Surge Reduction System Protects Formation, Helps Prevent Expensive Mud Loss

During the completion of oil and gas wells, casing is run in a well bore filled with fluids. The need for the fluid to easily enter casing that is being run in the hole (RIH) during the well completion process can sometimes be critical.

The act of running casing into a well creates a hydraulic, piston-like effect that causes well bore fluids to enter the casing from its lower end in a self-filling process called "auto-filling." By filling the casing automatically from the lower end, the need to manually fill the casing from the surface can be avoided. Operators like the process because it allows the reduction or elimination of costly rig time required to fill the casing from the surface.

However, in wells with tight annular clearances, problems sometimes occur as the casing is RIH because the well bore fluids cannot flow freely enough and fast enough to accommodate the string entering the well bore. Consequently, surge pressures on the formation are increased. If these pressures exceed the formation's fracture gradient, significant volumes of well bore fluids can be lost while the casing in being run. Also, if the formation is fractured, the risk of mud loss is increased and differential sticking can occur. In these situations, conventional floating and guiding equipment may not provide adequate fluid displacement for reducing surge pressures.

Prevention of mud losses also helps protect producing zones against porosity damage from the drilling fluid and prevent loss of circulation and the resultant failures of zonal isolation (i.e. voids in the cement sheath). Porosity damage can reduce hydrocarbon production and effectively reduce the world's oil and gas reserves. Where cement sheaths fail to provide 100% fill in the casing/well bore annulus, expensive remedial cement jobs (cement squeezes) may have to be performed to make the cement sheath complete.

The Early Years
Well bore fluids surge isn't a new phenomenon. Operators have been dealing with it for decades. In the early years, the most popular solution was to eliminate the check valve in the casing shoe and/or collar. Operators would run a guide shoe without a valve so the well bore fluids could flow freely into the casing. However, wells got deeper and string lengths increased.

Operators then began using latch down baffles and plugs. This didn't last very long because it wasn't very practical. A more reliable auto-fill method was required. The answer was a pressure-deactivated auto-fill. This device required that a ball be dropped or the cementing plug be used to deactivate the auto-fill. This solved the need for a reliable autofill while maintaining a backpressure mechanism. But, with the advent of longer liners and requirements to run thousands of feet of drill pipe in addition to higher rig rates, operators began demanding faster speeds for running casing into the well. This has led to the challenge of developing new surge reduction technology for completing deeper wells in deeper waters in shorter time frames.

Increased Auto-Fill Reliability
To answer the challenge, Halliburton has developed new surge reduction technology that provides greater reliability in the auto-filling process. It allows well bore fluids to enter the casing freely and exit the drillpipe, effectively reducing surge pressures and helping prevent expensive drilling fluid losses. And, the system is pressure-deactivated and designed to allow circulation prior to reaching total depth without affecting auto-filling. The SuperFill™ surge-reduction system consists of two main components:

Automatic fill equipment (collars or shoes)
A drillpipe diverter

Automatic Fill Equipment (AFE). The AFE assembly consists of an outer case or collar, with holes through which well bore fluids can freely pass, thereby reducing surge pressures. An internal sliding sleeve made from steel (or drillable materials) is held in position to ensure that ports in the collar remain open. After the casing string has been run into the hole, the sliding sleeve can be shifted into the closed position to cover the ports in the outer collar.

In addition to reducing mud losses, the AFE assembly provides casing fill while allowing the required circulation for RIH operations. The AFE is deactivated by pressure rather than flow, allowing operators to use full-bore, casing cementing plugs to shift internal components. Pressure deactivation also prevents the auto-fill device from prematurely closing during RIH operations.

Diverter. Under normal circumstances, conventional floating and guiding equipment fitted with special valves allows well bore fluids to enter the casing; however, on liners or subsea completions, the casing is run into the hole on the end of a drillpipe. Although the well bore fluid freely enters the casing through the AFE, the restricted ID of the drillpipe still increases the surge pressures on the formation. In these situations, the diverter can be installed below the drill pipe and just above the top of the running tool. Well bore fluids enter the casing through the collar and exit through the diverter.

This figure is of a drill pipe diverter tool that is run above the running tool near the bottom of the work string and provides a flow path into the annular space between the drill pipe and previous casing.

How the System Works
The surge reduction system is designed to provide a reliable means for well bore fluid to enter the casing or liner ID (auto-fill) while RIH. Enabling fluid to freely enter the casing with small casing/well bore clearances reduces the effective surge pressure on the formation. This reduction in surge pressure on the formation can reduce the volume of fluid lost into the formation while running casing.

The system is typically used in well bores that encounter very low-pressure formations or are run through tight clearance well bores. The AFE can be made up in the casing string as the casing is run in the hole. It contains fluid ports that divert the displaced fluid to the casing ID. This arrangement allows the casing to automatically fill as casing is run in the hole. Allowing the casing to automatically fill reduces the piston effect of the casing on the well bore. Removing these pressure surges helps retain expensive drilling mud that would otherwise be lost to low-pressure formations.

After the casing containing the surge reduction equipment reaches the required setting depth, a bottom plug is pumped to the AFE. After the plug lands, pressure is applied to the casing, shifting an internal sleeve and closing the fluid entry ports. Increased casing pressure ruptures the plug diaphragm, allowing drilling fluids to be pumped through the AFE and out the bottom of the casing. Drilling fluids are then pumped through the casing string to condition the hole for primary cementing operations. In addition to the plug- or ball-operated AFE mentioned above, other AFE has been designed that incorporate the activating ball retained inside to allow deactivation of the AFE upon first circulation.

This figure is of a AFE designed as a collar design that incorporates a ball in place that deactivates the auto-fill feature with first circulation. A shoe that incorporates a tapered composite nose to help guide the casing past ledges or obstructions in the well is also available.

Case Study
A major Gulf of Mexico exploration and production company planned to run an 11 ¾-in. liner into a hole located in a zone with typical mud losses of 1,000 bbl per job. The operator needed preventive float equipment that would be useful in a subsea setting, yet compatible with a hydraulic liner hanger. An AFE assembly was selected for reducing formation surge pressures. A sliding sleeve was used to cover the differential fill ports on the collar's body, and the bottom cementing plug was used to close the sleeve after the pipe had reached its total depth. A specially designed equalizer sub and a non-rotating subsurface release plug set were attached to a slick stinger on the liner-running tool. Then, the entire assembly was run into the hole during cementing. As a result of using the Halliburton surge-reduction system to help avoid mud losses, the operator (1) saved $60,000 in additional drilling fluid losses, and (2) protected his well bore from significant surge pressures while running casing.

Summary
Tight well bores with small annular clearances, as well as operations in which casing is run on the end of drill pipe, place significant surge pressures on the surrounding formation. If these pressures exceed the formation's fracture gradient, significant amounts of well bore fluids can be lost while the casing is being run into the hole.

In these situations, conventional floating and guiding equipment may not provide adequate fluid displacement for reduction of surge pressures. However, specialized auto-fill equipment that allows well bore fluids to freely enter the casing and exit the drill pipe can effectively reduce surge pressures, prevent expensive mud losses and reduce formation damage.