Drill-in solid liners have proven valuable for reaching target depths beyond trouble zones. When depleted zones, lost circulation zones or unstable formations are encountered, drilling

Figure 1. Baker Oil Tools’ drill-in sand screen liner system uses an inflow control-type screen design, coupled with a means to temporarily shut off the inflow passage, to eliminate the need for an inner isolation string. (Images courtesy of Baker Oil Tools)
with solid liners provides a means to complete wells that otherwise might not have been possible. Baker Oil Tools has combined sand control and liner drilling technologies to develop a drill-in sand screen liner system that can traverse collapsed hole sections without an inner circulation string, reduce rig time, and shorten the time to first production. The new system relies on a time- and temperature-sensitive protective film applied to the screen, an inflow control-type screen and a hydro-mechanical valving system to isolate the flow path through the screens when running in hole.

Drill-in screen system
Several challenges prevent operators from using sand screens when drilling. Screens are filtration elements not designed to contain pressure. Additionally, filtration media may plug when flowing cuttings up the annulus. Modern premium screens flow back mud during initial production, but formation fines created during drilling may cause plugging.

The predominant problem when running screen liners is borehole collapse in sensitive shale sections in the time interval between drilling the hole and running the screens. Failing to run the screen liner to total depth (TD) may have serious consequences, especially when zone isolation packers are spaced out in the liner to correspond to geologic features in the well bore. When this problem is encountered, the screen liner can be rotated and reciprocated in an attempt to pass through the collapsed section. However, risks of mechanical damage and plugging screen filtration media result. Normal screen running procedures caution against rotating the liner in open hole for this very reason.

Overcoming these challenges dictated designing a drill-in screen system around a positive displacement motor (PDM) and bit assembly rather than rotary drilling. Since torque would not be applied through the drill pipe and screen liner (except for the generated reactive torque from the mud motor) system components such as liner hangers, running tools and liner-top packers were already available. The screen design was altered to contain pressure during drilling and to supply a flow path for fluid to power the motor without the need for an inner workstring. Similarly, a temporary screen film was developed to protect filtration media from formation fines when running in the hole and drilling.

Eliminating the inner string
Selecting a PDM as the primary driver for the sand screen liner drilling assembly requires maintaining a closed flow path between the surface and motor when running the screen assembly in the hole. Traditionally, circulating fluid through a screen liner requires an inner workstring.

The inner workstring adds weight, which becomes a limiting factor in extended-reach wells. Additionally, rig time required to install and remove the inner string significantly increases cost, particularly offshore.

To overcome these issues, the company designed its drill-in sand screen liner system with an
Figure 2. The screen design uses a patent-pending, hydro-mechanical delayed-opening valve to temporarily prevent flow through the screen.
inflow control-type screen. After passing through the filtration media, fluid enters the screen through a single point of entry in the base pipe. This creates a more straightforward means of temporarily closing the flow path through the screen than does a traditional screen design with thousands of inflow holes in the base pipe. The inflow control-type screen design, coupled with a means to temporarily shut off the inflow passage, eliminates the need
for an inner isolation string and its resulting limitations (Figure 1).

Ensuring pressure integrity
To create a pressure-tight flow path to the drilling bottomhole assembly while allowing all screen joints to be simultaneously opened once the liner is in place, the company developed a hydro-mechanical delayed opening valve (patent pending) that temporarily prevents flow through the screens. The simplicity of hydraulic activation and mechanical reliability prevent premature or partial opening (Figure 2). 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 in each valve body add redundancy.

During run-in and drilling, the O-ring seal between the piston and valve body blocks the flow path through the screen. A shear pin prevents the circulating pressure inside the screen from shifting the piston. The pin is sized such that pressure pulses from drilling or any subsequent hydraulic operations can be completed without causing it to shear. Two ball bearings installed between the piston and valve body resist the force created by the compression spring used to open the valve, or any annular pressure that may act on the screen. Once the screen is in place and all required hydraulic events are completed, pressure is applied within the liner to shear the pin and shift the piston. The two ball bearings are pushed out of the counter bore in the valve body and attracted by magnets located on either side of the piston. The valves in every screen joint are then activated and the liner can still contain pressure. With the ball bearings no longer in place, the compression spring can eject the piston from the valve body after applied pressure within the liner is released. The flow path through all screen joints is now opened, and the well is ready to be put on production.

Temporary film resists plugging
Drilling fines and relatively high circulation rates around screen outside diameters pose a risk to the woven filter mesh during drill-in. To reduce this risk, Baker developed a water-
soluble film (patent pending) that is applied to the screen during fabrication. A shroud protects the film from wear during conveyance, and the interference of the shroud and mesh ensure that no voids are created once the film is dissolved.

Film removal is dictated by availability of free water in, and temperature of, the well fluid. Presently the film can be manufactured with a minimum solubility threshold of 160ºF to 200ºF (71°C to 93°C). If the well fluid is at or above the appropriate temperature for the film to dissolve, dissolution begins immediately, and total removal occurs within 6 hours. If ambient conditions are significantly below the solubility threshold temperature, or if there is not sufficient water present in the wellbore, the film will remain in place. Because oil-based muds do not dissolve the film, drilling operations may be conducted with oil-based muds or oil/water inversion emulsions without removing the film. When drilling is complete, the oil-based mud can be displaced or inverted to water phase, and the film will begin to dissolve once the fluid has reached solubility temperature. If low wellbore temperature or water deficiency prevents the film from dissolving, a contingency pill will remove the film within 24 hours. Qualification testing of the delayed opening valve and temporary film has yielded positive results.

More openhole applications
In addition to its originally intended purpose, the new drill-in sand screen liner system can be used to set hydraulic/hydrostatic openhole packers in the screen liner. Typically, these packers are isolated independently with a cup-tool or seal assembly in an inner string and set as the inner string is pulled out of the screen liner. This method requires adequate depth control to locate the cup tool/seal assembly in the packer and rig time to go through the setting process for each packer. Using the new, delayed-opening-valve-equipped screen, all the packers can be set simultaneously with applied pressure in the screen liner. The triggering pressure of the valves is not an issue because the valves continue to hold pressure after being sheared. The applied pressure to set the packers can be greater than the triggering pressure, if required.

The technology also can be used to float long screen liners in place. When running in the hole, the screen liner essentially 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.