JET LEACHING TOOL ACCURATELY UNDERREAMS SALT SECTIONS

Feb. 6, 1995
A new downhole tool cuts through salt faster and more efficiently than conventional drill bits, according to Corrosion Management International Inc. (CMI) of Lafayette, La. The jet leaching tool has applications in drilling gas storage wells, brine production wells, and conventional oil and gas wells that pass through large salt domes where salt creep can be a problem.

A new downhole tool cuts through salt faster and more efficiently than conventional drill bits, according to Corrosion Management International Inc. (CMI) of Lafayette, La.

The jet leaching tool has applications in drilling gas storage wells, brine production wells, and conventional oil and gas wells that pass through large salt domes where salt creep can be a problem.

The conventional approach to drilling through salt for a gas storage well involves underreaming with standard tools after a pilot hole is drilled. Many trips and underreaming tools may be necessary to give the borehole the required diameter. Conventional bit drilling can result in large washouts and a lack of directional control. Because of the casing string design, many gas storage wells require strict deviation control with an almost perfectly straight hole.

LEACHING PROCESS

For jet leaching operations, a pilot hole is first drilled conventionally. The bit is then tripped out of the hole, and a caliper log is run to determine the actual hole size and washing depths. The jet Teacher is then configured with appropriate sized nozzles to wash (underream) the hole to the required diameter.

The tool has one jet pointing down and four jets angled up at 30 from vertical. The jets can be blanked off or fitted with any size conventional bit nozzle (Fig. 1) (10562 bytes). A back pressure valve in the tool prevents the heavier saturated salt water in the annulus from flowing back up through the drillstring when the pumps are stopped.

The jet Teacher is then installed on the drillstring and run to bottom. Typically, a large sump (rat hole) is first made at the bottom of the well. This sump, usually about 100 ft long, catches debris, junk, or any insoluble material from the salt interval above. The tool is then pulled up to the appropriate depth to begin washing. The hole is underreamed while the tool is pulled up the well slowly.

During the washing process, freshwater is pumped through the tool at a specific flow rate and pressure. The drillstring is rotated slowly at a constant speed for a vortex effect during washing.

The specific gravity of the water pumped down the drillstring is measured and compared to that of the salt-saturated water exiting the well. Careful monitoring of the change in specific gravity and correlating that information with depth helps maintain accurate control on the diameter of the well and the amount of time the tool must work a particular interval (Table 1) (6174 bytes). When the tool is pulled from the well, a wire line caliper log is run across the washed area. The caliper log verifies the amount of leaching, and these data are compared to computer simulation data. If the hole needs additional opening or smoothing of any interval, the jets are reconfigured, and the tool is run back in the well. If ledges are found in the well bore, horizontal jets can be installed in the tool to work the area.

BRINE WELL

CMI recently used this jet leaching process on a brine production well for PPG Industries near Starks, La. This well required accurate deviation control (< 3/4 deviation), which may not have been easy with conventional underreamers, according to CMI. PPG said Well No. 13 would have required about 24 days of underreaming. The leaching process washed the hole to the required diameter in less than 2 days, saving about $250,000 on the well. Actual washing time on bottom was 26 hr.

The tool first opened a 4060 in. sump from 6,300 to 6,400 ft (Fig. 2) (16401 bytes). The nozzle configuration was changed, and the tool then washed an interval from about 2,600 ft to 6,400 ft, opening the well from 14 1/4 in. to 24 in. The tool was kept below the casing shoe to avoid washing out around the 16-in. casing. Several tight areas had to be reworked to open them to the appropriate diameter. The well was then completed with concentric strings of 13-1/8-in. and 8-1/8-in. casing.

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