Martin P. Coronado, Brad R. Pickle, Edward J. O’Malley, Paresh Patel - Baker Hughes
Drilling solid liners in place has had success in recent years, whether as a reaming application where a pre-drilled hole is present, or by drilling the borehole without a pre-drill. Drill-in solid liners are valuable in difficult drilling conditions such as weak matrix strengths, loss circulation zones, and tight pore pressure/fracture gradient windows. However, in sensitive shale sections, running screen liners risks borehole collapse. Failure to run the screen liner to total depth may have serious consequences, especially when zone isolation packers are spaced out in the liner to correspond to geologic features in the wellbore. In this scenario, mechanical damage and plugging of the screen filtration media can occur if the screen liner is rotated and reciprocated to pass through the collapsed section. Baker Hughes has developed a solution to this problem, with a simplified screen drill-in system that can open collapsed sections of the borehole in a pre-drilled hole.
Technical discussion
Limitations in the ability to rotate the screen, coupled with reduced torsional capability of the screen basepipe due to inflow holes, render a rotary drill-in method with screen liners impractical. The new system uses a mud motor and bit assembly in the liner assembly.
Current technologies can be used to develop a screen drill-in system. Since torque will not be applied through the drillpipe and screen liner, most of the system components are available already. However, current screen systems deficiencies are not compatible with a screen drill-in application. Since screens are not designed to contain pressure, the design must address pressure during drilling to supply a flow path for fluid to power the motor, or an inner string during run-in to isolate the screens, as well as formation fines created during drilling that may cause plugging.
Two technology developments address these deficiencies. A hydro-mechanical valving system has been incorporated into the screen design to isolate the flow path through the screens when running in the hole. This system eliminates the need for an inner isolation string inside the screen liner to provide a fluid conduit to the mud motor, reducing the liner assembly weight and eliminating the associated rig time required to run the inner string. A temporary screen film also has been developed to protect the filtration media in the screen from formation fines when running in the hole and drilling. This polymeric layer is built into the screen assembly during manufacturing and is soluble in water-based fluids once a predetermined minimum bottomhole temperature is reached, after the screen is in place, thus not impairing production flow through the screen.
This technology can also be adapted to other applications, such as setting hydraulic/hydrostatic zonal isolation packers remotely without an inner string, as a temporary well barrier, and to reduce drag forces for floating long liners into place in long horizontal wells.
Installation strategies
The primary choice for driving the drilling/reaming assembly is a positive displacement motor (PDM). A closed flow path must be maintained between the surface and motor when running the screen assembly in the hole. Traditional circulation methods incorporate a removable inner workstring run inside the screen and sealed off between the liner running tool and the end of the liner. An inner workstring presents two drawbacks: In long, extended-reach horizontal wells, the added weight limits how much pipe can be pushed into the hole with the available workstring weight; and the rig time required to install and remove the pipe.
One option for using a removable inner string for the drilling assembly is to run a disposable motor and reaming bit and leave them in the hole with the screen liner. This requires a stab-in seal system between the lower end of the inner string and the liner above the motor so the inner string can be recovered. This option requires a one-time motor investment.
An alternate allows recovery of the motor with a hydraulic or mechanical bit release tool that uncouples the inner sting and motor from the bit once the liner is on bottom. This system is more complicated mechanically.
One further alternative uses a permanent isolation string built into the screen itself. The inner string can be opened or closed selectively via wireline or coiled tubing, or hydraulically via a sliding sleeve run in the inner string. With all the sleeves in the closed position while running in hole, the screen liner provides a closed fluid path to the motor. Once the screen liner is in place, the sleeves can be manipulated open for production. This configuration also provides for selective production when coupled with zonal isolation packers set in the open hole-liner annulus. The drawback is the reduced bore through the completion due to the permanent inner string.
A new system that eliminates the need for an inner string to power the PDM temporarily plugs the flow path through the screen when running in the hole, which allows the screen liner itself to be the conduit for fluid flow to power the PDM. This design uses an inflow control-type screen, which has a single point of fluid entry into the screen basepipe after passing through the filtration media, which provides a more straightforward means to temporarily close the flow path through the screen versus a traditional screen with thousands of inflow holes in the basepipe. This design also provides a means to easily set hydraulic open hole packers, temporarily isolate the open hole section, and float long liners into place.
