STEERABLE BHAS DRILL STORAGE WELLS WITH DIFFICULT TRAJECTORIES

Hartmut Gomm, Lutz Peters
Kavernen Bau- und Betriebs-GmbH
Hannover, Germany

The use of steerable downhole motor assemblies allows greater variation in well bore trajectory for drilling gas and oil storage wells in salt domes in areas with surface site restrictions.

Modern directional drilling tools and techniques provide significantly greater penetration rates and can reduce total well costs, despite higher equipment rental costs, compared to conventional rotary drilling. In addition to economics, tighter environmental protection regulations have influenced the use of directional drilling technology in cavern boreholes.

With modern directional drilling tools, the cavern wells are drilled vertically, kicked off in an S turn, and then finished with a vertical section.

The last 100 m of a cavern well above the last cemented casing shoe must be vertical because of the technical demands of brining and completion.

In the past, the need to keep the last section almost perfectly vertical has required very tight deviation limits for the total length of the well because of limitations in the tools used. The large borehole diameters caused further difficulties for accurate vertical drilling.

To date, Kavernen Bau-und Betriebs-GmbH has successfully drilled and completed three directional cavern boreholes in Germany. These directional drilling techniques have also been used successfully for vertical boreholes with strict deviation limits.

In Germany, 174 caverns are currently in operation for storage of crude oil, gas, or various petrochemical products.1 Another 106 caverns are planned or are already under construction. Although approximately 60% of the currently operated caverns are used for crude oil and petrochemical storage, the new caverns are predominantly intended for natural gas storage.

CAVERN BOREHOLES

Compared to conventional oil and gas boreholes, cavern boreholes have several special characteristics:

• The inclination is limited to a maximum of 1.5, and measures must be taken to limit the inclination when it exceeds 1 from the borehole axis.

• The wells are relatively shallow total depth (1,000-2,000 m) with large diameters (14 3/4-17 1/2 in.).

• The external pressure gradients are relatively large (0.23 bar/m) in the last cemented casing string from the top of the salt. In gas caverns, the possibility of atmospheric inner pressure must be taken into consideration.

To understand the inclination requirements, one must briefly consider what is involved in the brining process.2 After the well reaches total depth (TD) in the cavern borehole, brining pipes, or two concentric casing strings such as 10 3/4-in. and 7-in. casing, are installed.

The brining process proceeds from the planned cavern bottom upwards. Freshwater or seawater is pumped in through the inner 7-in. string, and the resulting brine is displaced to the surface through the 10 3/4-in. x 7-in. annulus (Fig. 1).

If the borehole axis is steeply inclined between the planned cavern roof and the cavern sump, a marked displacement between the cavern axis and the borehole axis could occur during the brining process. The brining pipes initially lie directly on the sides of the borehole and can become unsupported as the salt dissolves. The pipes may then move into a more vertical position under the influence of gravity.

Consequently, strong loading on the brining pipes during running in and pulling out is possible, especially in the region of the cavern neck. Thus, the inclination is limited to a maximum of 1.5 to minimize these loads.

Experience has shown that this inclination limitation does not need to be applied to the total length of the borehole. Only the last 80-100 in above the casing shoe of the last cemented casing string and the complete height of the cavern must adhere to this limitation. Thus, reducing the overall inclination limitation allowed the possibility of directional drilling these cavern wells.

Because the cavern boreholes usually range about 1,000-2,000 in TD, they are generally drilled with medium-sized drill rigs with a hook load of approximately 120 tons.3

A typical rig, for instance, is the Cabot Franks 900. However, this type of rig or similar equipment may quickly reach the limits of the mud pump capacity when directional drilling equipment is used.

For essentially vertical wells, most medium-sized rigs have adequate reserve hook load capacity for running the heavy fast casing string required because of the high pressure gradient. The directionally drilled cavern boreholes, however, can have large frictional loads even at shallow total depths. Thus, the drilling rig must be carefully selected to have sufficient reserve hook load capacity.

