North German operator uses learning curve to improve horizontal drilling techniques

Dec. 1, 1997
The success of Mobil Erdgas-Erdöl GmbH's (MEEG) first horizontal well, the Siedenburg Z17, has promoted the widespread use of short, ultra-short, and medium-radius drilling technologies in North Germany. This well, drilled in 1990, initiated production at a rate of 17,000 normal cu m/hr (14.5 MMscfd), a six-fold increase compared to a vertical offset. Since then, MEEG has drilled more than 20 horizontal wells in North Germany (Fig. 1 [69,465 bytes] , Fig.2 [140,447 bytes]).
Juergen Schamp
Mobil International Drilling Services
Dallas
The success of Mobil Erdgas-Erdöl GmbH's (MEEG) first horizontal well, the Siedenburg Z17, has promoted the widespread use of short, ultra-short, and medium-radius drilling technologies in North Germany.

This well, drilled in 1990, initiated production at a rate of 17,000 normal cu m/hr (14.5 MMscfd), a six-fold increase compared to a vertical offset.

Since then, MEEG has drilled more than 20 horizontal wells in North Germany (Fig. 1 [69,465 bytes], Fig.2 [140,447 bytes]). True vertical depths (TVDs) for these wells range from 270 to 4,940 m, with build rates of 92°/30 m for ultra-short radius to 3-5°/30 m for long-radius wells.

Short-radius drilling technology

In the early 1990s, despite advances in short-radius drilling technology, its application still faced many challenges. Bottom hole assemblies (BHAs) reacted differently than standard directional BHAs and the short-radius portions of the hole required specially designed tools in order to pass through the curve.

Since then, sophisticated technologies have been developed to overcome these problems. In total, MEEG has drilled 10 short-radius and ultra-short radius wells in North Germany from 1990 to 1996.

Hamburg gas storage project

From 1992 to 1994, seven short-radius wells were drilled for a gas-storage project. The gas-storage facility uses a depleted oil reservoir that produced from 1937 to 1973. Because the area was environmentally sensitive, it became necessary to minimize new drilling and maximize the use of existing wells, an ideal application for short-radius technology.

Also, because the reservoir is densely fractured, horizontal drilling intersects more fractures than vertical drilling. Because of surface location restrictions, fixed targets, and complex geology, short-radius drilling with horizontal lengths more than 400 m became necessary. In some cases, it became necessary to turn the azimuth just before reaching the top of the reservoir and then drill a short-radius build section within the pay zone (Fig. 3 [21,010 bytes]).

Well planning had to be accomplished within several technical and geological constraints. Following are the main well procedures:

  • Maintain kick-off point (KOP) as deep as possible.
  • Maintain a maximum 40° inclination at 700 m TVD because of sucker rod restrictions.
  • Maintain a maximum 60° inclination prior to setting 95/8-in. production casing.
  • Utilize steerable BHAs up to the point of short-radius drilling.
  • Drill 81/2-in. hole without inclination and azimuth change in order to allow pipe rotation while cementing.
  • Maintain horizontal TVD target tolerance within 5 m.
  • Maintain good hole conditions for logging and stimulation of the natural fractures within the horizontal section.
The 7-in. liner, set through the boundary of the Tertiary clay/Cretaceous pay zone proved to be a critical well-design issue concerning proper zonal isolation. For complete gas shut-off, the 7-in. liner was rotated during cementation. In addition, an inflatable external casing-packer was run near the casing shoe to provide a contingency for any problems encountered during the cement job.

Torque and drag simulations concentrated on possible rotation of the 7-in. liner and the transfer of bit weight through the short-radius section to the bit.

The wells were drilled with a common small oil field rig, ITAG's National 108. The only additional equipment needed was a hydraulic top-drive, necessary for drilling the horizontal sections.

The short-radius BHAs developed by Baker Hughes Inteq consisted of the following major elements (Fig. 4 [40,462 bytes]):

  • A steering assembly, including a bit stabilizer, deflection element, and primary knuckle joint
  • A motor section with a 7/8-in. positive-displacement motor
  • Knuckle joints needed to decouple the motor from bending while passing through the curve radius
  • A short-radius measurement while drilling (MWD) assembly with a probe-type, positive pulse-telemetry system
  • Titanium flex-subs needed to withstand bending forces encountered through high-angle doglegs.
An additional near-bit inclination sensor located 1.6 m behind the bit made the steering process much easier. Modifications of the short-radius BHA allowed a change from fully oriented "snake" drilling (sliding) to rotary drilling with a slow rotational speed of 10-12 rpm. This method, combined with the use of the near-bit inclination sensor provided excellent steerability and led to a straight, smooth well path in the horizontal section.