Operational details
The screen assembly was designed to not use the drill-in feature except as a contingency and to not activate the inflow control device (ICD) valves in the screen. A minimum setting depth was established to enable either a tieback assembly or upper completion to be installed with appropriate casing scraper cleanup. Once the hanger is set, the ICD valves can be opened or left closed, acting as a temporary barrier. The reamer bit and motor are left downhole in a single-shot application; use of a more aggressive bit or the need to drill any amount of virgin formation would require further investigation into the effects of vibration and hole cleaning hydraulics, and the motor chosen must conform to API specifications and have known bending limits and flow rate ability.
The drill-in assembly will not be rotated into the wellbore; the ability to ream should be only a contingency. The screens may be rotated in quarter-turn increments to get over ledges and should be an integral part of the open hole decision matrix.
Modeling shows that drag increases with depth; the amount of drag that can be experienced before helical buckling becomes of concern dictates how much further the assembly can be run before lock-up. In the modeling example, 25,000 lb of drag above predicted was acceptable and the assembly reached planned hanger setting depth. However, if modeling has shown that a buckling limit of only 15,000 lb above predicted could be tolerated, then the assembly would have had to be set higher up or pulled to surface if the minimum installation depth had not been reached.
Advanced torque and drag modeling results, both with and without an inner washpipe for pump-through ability, showed that, although the pick-up and slack-off weights follow the same trend, when washpipe is run the added drag is high enough to decide whether or not the assembly can get to bottom with the weight available from the landing string and block. In this case, if washpipe was run, the screens could not be installed since less than 5,000 lb of drag would have induced helical buckling in the landing string. The screens were installed without running washpipe, allowing over 15,000 lb of nominal drag without experiencing helical buckling. However, the sandface was completed without pump-through ability.
Using washpipe may limit lateral length of a sandface completion. Running the screen assembly without washpipe saves on associated rig time, but also minimizes the time the reservoir is left open. While this is important with unstable sections of reservoir or shales, this does not allow any pump-through ability for displacement or motor operation for a drill-in requirement when using normal screens. The use of valves in the screen allows pump-through and, therefore, motor activation.
Screen design for pressure integrity
A basic drill-in requirement is a method to temporarily prevent flow through the screens and to create a pressure-tight flow path to the drilling BHA while allowing all screen joints to be opened simultaneously once the liner is in place. A new hydro-mechanical delayed opening valve combines the simplicity of hydraulic activation and mechanical reliability to prevent premature or partial opening of the valves. Each screen joint is fitted with a single valve body between the inflow area of the screen and the inflow ports of the base pipe. Multiple, independently acting valves add redundancy.
During run-in and drilling, the O-ring seal between the piston and valve body blocks flow through the screen. A shear pin prevents the circulating pressure inside the screen from shifting the piston. Pressure pulses from drilling or any subsequent hydraulic operations can be completed without pin shear. Two ball bearings between the piston and valve body resist the force of the compression spring or any annular pressure that acts on the screen. Pressure then is applied within the liner to the predetermined activation pressure. The shear pin is sheared and the piston shifts. The ball bearings are pushed out of the counter bore in the valve body and attracted by magnets on either side of the piston. At this point, the valves in every screen joint are activated and the liner is able to contain pressure. With the ball bearings no longer in place, the compression spring is able to eject the piston from the valve body after applied pressure within the liner is released to open the flow path through all screen joints.
The locking and releasing mechanism developed for this valve requires the ball bearing in the pistons to remain aligned with the magnets. This is accomplished by incorporating a rectangular head on the pistons which prevent rotation of the piston once assembled due to the body and housing diameters.
Screen design for plugging resistance
To reduce the risk to the woven filter mesh of drilling fines and high circulation rates, a degradable film has been developed that can be included in screen construction. Degradable film is placed around the outermost filtration layer and captured via mechanical interference of a shroud. The shroud protects the film from wear during conveyance, and the interference of the shroud and mesh ensure that no voids are created between the shroud and mesh once the film is dissolved.
