FLOATING JUNK BONNET PROTECTS LINER TOP FROM DEBRIS

George Telfer
Nodeco Ltd.
Aberdeen

Dave Sweeney
Enterprise Oil Plc.
Aberdeen

A new design of debris protection system, the floating junk bonnet for liner tops prevents welL debris from entering the liner top polished bore receptacle (PBR) and sticking the setting tools. The floating junk bonnet improves the recovery of liner setting tools from the well no matter how severe the debris problem is.

Well debris (formation cuttings, fines, mud solids, cement solids, foreign bodies, etc.) entering the liner top PBR during cementing operations has been a serious problem for many operators. In some cases, this problem has led to liner hanger setting tools sticking in the liner top PBR. The setting tool could then become cemented in place, resulting in an expensive fishing job or sidetrack.

A prototype debris protection system was developed and tested prior to offshore application.

The liner protection tool was subsequently successfully used on Nelson project development wells in the U.K. North Sea.

DEBRIS ACCUMULATION

Enterprise Oil Plc. had several problems with well debris entering the liner top PBR and sticking the liner hanger setting tool.

During drilling of the early Nelson development wells, severe problems were experienced from well debris entering the liner top PBR. As a result, difficulties occurred retrieving the liner hanger setting tool. Although in all cases the setting tools were successfully recovered, there had been a significant risk of cementing the liner running string into the liner top PBR.

In all the cases, the well debris entering the liner top PBR consisted of fine cavings of shale from the base of the Balder formation (Fig. 1).

The setting depth of the 9 5/8 in. casing strings on the Nelson development wells is governed by two factors:

The Eocene formation must be cased off prior to drilling of the Forties formation.
Casing must be set prior to drilling into a loss zone 85 ft (TVD) below the top of the Balder formation.

The Eocene formation consists of highly reactive and over pressured shales which become unstable if drilled with mud weights less than 14.0 ppg. The Forties formation is a normally pressured sandstone in which differential sticking may occur with mud weights greater than 10.0 ppg.

Therefore, to allow the Forties sandstone to be successfully drilled without inducing borehole collapse in the Eocene, the 9 5/8 in. casing string is set in the Balder formation. In addition, if the mud weight is greater than 14.0 ppg, the Balder also contains a known loss zone above which the 9 5/8 in. casing must be set.

These criteria dictate the setting depth of the 9 5/8 in. casing shoe.

After the 9 5/8 in. casing string is run and cemented, the 14.0 ppg mud used to drill the 12 1/4 in. hole section is displaced with 10.0 ppg mud prior to drilling of the 8 1/2 in. hole section. This mud weight, however, can induce borehole instability in the Balder because this formation should ideally be drilled with 12.0 ppg mud.

To date, drilling the Balder with 10.0 ppg mud has caused no fluids production but has led to the production of a small volume of fine shale cavings which have deposited on top of the liner top PBR. The cuttings transport efficiency drops significantly here because of the increase in annular volume (Fig. 2).

The typical displacement rate for the 7 in./5 in. tapered liner cement job was 10 bbl/min. This pump rate provided an annular velocity of 432 fpm in the 9 5/8 in. x 7-in. annulus. When fluids passed the liner top PBR and entered the 9 5/8 in. x 5 in. drill pipe annulus of the liner running string, however, the annular velocity dropped to 215 fpm. This drop in the annular velocity led to some cavings dropping onto the liner top and into the PBR and becoming compacted around the setting tool.

The 9 5/8 in. casing setting depth was adjusted; however, this change failed to prevent the production of shale cavings.

Various methods, such as filling the PBR with cross-linked fracture material proppant, were also tried to keep the cavings away from the setting tool. These attempts also were unsuccessful.

The only solution was to isolate the setting tool mechanically from the well bore fluids. This solution led to the design, testing, and introduction of the floating junk bonnet.

JUNK BASKET

The junk basket principle is a well known means of collecting loose items of junk from the bottom of a well (Fig. 3). Simply stated, the high annular velocity around the larger outside diameter (OD) part of the tool lifts the debris, and when the annular velocity is reduced at a smaller OD, the debris drops out of the fluid into the basket or catcher.