DIRECTIONAL DRILLING

Conventional directional drilling technology involves the use of a fixed downhole motor in connection with bent subs and measurement while drilling (MWD) tools. The bent sub and a suitable arrangement of stabilizers give a set build up or build down rate. The planned azimuth is adjusted by varying the hook load and therefore using the back torque of the downhole motor. The azimuth is monitored at the surface via the MWD tools.

A disadvantage of the conventional directional drilling technology is the necessity for extra round trips required to change the bottom hole assembly (BHA). If the expected build up or build down rate cannot be achieved or if the planned azimuth cannot be maintained, the entire drilling assembly must be pulled out of the hole. The stabilizer arrangement must be adjusted or a bent sub with a different deviation angle must be fitted to the BHA to attempt to drill the planned trajectory.

Modern directional drilling technology does not use a bent sub. The bent sub is replaced by the bent housing of the variable downhole motor. The deviation angle of the bent housing can either be preset and adjusted in the workshop or set with stages adjustable at the drilling location. The BHA consists of a variable downhole motor, the MWD tool, and an arrangement of stabilizers.

A large advantage of the bent housing is the relatively small deviation angle compared to the bent sub. Because the small deviation angle is so close to the drill bit, the effect on the build up or build down rate is the same as that from a bent sub with a significantly larger deviation angle higher up in the BHA.

The small angle also makes rotary drilling possible, which can cancel the influence of the bent housing in building up or building down a deviation angle. Thus, it is possible to build up a deviation angle, then upon reaching the angle required, to drill straight and subsequently to build the angle down again (Fig. 2). This type of S-shaped borehole can be drilled without any intermediate round trips, providing trips are not required for other reasons, such as bit wear.

INITIAL EXPERIENCE

The first deviated cavern well had to be drilled as such because of environmental protection regulations and other local authority requirements with respect to the borehole location.4 Specifically, the planned vertical borehole location was initially situated directly above the cavern site which was selected based on rock mechanics and the distance from neighboring caverns. This location was in a natural park with protected wet land and young trees and in the exclusion zone of nearby housing (Fig. 3). Thus, the surface location had to be shifted and a deviated well planned.

The well was planned to have a deviation of 136 m and an azimuth of 293. The top part of the well was rotary drilled vertically to 350 m with a 20-in. roller cone bit. The surface casing was then run and cemented to surface.

Subsequently, with modern directional drilling technology, the S-shaped part of the borehole (14 3/4 in. diameter) was drilled into the salt. The directional drilling took 5 days with no bit changes, and the penetration rate averaged 7.8 m/hr.

The directional drilling string was also used to drill from 940 m to 1,475 m TD. To drill this section, the deviation angle of the bent housing was changed from 0.75 to 0.5. Ten cores in this zone were cut with an 8 1/2-in. conventional corer. After each core, the well was opened from 8 1/2 in. to 14 3/4 in., during which there was a tendency for the borehole to build up angle. By using the directional drilling string, this angle could be returned to normal after just a few meters were drilled. This section of the borehole was drilled in less than 12 days with an average penetration rate of 6.7 m/hr.

Fig. 4 compares the penetration rate of Borehole A with the conventionally drilled cavern Borehole N. Borehole N was used as a comparison because it had a target radius of 2.5 m and can thus be described as "a vertical directional well."

Eight drilling days were saved on Borehole A even though its TD was 100 m deeper than that of Borehole N. The extra costs for the use of the directional drilling equipment were approximately canceled out by the savings on rig costs.

VERTICAL DIRECTIONAL HOLE

This directional drilling technology was therefore selected to drill a vertical borehole (B) because of the significant savings in drilling time and thus rig costs. The experience gained in drilling the vertical section of Borehole A was applied in drilling Borehole B.

Well B was rotary drilled with a 22-in. roller cone bit to 622 m (the setting depth for the 18 5/8 in. surface casing) into the caprock. The next 45.5 m to the top of the salt were rotary drilled with a 17 1/2-in. roller cone bit. This section consisted of anhydrite that could only be drilled at a relatively low penetration rate.

Upon reaching the salt, the directional drilling assembly drilled from 667.5 m to 1,411.0 m. Nine 8 1/2-in. diameter conventional cores were cut in this section.