Recent development efforts have focused on a film that is removed by interaction with water. Cleanup of this film is dictated by availability of free water in the well fluid and the temperature of that well fluid. The film can be manufactured with a minimum solubility threshold in the range of 160º F to 200º F (71º C to 93º C). If the well fluid is at or above the temperature for the film to dissolve, dissolution begins immediately, and total removal of the film occurs within six hours. If ambient conditions are significantly below the solubility threshold temperature, or if there is insufficient water in the wellbore, the film will remain in place. Oil-based muds do not affect the water-soluble film, so drilling operations may be conducted with oil-based muds, or oil/water inversion emulsions without removing the film.
When drilling/reaming is complete, the oil-based mud can be displaced or inverted to water phase, and, once the fluid has reached the solubility temperature, removal of the film commences. A contingency fluid pill is available; to treat the screens, a minimum of two interval volumes of the contingency fluid are pumped to the screen interval, for at least four hours per interval volume.
Preliminary work is also under way on films that are degraded by any well fluid. To date, experiments with this new film have been conducted using LVT-200 at temperatures ranging from 180º F to 220º F (82º C to 104º C). Unlike the water-reactive film, the removal mechanism for oil-based mud films is thermal degradation, so the rate of removal correlates with downhole temperature. Contingency fluids for these films have yet to be investigated, but are likely to include acids.
Additional applications
This screen design can be used for other applications. With the valves providing pressure integrity, hydraulic operations can be performed as long as the applied pressures are within the burst capability of the screen assembly. If repeated applied pressure cycles are required, the pressure level must not exceed the triggering pressure of the valves, since this pressure will cause the valves to open when applied pressure is bled off.
Setting hydraulic/hydrostatic open hole packers in the screen liner is a primary application for this screen. Using the new, delayed-opening-valve-equipped screen, all the packers can be set simultaneously with applied pressure in the screen liner.
In this application, the triggering pressure of the valves is not an issue since the valves hold pressure after being sheared, allowing the applied pressure to set the packers to be greater than the triggering pressure, if required.
Floating long screen liners in place also is a possibility. When running in the hole, the screen liner acts as solid liner, so any lighter-weight fluid system can be displaced into the liner to gain buoyancy advantages to help reduce drag forces. The valve system is compatible with higher annular pressures.
An additional benefit of having the screen joints temporarily plugged is for flow control during the upper completion of the well. Traditionally, inflow or fluid loss control in sand control applications is accomplished with a barrier valve installed below the packer, which allows normal operation of the service tool string during installation of the screen liner, but closes when the service tools are removed; this barrier prevents fluid loss to the formation or premature inflow when installing the upper completion.
The new screen design may eliminate the need for the barrier valve since the screen liner does not permit flow either through the screens or in from the formation until the valves in the screens are opened. Valve opening can be delayed until after the upper completion is installed, providing the same flow control features as a barrier valve, although the valves, once opened, cannot be re-closed.
Conclusions
A screen design has been developed and tested that makes drilling/reaming a screen liner into the hole through collapsed sections of hole without an inner string a possibility. In addition, a dissolving screen mesh protective film has been developed for installing screens in high-solids environments, such as a drilling/reaming screen liner.
References
1. R. Billa, D. Watkins, M. Weissman, L. Clark, E. McClain, J. Sessions and K. Evans: “Reducing Well Cost Utilizing Liner-Drilling Operations in South Texas,” paper IADC/SPE 99110 presented at the 2006 IADC Drilling Conference, Miami, Florida, 21-23 February.
2. A. Durkin and M. Myhre: “Liner Drilling Feasibility Study,” Baker-Hughes study report, 17 March, 2006.
3. McElfresh, P. and Welch, J., “Protecting Sand Screens Against Solids Plugging During Installation,” SPE 112314 presented at the 2008 International Symposium and Exhibition on Formation Damage Control, Lafayette, Louisiana, February 13-15.