As a result, it was possible to increase the horizontal section length beyond 600 m. It would have been possible to drill the horizontal well sections further, as long as the string could be slowly rotated.

For acidizing the natural fractures using a straddle-packer assembly, the wells were reamed with a specially designed watermelon mill assembly. This assembly was designed to withstand bending stresses encountered during reaming. Motor stalling was encountered during heavy use, but each horizontal section was successfully reamed and no problems were encountered when the straddle-packer assembly was run in the hole.

Hydraulic mining rig

The Voigtei 115 oil well, completed in November 1993, was drilled as a horizontal producer into the shallow and depleted Voigtei oil field, located west of Hanover. The goal was to improve recovery by doubling the horizontal section in comparison to the well's TVD. The target interval was only ±1.5 m TVD.

In order to keep the well within budget, it was necessary to use a hydraulic mining rig equipped with a top drive unit.

A build section to 7° followed the initial KOP at 116 m. At 236 m, the hole was kicked off again and inclination was increased to 90°. To avoid the possibility of cutting new hole in soft sediments while tripping through the radius, 7-in. production casing was run through the curve.

To avoid getting stuck, the hydraulic mining rig was equipped with a specially designed pull-down mechanism (Fig. 5 [28,527 bytes]). Two mining air-winches were placed at both sides of the rig's rotary table with chain hooks fixed to the rail-guided top drive. By applying an additional 25 tons to the top of the 7-in. casing through the pull-down mechanism, it was possible to push/pull the casing into the hole.

Staying within the narrow target while drilling the horizontal section presented no problems, and with guidance from the near-bit inclination sensor, the well was easily steered upwards. The string was continuously rotated at 12 rpm while the pull-down mechanism applied additional bit weight.

The dual-lateral Siedenburg Z6

The project goal for the Siedenburg project was to produce the largely depleted sour gas field by drilling a reentry-well, the Siedenburg Z6, with two horizontal sections (dual-lateral).

For this low-budget well completed in 1996, it was necessary to cut drilling and completion costs in half by using open hole-completions and dual-lateral techniques. In addition, having two lateral sections within the pay zone reduced the geological risk.1

The geometric tolerance for the re-entry was tight, with only a 42 m zone of anhydrite from which to achieve the two build sections. Target tolerances for the pay zone were only ±2.5 m within the porous part of the reservoir. The TVD for the Main dolomite pay zone was 3,578 m.

The first lateral had an ultra-short build section of 98°/30 m and a turning radius of 18 m. The second lateral had a short-build section of 57°/30 m with a turning radius of 30 m.

While drilling the ultra-short radius section, severe hanging problems occurred. Thus, it became necessary to modify the placement and configuration of the stabilizer ODs. Because of budget restraints, only 41 m of the planned 150 m were drilled.

The second lateral used a retrievable whipstock in order to cut a casing window. After drilling the short-radius curve, 370 m of horizontal section was drilled without severe problems. The retrievable whipstock could not be recovered, and the entry to the first ultra-short radius lateral could not be reopened. The acidizing job was done by bullheading the formation.

Despite this drawback, the well was economical with a production rate of 12,000 normal cu m/hr (10 MMscfd). This was a six-fold increase in production as compared to a vertical well. This successful application demonstrated that ultra-short and short-radius drilling is a viable method for drilling in this area of North Germany.

Medium-radius wells

In the Siedenburg sour-gas field, four horizontal producers were drilled from 1990 to 1996 in the Main dolomite formation. These wells targeted high porosity and permeability zones located at the top of the reservoir. Other than the dual-lateral Siedenburg Z6a well, all wells used medium-radius technology in the hard, partially fractured limestone formations (Fig. 6 [32,628 bytes]).

In this area, sour gas was discovered in the late 1950s.2 The reservoirs have an average depth of 3,400 m and the original gas-in-place is about 40 billion cu m (1.5 tcf). Mobil operates the eastern part of the field.