Drilling through the salt took 13 days at an average penetration rate of 5.1 m/hr. A comparison of the time vs. depth diagrams for Borehole B and Borehole N revealed no apparent time advantage for Borehole B.

During the drilling of Borehole B, however, 6 days were lost because of repairs to a mud pump and elimination of safety deficiencies in some drilling equipment and because the caprock was significantly thicker than anticipated.

After these problems are taken into consideration, there was still a significant time advantage gained by using the directional drilling equipment.

LATER EXPERIENCE

Boreholes A, B, and N were drilled in areas with fairly well understood geology. The geological risk for Borehole C was significantly higher.

It was possible that the salt to be drilled may not have been suitable for brining. Thus, this borehole was planned basically as an exploration well, although the surface casing was designed in the same way as for a conventional cavern borehole.

The section from the surface casing shoe to TD was drilled as an exploration borehole with a small diameter hole. If the results from this section indicated that the cavern could be brined out, then the hole was planned to be enlarged. If not, the borehole would be plugged and possibly deviated in another direction.

The drill site could not be constructed directly above the planned cavern for legal and environmental protection reasons. Therefore, the borehole was planned to have a deviation of 300 m and an azimuth of 80 (Fig. 5).

The surface casing section was drilled from a depth of 180 m to casing shoe depth at 824 m with a deviation of 185 m. This involved the use of conventional directional drilling technology with a downhole motor, bent sub, MWD tool, and an appropriate stabilizer arrangement. The deviation angle buildup to 25 took 4 days with a 20-in. roller bit with an average penetration rate of 8.7 m/hr. The subsequent holding stretch was rotary drilled with an angle-holding assembly. However, the selected stabilizer arrangement did not initially give the result planned; the BHA had to be pulled out and the stabilizers rearranged.

After 3 days, a flint layer was encountered at a depth of 824 m, and it could not be drilled through with the roller cone bit. Because this depth was very close to the planned setting depth of the 16-in. surface casing string, the casing was run and cemented.

After the cement was drilled out, the steerable BHA (downhole motor with bent housing, MWD, and stabilizer) was made up and run in the hole. The flint layer was drilled with a 14 3/4-in. button bit without any significant problems. The 14 3/4-in. BHA was used to drill from 824 m to 1,000 m, and the build down of the deviation angle was begun below 950 m.

An 8 1/2-in. BHA was used to drill from 1,000 to 1,800 m TD. Thirteen cores were cut with an 8 1/2-in. coring string in this section. The cores and the borehole logs indicated that brining out the cavern was possible. Therefore, the borehole was enlarged to 14 3/4 in., and the 11 3/4-in. casing string was run down to 1,410 m, set, and cemented.

WASTE DISPOSAL

Many of the current drilling mud and cuttings disposal sites in Germany are operated by large oil and gas companies which use them to dispose of their own mud and cuttings.4 Because the approval process for the construction and operation of new disposal sites is very lengthy, it is understandable that these companies intend to use their disposal sites for as long as possible. Therefore, the charges for other companies to use the disposal sites-if at all possible-are quite high.

The disposal costs approximately DM 250/cu m ($150/cu m), excluding transportation costs. Thus, one of the most important considerations in planning a cavern borehole is determining the least expensive, but also environmentally friendly, method for disposal of cuttings and drilling mud.

In the north German plain, the overburden has a thickness of about 300-700 m. The overburden generally consists of Quaternary and Tertiary clays and sands, upper and lower Cretaceous formations, and the gypsum and anhydrite caprock. From there down to TD (1,000-2,000 m) is the Zechstein salt zone.

For these depths and formations, muds with special temperature stability or with very low water loss characteristics are not required. Relatively simple muds are acceptable and are especially beneficial considering environmental regulations.

For Borehole A, the cuttings from the cover rock could be disposed of in a nearby domestic refuse site.

The following conditions had to be met for approval of this disposal method:

• The material had to be compact when delivered.

• A set chemical oxygen requirement (COR) value had to be met.

• The waste could contain no soluble constituents (for example, salt).

The use of centrifuges, intermediate storage, and dewatering of the cuttings from the shakers and the mud cleaner helped compact the waste.

