembly included Halliburton's TRACS inclination control system.
- Offshore pumping: Two BJ Services stimulation vessels, the Vestfonn and Renaissance, conducted the largest volume fracing of 28% HCl in the North Sea and the first time for two such vessels to be engaged. The field was Phillips Joanne.
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