The pore pressure in the reservoir was depleted to a gradient of 0.3 bar/10 m. Reservoir analysis indicated that a potential five to eight-fold production increase was possible using 400 m long horizontal sections connected across several fault systems.2 Build rates were 5-13°/30 m, and total measured depths were 3,300-3,700 m.

The geological marker for all directional work in this area is the Basal anhydrite, a thin layer below the Zechstein salt. The last casing string is set in this layer prior to drilling the reservoir.

All new wells and re-entries are surveyed with gyroscopes to ensure the highest well path accuracy prior to drilling the pay zone. Survey data are a mandatory requirement by the German mining authorities, in case a relief well needs to be drilled because of a severe blowout.

The sour gas environment in this area had an average H2S content of 6.7% and required the use of special downhole tools and equipment as follows:

  • Grade X/CX drillpipe; 31/2 in., 13.3 lb/ft
  • Special elastomers for the positive-displacement mud motors
  • Corrosion-resistant alloy (CRA) material for the slotted liners.
The depleted reservoir was drilled with water-based mud. Despite the use of a low-density drilling fluid (1.05 kg/l.), the hydrostatic column remained over-pressured by 240 bars.

The drillstring and logging string became stuck several times during the program and could not be freed by jarring or by spotting oil pills. With a low reservoir pressure, it became necessary to reduce the hydrostatic head to 0.4 bar/10 m (3.3 ppg) using gas. Thus, the strings had to be freed by pumping nitrogen into the annulus. This technique was applied several times with success.

There were also problems with partial and total mud losses in the densely fractured areas of the Main dolomite. Formation fluid influx had to be strictly avoided by continuously filling the annulus while drilling and tripping.

MWD operators also had to develop special pulsing techniques to provide directional survey information to the surface. Mud/gas separators, standard for drilling in the pay zone, and additional diverters improved well-control safety.

Well tests confirmed high production rates for these wells and made the projects attractive for future operations.

Long-radius wells

In the deep Permian Rotliegendes Wechselfolge play, harsh drilling conditions make operations difficult. Formation hardness and abrasiveness strongly influence directional drilling. Low penetration rates have a negative impact on the directional performance of steering devices like bent, adjustable kick-off (AKO), and bent-housing subs.

High temperatures and pressures also place operational stresses on downhole tools like mud motors and MWD tools. In addition, downhole vibrations can lead to early failures of the MWD pulsers, requiring long and costly trips.

The wells are drilled in an alternating sequence of hard claystones and tight, abrasive sandstones. The sedimentary profile is similar to the North Sea, but the horizons are deeper, ranging from 4,000 to 5,000 m TVD. The deepened sections drastically reduced the drillability of the rocks.

Bunter and Wechselfolge sandstones and claystones can only be drilled with impregnated diamond bits because PDC bits wear out too fast in these highly abrasive and hard formations.

Well paths were designed to take advantage of the geology. The KOP was usually from 3,000 to 3,500 m so there was enough room to achieve the desired horizontal displacement.

The first build sections were located in the salt sections of the Muschel chalk or Upper Bunter sandstone formations. To maintain a drillable hole in the deeper part, build-rates were limited to 3°/100 ft. In slanted wells like the Walrode Z7, the target was reached with a long tangential section with an inclination of about 45-60°.

Correction runs were done in the Zechstein salt, or if necessary, prior to reaching the target. The horizontal wells reached the base of the Zechstein formation either vertically or at a 45° inclination. The final build is done in the Wechselfolge formation, with built rates limited to 4-6°/100 ft. Maximum doglegs of 9°/100 ft have occurred while drilling, but depend on the hardness of single layers. The variance of doglegs increases with increasing build rates.

Deep drilling equipment requirements

There are several equipment requirements necessary for deep-deviated drilling operations. Rigs should have a top drive for back-reaming in critical situations.

The two major drilling contractors in Germany, ITAG Tiefbohr GMBH and Deutag Tiefbohr GMBH, are already equipped with hydraulic top drives. Although rig rates have increased because of these additions, critical fishing jobs have proven their necessity because of enhanced back-reaming and circulation capabilities.

In addition, hydraulic performance has not been satisfactorily solved. Almost all rigs are limited to a maximum pressure of 350 bar with a working pressure of about 320 bar. This is not enough pressure optimally to run the high-torque/high-flow tandem downhole motors. One of MEEG's partners currently plans to build a new land rig with a maximum pressure capability of 500 bar.