The annular velocity in the 9 5/8 in. x 7 in. annulus is 432 fpm at a 10 bbl/min circulating rate. On conventional liner systems, the maximum OD usually occurs at the PBR (for example, 8 1/4 in. OD inside 9 5/8 in, casing), and in this situation annular velocities can drop from 2,100 fpm to 215 fpm at a 10 bbl/min circulation rate.

This phenomenon has dictated the use of some form of junk bonnet at the liner top to prevent debris from entering the PBR and fouling the liner setting tool.

Fig. 4 illustrates a typical junk bonnet design, which consists of a metal cap used to bridge the gap between the inside diameter (ID) of the PBR and the OD of the setting tool extension.

These devices have been reasonably effective in normal circumstances, but in situations where the debris or clay fines accumulations are severe, they do not prevent entry into the PBR. The major limitation of existing designs is that they must not form a pressure seal between the ID of the PBR and the setting tool extension. Should this seal occur, the PBR could collapse because of the inability to allow hydrostatic pressure equalization, or the setting tool could become stuck because of "fluid locking."

Overpull during the release of setting tools from liner tops has been an industry problem for many years. In some cases, the liner is pulled back up the well before "dropping off," or the liner may be pulled to surface if the problem is serious enough.

Filling the PBR with a highly viscous fluid prior to being run in the well is often tried but is not particularly effective.

FLOATING JUNK BONNET

The floating junk bonnet (FJB) has been used successfully in the Nelson field and elsewhere (Fig. 5). This floating system is considered an improvement over other junk bonnets and has the following advantages:

The FJB forms a seal at the PBR top and thereby prevents entry of any debris into the liner top.
The FJB does not move when the drill pipe/setting tool is stroked up or down.
Backreaming blades (during rotation) allow easier removal of packed off cuttings and debris.
The emergency release sleeve allows the fluid lock to be broken, even with severe accumulations of cuttings and debris.
Circulation through the emergency release sleeve port can clear debris from the liner top.
The FJB has a smaller OD than conventional junk bonnets and is therefore less likely to be fouled when it is pulled away from the liner top.

DESIGN

Prior to running of the finer hanger, the FJB system is prepared on the rig floor with the inside of the PBR rifled with water through the water inlet (Fig. 5). When water is expelled through the air outlet, the PBR is full. The water inlet and air outlet are then plugged off, allowing the junk bonnet to form a seal between the setting tool extension and the inside of the PBR.

Because of fluid locking, the FJB cannot be pushed farther into or pulled out of the PBR. As the liner is run in the well, the gradual increase in hydrostatic pressure forces the junk bonnet downward, thereby pressuring up the water volume in the PBR until the pressures above and within the PBR are equalized. This downward movement of the FJB will be partially offset by the thermal expansion of the water trapped inside the PBR; the net effect is calculated to be 1 2 in. of downward movement.

Before the liner is cemented in place, the running tool is released by right hand rotation of the drill pipe and then picked up a few feet to check for proper release. This action is considered partly responsible for cuttings and debris entering the PBR when conventional junk bonnets are used.

The FJB will stay in place, however, because the diameter of the setting tool extension is the same as that of the stinger pulled through the internal seals of the retrievable seal mandrel (RSM) pack off which is locked in place. (The RSM pack off is a retrievable bushing used to effect a seal between the drill pipe running string and the liner. This type of sealing arrangement, or the commonly used slick stinger with drillable pack off, must be used to allow use of this junk bonnet.) In other words, the volume within the PBR remains the same. Thus, cuttings and debris are prevented from entering the PBR and interfering with the functioning of the setting tool.

The junk bonnet is pulled out of the PBR once the bypass slots pass under the internal O ring seals in the junk bonnet, thereby allowing the necessary fluid to enter the PBR (Fig. 6). Continued upward movement pulls the junk bonnet clear of the PBR.

If a severe amount of cuttings and debris has accumulated at the liner top, and the bypass slots plug, then a combination of pulling and drill pipe rotation should be applied to remove the debris. In this case, the rotation lugs engage corresponding slots in the junk bonnet, rotating the junk bonnet to assist in the removal of the debris. If the junk bonnet should remain stuck in the PBR, then continued upward pull of the drill pipe allows the emergency release sleeve to be sheared at a preset overpull. Fluid can then enter through the port to break the fluid lock (Fig. 6). Additionally, circulation through this port helps in the recovery of the setting tools from the well.

