SLANT RIGS OFFER BIG PAYOFFS IN SHALLOW DRILLING

Jeff Smith
George E. Failing Co.
Enid, Okla.

Bob Edwards
Sierra Drilling Co.
Calgary

Slant hole drilling technology can result in considerable savings over conventionally drilled deviated holes because mud motors and deviation control with measurement while drilling tools are usually unnecessary.

The benefits of using slant hole rigs for development drilling improve after the bit walk tendencies and the correct bottom hole assemblies have been determined for a particular area.

Slant hole drilling has been available for several years, and over the last 10 years over 1,000 slant or angle wells have been drilled primarily, in Canada, Venezuela, and China.

This article discusses three recent drilling operations that successfully used slant drilling technology on land-based projects: drilling for heavy oil in Alberta, drilling for gas in Alberta, and drilling a river crossing for a gas pipeline in British Columbia. These examples demonstrate the flexibility of slant drilling technology.

A distinction must first be made between directional and slant or angle drilling.

A conventionally drilled directional well is typically started with a vertical section from surface. Specially designed bottom hole assemblies or mud motors are then used to deviate the well at an appropriate angle to reach the target. The conventional directional drilling method increases the measured length of the well because it does not follow the shortest distance between two points, a straight line. The additional length plus the necessity for specialized equipment results in higher drilling costs.

In contrast, slant hole drilling attains the shortest drilling distance by allowing the well to be spudded at an angle (usually a maximum of 45) aimed directly at the target (Fig. 1). With the proper drilling rig, the well can be drilled straight to the target (Fig. 2). The slant hole path can be deviated additionally with the use of conventions directional tools, including into a horizontal direction.

The major applications for this technology have occurred in the shallow depth, heavy oil areas. The multiwell pad approach of slant technology is conducive to the drilling of the numerous wells required in these areas. Other applications include wells that require considerable horizontal displacement, especially at shallow depths.

Future development of slant drilling rigs is expected to result in equipment capable of operating in the 4,0005,000 m true vertical depth range. The use of slant technology will require an increased number of drilling rigs capable of operating in either the vertical or slant mode.

MULTIWELL PROJECT

A slant hole drilling project was engineered for the drilling of 23 wells from one pad near Wolf Lake in the northeast portion of Alberta. This area of Canada has one of the world's richest heavy oil deposits.

The plan required 10 slant wells, of which 6 wells were spudded at 30, 2 at 29, 1 at 27, and 1 at 26. The remaining 13 wells were started vertically and kicked off with conventional directional drilling tools.

The following objectives were established for the drilling plan:

True vertical depth of 414 m
Maximum measured depth for the slant wells of 567 m
Maximum measured depth for the directional wells of 523 m
Primary drilling target of 13.5 m x 6 m
Secondary drilling target of 27 m x 12 m
Maximum spud angle 30
Maximum angle at the target of 45
Maximum angle build of 4/30 m
Maximum dogleg severity of 5/30 m.

The operational portion of the program included the following drilling criteria: the surface hole diameter was 343 mm (13.5 in.), the main hole diameter was 251 mm (9.875 in.), all wells were logged, and three slant-drilled wells and three vertical drilled wells were cased with 177-mm casing.

DRILLING OPERATIONS

For directional control, the mud motors were oriented to build and maintain drift angle as well as make minor azimuth adjustments. After both the angle and direction requirements to achieve the target parameters had been met, the drillstring was rotated slowly to negate the effects of the deflecting action of the motor.

Each well was drilled with one bottom hole assembly. No trips were necessary for bottom hole assembly changes, and penetration rates were considerably higher than with conventional rotary assemblies because of the high speed (300-500 rpm) of the mud motors.

The multiwell plan for the drilling location incorporated a central mud system which further improved the overall economics of the project. This configuration allowed the centralized mud system, generator, boiler, and mud pump house to remain in one location on the pad for the duration of the project (Fig. 3). The mud system consisted of a desander, desilter, and centrifuge.