Finally, there is a need for drillstring design improvements. Larger drill pipe sizes (o 51/2 in.) and higher-grade drill pipe (S-135) should become available for high-torque applications so that future projects with displacements of 4,000 m can become technically feasible.

Drill-in fluids

Deep, low-permeability reservoirs may require the use of special drill-in muds to avoid sticking and to reduce reservoir damage. In the case of the Walsrode Z7 well, drilled in 1995-1996, a first approach to a new drill-in fluid was done for directional drilling operations in the Rotliegend Wechselfolge area.

The typical approach was to drill the Wechselfolge formation and pay zone with a potassium chloride chalk mud. However, because it was necessary to achieve a mud weight of 1.55 kg/l., this mud was inappropriate. Instead, a potassium formate mud was designed and used, resulting in superior performance.

Advantages of the low solids system include:

  • A high-density, extremely low-solids content with a thin and smooth filter cake. These properties substantially reduced the tendency for differential sticking
  • Temperature stability up to and possibly greater than 150° C., depending on the formate bring saturation
  • High thixotropy, extremely shear thinning
  • Low fluid loss under conditions of high pressure, high temperatures
  • No formation damage because of a high skin factor
  • Good fluid compatibility to other chemicals and materials and low corrosion
  • Good health, safety, and environmental properties.3
A disadvantage of the mud system was its high cost. This was partially offset by the low-treatment costs while drilling. Well production showed favorable results. Restrained production began at 50,000 normal cu m/hr (45 MMscfd) although the well was capable of 70,000 cu m/hr (59 MMscfd). No skin or reservoir damage was observed.

In the Soehlingen Z3a well, a depleted reservoir had to be drilled highly overbalanced with a mixture of potassium-sodium formate at 1.40 kg/l. and a pure sodium formate mud at 1.23-1.35 kg/l. without experiencing severe differential sticking problems. A 25% reduction in pump pressure was observed after changing from the standard mud to the formate system.

New fishing tool

The Soehlingen Z3a well, drilled in 1996, was planned as a deep re-entry into an existing producer. The goal was to cut a window in the 7-in. production casing at 4,560 m TVD, build angle within the hard Rotliegendes sandstones, and finally drill through several unproven layers of dune sands at an inclination 82°.

Window milling and drilling went well until open hole logging at 1,200 m using coiled tubing (CT). The CT got stuck at about 5,260 m at the base of the first sandstone layer and could not be retrieved using normal methods. While trying to pull the CT out of the hole, the string parted, leaving 5,200 m of 13/4-in. CT with a 7/16-in. wire line and 40 m of 41/2-in. logging tools in the hole.

For safety purposes, the wire line was cemented within the CT. During the cement job the CT partly collapsed because of downhole differential pressures, caused by the collapse of hollow beads within the cement slurry.

Fishing for a collapsed and uncollapsed fish required a newly designed fishing tool with a single, spring-loaded flapper. The spring-loaded flapper with hardened teeth on the rounded contour was designed to slip over the fish while running in hole. When pulling upwards, the teeth engaged the fish.

Unlike other conventional fishing tools, this prototype was able to fish for different CT sizes and shapes. The fishing tool retrieved 190 m of round and deformed CT, until the open-hole section was reached and a safe kick-off from the cement plug was achieved. This soon-to-be patented tool greatly improved the chances of successful CT fishing operations.4

Multifrac well

The objective of the pilot-project Soehlingen Z10, drilled in 1994, was to economically produce the untapped gas potential of the Rotliegendes formation's Main sandstone (Fig. 7 [48,733 bytes]). With more than 40 billion cu m gas-in-place, this zone had reduced permeabilities of 10-20 micro darcies because of illite pore-space plugging. Offset wells indicated that economic production rates could be obtained through hydraulic fracing.

The goal was to combine two existing technologies: horizontal drilling and hydraulic fracing. By drilling a 1,000 m horizontal section at 4,780 m TVD and casing it off with a cemented 7-in. liner, the gas should be produced through four hydraulic fracs perpendicular to the well bore axis (Fig. 8 [81,943 bytes]).

The well, having a planned MD of more than 6,000 m, required directional work in the top part of the hole in order to maintain verticality and minimize torque and drag near TD. Inclination was kept in an acceptable range of 0.25-0.5 using a conventional, double-tilted, universal joint housing.