TEST PROCEDURE

To function test the FJB, a special test fixture was constructed (Fig. 7). The purpose of the test was to demonstrate that an FJB for a 7-in. liner top, which forms a seal between the 7 1/2 in. ID of the PBR and the 100 mm OD setting tool extension would perform the following:

Move downward as a simulated hydrostatic pressure of 5,000 psi is applied above the PBR
Allow normal function of the liner setting tool
Stay in place as the setting tool is released then moved upward and downward
Not be removable unless fluid is allowed to enter the PBR.

The following procedure was used to test the prototype FJB:

The junk bonnet was pressure tested internally to 750 psi, proving the integrity of the inner O rings which seal around the setting tool extension and the outer O-rings which seal on the inside of the PBR.
Area X in Fig. 7 was filled with cold water and pressure tested to 2,000 psi through Line 1 to prove the integrity of all the threaded connections on the liner setting tool assembly.
Area Z in Fig. 7 was filled with cold water, the fill line through the junk bonnet was plugged off, and the assembly was placed inside the test fixture.
Area Y in Fig. 7 was filled with cold water.
Pressure was applied through Lines 1 and 2 simultaneously. The indicator rod (tell tale pipe) connected to the junk bonnet moved inward approximately 4 in. before pressure started to build because of air in the system. Pressure was applied in 1,000 psi increments until 5,000 psi was reached; the indicator rod moved inward approximately 1 1/4 in.

This movement was close to the calculated 1 in. of movement required to pressurize the fluid volume in area Z to 5,000 psi. The gauge attached to the indicator rod read approximately 4,850 psi, with 5,000 psi applied above the junk bonnet. This 150 psi discrepancy was attributed to the frictional resistance of the O rings on the junk bonnet. (Both gauges were calibrated before the test.)

Pressure was bled off to zero, and the indicator rod returned 3 in. The net effect of these movements was that the junk bonnet had remained 2 1/4 in. lower than the initial position, which was assumed a result of air in the system remaining partly compressed.

This step was then repeated, with similar results.

The liner setting tool was backed off.
The liner setting tool was stroked 18 in. The indicator rod remained stationary because the OD of the setting tool extension has the same OD as the stinger through the RSM pack off.
The liner setting tool was pulled back until it contacted the junk bonnet (Fig. 8). A substantial pull was then applied, and the indicator rod did not move.
The pressure gauge was removed from the indicator rod to allow air into area Z.
The liner setting tool was pulled again. The indicator rod moved outward indicating the junk bonnet was being pulled clear of the PBR.
The running tool was removed from the test fixture and examined for damage. No damage was found.

TEST RESULTS

The conclusions drawn from the test observations can be summarized as follows:

The FJB will move downward at approximately 1/4 in. per 1,000 psi of simulated applied hydrostatic pressure.
The functioning of the liner setting tool is not affected by the presence of the FJB.
The FJB will not move while the tool is stroked to check for release.
Air or liquid must be allowed to enter area Z to break the fluid lock and allow removal of the FJB from the PBR (Figs. 7 and 8).

The test was considered successful. A production model based on the design shown in Fig. 5 was manufactured for downhole use.

FIELD RUNS

The FJB was first used by Enterprise Oil in September 1992 in the Nelson field. Since then, the tool has been used successfully on 75 occasions (Table 1).

When the FJB was used, no overpulls have occurred while checking for release of the setting tools. In addition, the junk bonnets have stayed in place, as verified by the preset overpulls at the anticipated pull back distance.

Analysis of the liner hanger running tools brought back to surface showed that the emergency release sleeve sheared on approximately 80% of the runs. Also, the bypass slots were often packed with cuttings, debris, or cement.

ACKNOWLEDGMENT

The authors would like to thank Graham Ross, Bob Lyons, Andrew Paterson, and Glenn Harris of Enterprise Oil Plc. for their cooperation and encouragement and also Mark Carmichael of SPS for his input.