Down to a depth of 278 m in the caprock, the drilling fluid was a bentonite suspension-carboxymethyl cellulose (CMC) was deliberately not added. CMC and other similar mud additives significantly increase the COR value of the mud. Because the geology was very well known based on offset wells, the transition from caprock to salt could be determined very accurately. Knowing the transition point ensured that no salt was mixed in with the material to be disposed of.

SALT CUTTINGS

The cuttings from the salt zone were pumped into solution tanks via pumping trucks. The salt cuttings dissolved, and the brine was pumped into a brine pipeline. Disposing of brine into an adjacent river required meeting the following regulations: The solids content of the brine had to be kept low, and the COR value of the brine had to be low.

The salt cuttings were dissolved in two tanks. A large part of the insolubles (clays, anhydrite) sedimented out in the first tank. In the second tank the brine was sucked out from the upper levels.

This procedure kept the solids content less than the set limits. The small volume of insoluble sediments was removed for disposal when the tanks were cleaned.

To keep the COR value as low as possible, a special saltwater drilling clay-sepiolith (SWDC-S) was used for the clay-saltwater mud for drilling the salt zone. The SWDC-S mud requires no protective colloids, such as CMC.

Another advantage of SWDC-S was its low concentration in the mud compared to conventional SWDC. Only 4.5% SWDC-S was used, whereas conventional SWDC requires concentrations of 8% to achieve the same viscosity and weight suspending properties (Fig. 6).

All of the mud in the borehole from surface to TD was replaced with freshwater at the end of the drilling operation. The mud was sent to a mud company for further use.

The cuttings and the mud from Borehole B were transported by suction trucks and trenches to a neighboring cavern. There, the cuttings were thinned with mud and freshwater and pumped into the inner brining pipe by a cement pump. The brining rate of the cavern was reduced from 250 cu m/hr to 50 cu m/hr during the pumping.

The mud and cuttings were pumped directly into the sump of the cavern and sedimented there because the current velocities in the cavern were very low.

The mud used in the overburden and also in the salt zone did not contain any protective colloid additives, thus preventing an increase in COR values in the brine pumped out of the cavern.

In Borehole C, the cuttings and mud from the overburden and the salt zone were stored separately in temporary collecting basins. The basin for the salt zone cuttings was sealed with an impermeable membrane. The cuttings from the cover rock were to be transported to a building materials waste disposal site. The cuttings from the salt zone were pumped together with the clay saltwater mud into the cavern sump as soon as it was brined out.

OUTLOOK

The use of modern directional drilling equipment to drill cavern boreholes allows a site to be selected with respect to local environmental conditions (forest, nature reserve, etc.), a well to remain within the given tolerances yet to have a high penetration rate, a cavern with a damaged access borehole to remain in operation while a new well is drilled before the old well is plugged.

There are several viable possibilities for the disposal of cuttings and mud. Taking into consideration the relatively simple mud composition for a cavern borehole, meeting disposal requirements is usually not a problem.

Taking into consideration land use, the pipeline network between caverns, and rock mechanical demands for the separation between caverns, it is possible to lay out a cavern field such that the caverns can all be drilled with directional drilling technology from a central location.

Fig. 7 is a hexagonal grid pattern laid out with a maximum of seven caverns at similar depths. Projects using this concept are now being planned.

REFERENCES

1. German Mining Yearbook 1991.

2. Gomm, H., Hieblinger, J., and Kuhn, G., "German caverns store 60-million-bbl oil reserve," OGJ, July 3, 1978, pp. 60-64, and July 10, 1978, pp. 153-158.

3. Gomm, H., and Quast, P., "Status of Gas Storage in Salt Caverns in West Germany," SPE paper 19084, presented at the SPE Gas Technology Symposium, Dallas, June 7-9, 1989.

4. Beck, J., Gomm, H., and Peters, L., "Cavern Well Lesum 203-Application of Modern Drilling Methods to Protect the Environment," Erdol, Erdgas, Kohle, Heft 11, 1991, pp. 456-459.

Copyright 1993 Oil & Gas Journal. All Rights Reserved.