Build rates in the Rotliegendes formation were designed at 4.5°/100 ft, which later proved to be an acceptable value for directional work in these formations. Directional assemblies consisted of Navidrill Mach 1C positive displacement motors with an AKO Bent Housing set at 1.2-1.5° for the build section and 0.9-1.2° for the hold sections.

Borehole-stability analysis

An important area of concern was well-bore stability. For proper frac isolation in the horizontal section, perfect cementation of the 7-in. liner was essential. 5 Therefore, an in-gauge hole had to be drilled within the pay zone in order to obtain a turbulent flow regime while displacing cement.

Rock properties from an oriented core drilled in a previous well were analyzed. A model of the in situ stress distribution was then derived, and the data were used as input for Mobil's in-house borehole stability analysis software.

The program calculated the minimum mud weight to be used in order to avoid borehole breakouts. The success of the fracs and cement job showed that the latest developments in rock mechanics play an important role in drilling and fracing technology.

Because high mud weights of 1.50 kg/l. (pore pressure = 1.26 kg/l.) were required, oil-based mud (OBM) was used to lower the risk of differential sticking.

The following measures were taken to ensure a successful job:

  • Centralize the 7-in. liner with standoff 80% using one semirigid centralizer per joint.
  • Use additional aluminum-rigid spiral centralizers near the frac zones.
  • Design the cement slurry with 0% free water and 0% settling tendency.
  • Optimize the displacement efficiency with a spacer train because of the use of OBM in the pay zone.
  • Pump the spacer and mud in a turbulent flow regime.
The Soehlingen Z10 well broke several world records, including deepest horizontal well, deepest cemented liner, and deepest sidetrack. In the horizontal section, the plug-back and sidetrack operations were carried out successfully. Production began in early 1995. All four fracs began producing at a rate of about 20,000 normal cu m/hr in 1995 (16.9 MMscfd), 33% more than expected, and is currently producing at 17,000 cu m/hr (14.4 MMscfd). The project was an overall success and may lead to future similar projects for tight gas on a worldwide basis.

Future projects

Future developments in directional and horizontal drilling may lead to even larger horizontal displacements. Some of MEEG's attractive leases lie directly under areas with restricted access, including military locations and cities, which can only be reached with horizontal displacements in the range of 4,000 m.

A risk-assessment study is currently under way to determine the potential of the Soehlingen tight gas reservoir using several horizontal-multifrac wells of the Soehlingen Z10 type.

Other planned projects include re-entering depleted reservoirs with underbalanced drilling technology in order to increase total recovery. Planning is already under way for a deep Rotliegend well to be drilled with this technology.

The application of performance drilling using steerable systems consisting of high-torque motors, adjustable stabilizers, and newly designed bits are either already available or in development and will help to keep the cost of these challenging projects within economical limits.

Acknowledgment

The author wishes to thank Mobil Erdgas-Erdöl GmbH for permission to publish this article.

References

  1. Müller, M., and Wendt, K., "Planning and Design of a Completion Concept for Dual Lateral Wells shown at the Sour Gas Well Siedenburg Z6a," DGMK meeting, April 1996.
  2. Schüler, S., "Horizontal well improves recovery in deep sour gas field," OGJ, Mar. 23, 1992.
  3. Trautwein, R., Sundermann, R., and Bungert, D., "Potassium-formate brines for drilling and completion fluids in deep Rotliegend Gas Wells," DGMK meeting, Celle, Germany, April 1996.
  4. Suryanarayana, S., and Schamp, J., "Soehlingen Z3a-Coiled Tubing Logging Post Job Analysis," MEPTEC/MEEG internal report, December 1996.
  5. Pust, G., and Schamp, J., "Soehlingen Z10: Drilling Aspects of a deep horizontal Well for Tight-Gas," SPE paper 30350, Offshore Europe, Aberdeen, October 1995.
Juergen Schamp is a senior drilling engineer for Mobil Corp. He studied petroleum engineering at Technical University of Clausthal and Colorado School of Mines and received his MS and PhD from Clausthal University in 1985 and 1990, respectively. After working 7 years as a drilling engineer for Mobil Erdgas-Erdöl GmbH in Germany, he was assigned to the Mobil International Drilling Services group in Dallas, preparing drilling projects in the Russian Far East and Siberia.

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