A flexible "utilidor" connected the drilling rig to the central mud system. The utilidor allows for transfer of fluids during drilling and facilitates rig moves from well to well around the pad.

The drilling program was conducted during midwinter conditions, yet the overall average drilling time on the 10 slant wells was 2.2 days per well, from spud to rig release.

The 26 slant hole was the first hole drilled on the pad. Including the long rig move and the time for rigging up on the first hole, the well was drilled in 2.80 days.
The six 30 slant holes required a total of 12.46 days to drill for an average of 2.08 days per hole from spud to rig release.
The two 29 slant holes were drilled in 4.30 days for an average of 2.15 days per hole.
The 27 slant hole was drilled in 2.6 days.
The 13 conventional directional holes were drilled in a total of 24.10 days. This included two wells that required the cutting of nine cores. The average drilling time for each hole was 1.85 days.

The program was approached as a pilot project for future reference; consequently, the drilling schedule was excessively budgeted to take approximately 120 days. However, only 46 actual drilling days were required.

SHALLOW GAS WELL

A second slant hole drilling project involved a shallow gas well drilled for Grand Prix Natural Gas Ltd. near the town of Brooks in southern Alberta. The gas field is located in rich, irrigated farmland; therefore, direct vertical access to the target was not permissible.

The operator chose to spud the well at an angle of 45 and then use directional tools to build angle to a maximum of 78 to reach the target area (Fig. 4). The well was drilled by Sierra Drilling Co. Rig No. 2.

The drilling design for the well included:

True vertical depth of 575 m
Measured depth of 1,400 m
Horizontal displacement of 1,280 m
Maximum spud angle of 45
Surface hole size of 251 mm drilled to 123 m true vertical depth
Surface casing (178 mm H-40) set at 181 m measured depth
Maximum angle build of 4/30 m
Maximum angle reached of 78.25.

Downhole motors were not required on this project. Instead, experience gained on previous projects allowed the use of more conventional and less costly bottom hole assemblies.

The well was drilled under budget and inside of the expected time schedule for the project.

From spud to rig release the drilling operation lasted 5.42 days. This well is believed to be one of the first to achieve such great horizontal displacement at such a shallow depth.

RIVER CROSSING

Although outside of conventional petroleum drilling practices, slant hole drilling technology was used to drill the Pouce Coupe river crossing in British Columbia. The objective of the project was to install a buried 406-mm pipeline under the Pouce Coupe River and flood plain for an approximate distance of 1,740 m.

The operation involved the drilling of two holes, one of which started from the raised north bank, passed under the river, and exited on the dry flood plain. The north bank section required a measured length of 867 m (Fig. 5). The second hole started on the raised south bank, passed under the flood plain, and exited short of the river adjacent to the north exit; this section had a measured length of 866 m.

After the slant rig was moved on the north location, the crossing was spudded at 45. A 251-mm pilot hole was drilled to 867 m and exited at an angle of approximately 105. This operation took 42.75 rotating hr. The 251-mm hole was then enlarged to 444.5 mm during 58.5 hr of drilling. The hole was then enlarged to 600 mm during 62.5 hr of drilling. At this point, the prewelded 406-mm pipe was pulled into the hole (a 26-hr operation).

The south crossing operation was similar. The rig was moved on location, and the hole was spudded at 45. The 251-mm pilot hole was drilled 866 m during 52.5 rotating hr. The hole exited at an angle of approximately 106, near the exit point of the north crossing. The hole was enlarged to 444.5 mm in 71.5 hr, and then to 600 mm in 52.75 hr. The pulling operation for the prewelded 406-mm pipe lasted 24 hr.

At the point of adjacent exits, the pipeline sections were tied-in in open trenches. After the appropriate connection was finished, the trench was backfilled; the continuous underground pipeline ran approximately 1,740 m.

The application of slant technology on this project improved with the learning curve as operations progressed.

The completion of this project demonstrated the benefits of blending technologies from conventional, directional, slant, and horizontal drilling practices.