Lubricator system uses liquid seal for frictionless intervention.

A new technology is meeting the challenges of high-profile and difficult well interventions performed by slickline.
The Schlumberger Liquid-Seal Slickline Lubricator system is designed to contain well fluids and reservoir pressures during slickline operations using a liquid, rather than mechanical, seal around the work line. Its use allows environmentally secure downhole operations by maintaining a virtually frictionless liquid sealing mechanism in the control head at surface. The design eliminates mechanical packing and the problems associated with induced friction and temperature from such packing styles.
The system's design originally was conceived to allow slickline operations in high-pressure gas wells containing H2S. However, oil and gas development and production has grown increasingly challenging over time, with deep, high-pressure field conditions becoming more the rule than the exception.
The system's liquid injection control head enables improved slickline interventions in challenging applications. This is mainly due to significantly less friction induced at the control head, thereby making slickline manipulations more efficient and predictable. Additionally, slickline interventions can be conducted under critical environmental conditions, such as offshore fields and low-ambient temperature areas. The system consists of four main components (Figure 1).
Pressure control head assembly. This unit's design includes components for reducing pressure and separating gas from liquid. Segmented pressure-reduction modules are designed to reduce pressures from injection pressure at the entry port to atmospheric pressure at the final exit. The gas separation mechanism in the control head isolates gas bubbles from the line so that only seal fluid fills the void between the slickline outer diameter (OD) and the seal bores.
Slickline blowout preventer (BOP). The BOP is a hydraulically actuated, dual-ram, solid block 15,000-psi working pressure unit having equalizing valves and a 3-in. working bore. It is tested to 22,500 psi, with the pressure being applied to each ram as well as the entire body.
Lubricator sections and head. The lubricator sections are pressure-retaining tubes constructed and designed for a safe test pressure of 1.5 times working pressure. Therefore, 15,000-psi lubricator sections are tested to 22,500 psi. The lubricator head assembly is a hydraulically operated packing gland unit that contains a heavy-duty aluminum sheave with a sturdy design that provides robust bearings and support structure. It also serves as a stand-by BOP.
Control console and injection test skid. The lubricator system's filling, testing and operating activities are routed through the control console and injection test skid, which contains gauges, valve controls, a diesel power plant and hydraulic pumps and motors. The control console skid centralizes all system power, controls and pressure gauges in one location. All pressures can be monitored at the console. Holding tanks separately stock the seal- and pressure-testing fluids.
Control head design
In addition to pressure control, the control head assembly eliminates friction. This reduces the required toolstring length, which in turn reduces its weight. This allows operations to be conducted lower in the line's elasticity curve, thereby prolonging line life. Extended line life allows nonstop operations, which translates into reduced downtime for line replacement. Also, shorter toolstring length permits slickline operations in restricted height situations.
Additionally, the absence of friction means a minimal string weight can be calculated accurately for specific conditions prior to the job, which benefits deep well operations, in particular. Decision-makers are provided with accurate information on downhole operating parameters.
Initially, a pressure-control liquid, and inhibitor as needed, is injected into the head. Above the injection point, the liquid seal is created between the wire OD and multistage seal bore. Below the injection point, a controlled amount of the injected fluid is allowed to enter the well tubing during operations. The pressurized fluid surrounds the slickline, providing an effective yet frictionless seal.
The elastomers and mechanical packings used in conventional stuffing boxes can be problematic. Packing drag and heat can cause leaks. The increased friction generated by this type of packing requires more weight on the toolstring, which generates heat that can weaken the wire and jeopardize the operation's success. Furthermore, seal frictions are difficult, if not impossible, to estimate and predict during slickline operations.
By removing well pressure-actuated stuffing box packing elastomers, weight, strain and loads can be determined because they are not influenced by friction changes. Pressure containment with sealing fluids, rather than elastomers, allows slickline operations to proceed for the job's duration without interruptions for maintenance.
The liquid-seal system can be applied under a range of pressures from 1 psi to 20,000 psi. Pumping the seal fluid mixture into the control head at a pressure greater than well pressure contains the well fluids and coats the slickline as it enters or exits the well tubing. To neutralize H2S, CO2 and other corrosive or toxic gases and fluids, the seal fluid can be premixed with the appropriate corrosion inhibitor. While coming out of the hole, contaminated fluids are removed, and the wire is coated with fresh inhibitor before it is spooled onto the reel.
The surface system and components are manufactured using materials and processes specified for H2S service. Also, the system includes a dual-ram (hydraulically or manually actuated) BOP specifically for H2S applications.
A built-in ball check closes immediately in the event that the slickline breaks and comes out of the lubricator. When closed, the ball check isolates well pressure from all lines attached to the control head assembly.
Pressure control head details
The pressure control head has two distinct sections: the pressure reduction section and the degasser section. The pressure reduction section is a series of slickline seal bores; the degasser section is essentially a gas control mechanism. The design combination permits minimum control head length.
After the seal liquid is injected, it flows in two directions from the injection point, up through the pressure reduction section and down through the degasser section.
Pressure reduction section. Multiple precision-bored pressure-drop tubes make up this section, which consists of primary- and secondary-pressure reduction sections. The precision bore in each tube is about 2 in. long. Each tube's exit connects to a module having an enlarged bore. The increased flow area within the enlarged bore significantly reduces the velocity of the injected fluid. The velocity reduction that occurs in each module has the most pronounced effect on the pressure drop. Another significant pressure drop results from viscous flow through the small clearance formed by the wire OD and the seal bore.
Pressure drop through an orifice is a function of its area and the flow rate. All fluid that exits one pressure drop tube enters and flows through the next tube. Since the same amount of fluid flows through each tube, the flow rates through the tubes are equal. Controlled seal bore dimensions and minimal wire OD variations mean each tube's flow area is essentially the same, and thus the pressure drop across each tube is almost equal. Therefore, a liquid injection pressure of 9,000 psi causes a pressure drop of 1,500 psi across each tube (9,000 psi divided by six pressure-drop tubes), the last tube's exit being at atmospheric pressure.
Degasser section. Below the injection port is the degasser section, constructed to only slightly restrict the liquid's flow. Its design ensures that some liquid flows into the well at all times. A liquid chamber within the degasser continuously floods the wire's final entry into the control head. This flooded condition allows gas bubbles, which may enter at the lower point, to rise up to the liquid's surface and away from the final wire entry. Gas is not permitted to accumulate near the final wire entry. Therefore, very little, if any, gas is allowed to enter it. For additional protection, several up-angled holes in the final wire entry serve to scavenge any gas bubbles that may enter inadvertently.

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