. Picture of the yard test of the new seal elements.

Production logging is difficult in complex well designs such as horizontal multilateral (ML) wells with jagged Level 2 exits. The ML exits can obstruct the access of logging tools due to damage around the milled windows. Once inside a low-flow, high-watercut reservoir, the challenge becomes measuring an oil inflow profile in stratified flow.

The access issue is worse in sub-hydrostatic wells completed with artificial lift. Horizontal gas-lifted wells are easily choked by the presence of coiled tubing (CT). Y-Tools can be deployed in electrical submersible pump (ESP) wells to provide bypass tubing for logging strings necessary to log while pumping, although the system has not been successful in low-rate wells due to re-circulation across the dynamic seal in the Y-Tool. This recirculation invalidates the log data and overheats the motor. As in gas-lift completions, the low-rate ESP completions also suffer rate reduction caused by the presence of CT.

Several methods were used to overcome these access and production logging challenges in North Oman. A new tool configuration that comprises a vertical array of mini-spinners and fluid-type sensors was used to detect a 2% oil holdup in a stratified flow. It has a surface-controlled collapsible spinner cage that makes the tool slick to maneuver around obstructions in the well. This has overcome some of the Level 2 bypass problems in ML wells. A temporary ESP and Y-tool upper completion have been used to log a three-leg ML backbone successfully. To fix the recirculation problem, a modified Y-tool plug was designed to reduce fluid leakage in the dynamic seal. A variable speed drive (VSD) at the surface controlled the ESP motor speed, adjusting the pump rate to compensate for the rate reduction caused by the presence of the CT. Wireline-conveyed tractors have overcome some of the choking limitations of CT in gas-lifted wells.

Detecting thin oil
Much has been published about stratified flow in multiphase horizontal wells and the need for a direct measurement of each phase holdup and phase velocity. Without direct phase measurements, it is complicated to model the complex interactions between the borehole deviation,
fluid densities, viscosities, and shear friction that produce the slip velocity and holdup between the phases. One of the latest PLT tools that attempt to address stratified flow is the FloScan Imager. In this tool, four mini-spinners and five electrical and optical probes are located on a retractable arm that extends across the well bore from an eccentered tool body, and one mini-spinner and one electrical and optical probe are located directly on the tool body. The uppermost sensors of this tool are closer to the top of the hole than the previous generation, with the tool arm kept vertical by the weight of the tool body. This feature can make a significant difference to the quality of PLT data obtained in a high-water-cut horizontal well.

Logging through backbones

A useful application of the new PLT retractable arm is negotiating a way past jagged Level 2 exits, which are milled windows in the cased backbone of multilaterals. Each exit was drilled with a 2? or 3? angled whipstock set sideways using a measurement-while-drilling tool face. The objective of logging was to measure the water cut of each of the three legs. To obtain this information, it was necessary to deploy the PLT down the 7-in. cased backbone past the exits at legs two and three. The retractable arm can be closed while running into the well. A slick tool lying on the bottom of the casing is more likely to remain in the cased backbone than a tool with a centralized spinner cage. This is particularly true if there is a groove in the bottom of the backbone caused by the drill pipe tripping in and out of the well or while drilling each lateral.

Y-Tool logging for low flow rates

The Y-tool is a mechanical device used in the upper completion to connect bypass tubing and a pump. This bypass tubing enables access to the reservoir past the pump for interventions without the need to pull out the completion, enabling logging under dynamic conditions. Normally there is a blank plug in this bypass tubing to avoid hydraulically short-circuiting the pump. During logging operations, when the pump and logging measurements are concurrently active, this solid plug is replaced with a hollow device that allows passage of a cable or coil. If the well is horizontal and needs CT conveyance, the plug has to be a CT plug. The external sealing elements of the CT plug are identical to the blank plug it replaces. An additional dynamic seal is seated inside the CT plug that seals the gap between it and the moving CT.

Traditional CT logging plugs rely on a narrow gap in a brass bushing to provide this hydraulic seal. Ovality and wear of coiled tubing as well as the limited length of seal due to the short plug creates a sizable leak path for recirculation of the pumped fluid. In wells flowing 53,000 to 70,600 cf/d (1,500 to 2,000 cu m/day), a leak of 21,200 to 28,000 cf/d (600 to 800 cu m/d) can be tolerated, providing enough flow for the production logs without overheating the ESP. However, in low-flow-rate wells, all the fluid would recirculate, overheating the ESP motor and invalidating the log.

A new high-integrity internal seal was designed and fabricated to minimize this leak. The design concept was to treat the CT as a very long piston. The sealing mechanism had to be flexible enough to accommodate the changing ovality, yet hard enough to withstand the continuous wear due to the thousands of meters of coil movement. The yard tests used a pair of 2-in. CT reels and strippers straddling a bypass tubing and CT plug. A differential pressure of 1,500 psi was applied to the CT plug, and the leakage rate of water was measured for 2,000 ft (610 m) of coil movement at 100 ft/min (30.5 m/min) in both directions.

VSD compensates for CT choke effect

When logging with CT, it is necessary to compensate for the effect of the coil restriction on the production rate. With ESP-lifted wells, it is possible to do this with a VSD. It is common to have permanent downhole monitors such as discharge and intake pressures and temperatures across the ESP. In this case, the VSD frequency can be regulated to compensate for the additional friction introduced by the coil by keeping the pump intake pressure at the same value before and during the logging job.
Wireline tractor eliminates CT choking CT can choke production during logging due to the relatively large proportion of completion tubing occupied by the coil. Unlike ESP
completions, the vertical head in gas-lift wells is not easily altered. A logging cable has a much smaller cross-sectional area, 0.23-in. diameter, instead of 1.5-in or 2-in coil. Using a tractor rather than CT to deploy the logging tools can solve this problem.

New approaches have overcome Several flow-regime and deployment challenges:
• 98% water holdup in a horizontal well;
• ML backbone past Level 2 exits;
• Y-tool with modified 2-in. coil plug for low flow rates;
• VSD with ESP pumps to compensate coil choking; and
• Tractors for gas lifted horizontals with small tubing.