Horizontal well strategy taps attic oil

Fred Morissey
Numac Energy Inc.
Calgary

Fig. 2 -- Modified New Well Profile (157667 bytes)

Fig. 5 -- Reentry Drilling and Cost Curves (52661 bytes)

The use of horizontal well technology has allowed pursuit of a sometimes elusive formation to search for attic oil through the hills and valleys of a Precambrian landscape (Fig. 1 (22810 bytes)). These wells are in the Granite Wash formation in the Peace River Arch area of northern Alberta. The target is atypical of many horizontal well schemes.

With increased understanding of the target formation, the program has seen an evolution in well design to improve economics. Some features may appear unconventional for horizontal drilling but are particularly suitable in this case.

The Otter and Red Earth areas of Alberta have been core producing properties for Numac Energy Inc. for a number of years. Located approximately 350 km northwest of Edmonton, these areas are prone to muskeg conditions at surface and are best suited to winter drilling.

In the past 2 years, Numac has drilled nine new and eight reentry horizontal wells in the Granite Wash formation in the area (Table 1 (55478 bytes)). The program began with new-drill wells, but the opportunity to lower well cost has shifted the focus to the reentry method where possible.

With the help of 3D seismic interpretation, the objective of these wells has been to tap into structural highs that will not be drained by vertical wells. As the reservoir has an active bottom water aquifer, the vertical wells can also have high water cuts, which may be reduced by accessing the higher structure.

An evolution in well strategy has developed through the course of the program. It is now recognized that each well may have significant site-specific requirements.

In this program, wells have more an "exploratory" component than one would normally expect in a horizontal "development" program, to the extent that some of these wells have been unsuccessful in finding commercial reservoirs and were abandoned.

It is an area where a reservoir is often not found precisely below where the drilling rig is located, and plug backs and sidetracks on vertical wells are common. In this sense, horizontal well technology is being used as a hedge in exploitation strategy to reduce the risk of missing a target, while offering the ability to access the highest part of a pool's attic oil.

Geological setting

The Granite Wash formation at 1,600 m true vertical depth (TVD) is the main producing horizon in the Otter/Red Earth area. The formation is generally a highly porous and permeable sandstone of varying thickness. The sand is derived from Precambrian highs, around which it is draped.

A nonproductive regolith, often located at the basement contact, is very similar to the Granite Wash sand on seismic. As these prospects are defined with 3D seismic interpretation, this feature can complicate the selection of drilling candidates.

The irregular geographic distribution of the Granite Wash formation is defined by Precambrian topography, which has resulted in no preference in well bore orientation.

The Granite Wash pools have been approached in as many ways as there have been wells drilled, in order to tap into attic oil. Uphill, downhill, and sidehill trajectories have all been used.

Because of the seismic definition of the reservoir target, geometric buffers are required in build-section profile design to make allowance for the seismic resolution, as the reservoir may come in either high or low to prognosis. The proximity to the basement is also a concern to avoid drilling granite ledges, which has happened on occasion.

The immediate overlying formations (Granite Wash zone, Keg River, and Muskeg) are reasonably consistent in thickness moving up from the basement. These formations provide lithological and gamma ray markers to assist with course corrections in the last 20-35 m TVD prior to entering the Granite Wash target.

Drilling problems in the vertical and build sections of wells in this area vary in severity. They include shallow gas in the Bluesky formation, lost circulation in the Wabamun and Winterburn formations, deviation through the Ireton shale, and anhydrite in the Muskeg formation.

Well profiles

The new-drill wells have surface casing (245 mm) set at 300 m, which satisfies ground water isolation requirements. A 222-mm intermediate hole is drilled vertically to the kick-off point (KOP) at 1,400 m, where directional tools (including gamma ray measurement while drilling tools) are picked up to drill a 200-m-radius build section. Figs. 2 and 3 are schematics of a typical new horizontal well and a typical reentry horizontal well, respectively. The original well plans were for open hole completions, but the modified design included a liner in each.

Intermediate casing (178 mm) is set into the Granite Wash at 90, although the actual landing angle varies, depending on the local geology. The horizontal sections (159 mm) have been drilled from as little as 60 m to more than 600 m, with horizontal length also being specific to geological conditions.

The intermediate hole has been drilled with gypsum mud because of the amount of anhydrite in the Muskeg formation. A clay-free polymer water fluid has been used to drill the Granite Wash horizontal sections. At total depth (TD), the wells have been displaced with produced water, and a bridge plug set at the KOP prior to moving off the drilling rig.

The new wells have required an average of 22 days to drill and cost about $1.1 million (Canadian) to drill and complete (Figs. 4 and 5).

Reentry horizontal wells

The reentry horizontal wells drilled in Otter/Red Earth have mostly been in 140-mm casing. They have required about 16 days to drill, at an average cost of about $750,000 to drill and complete. In reentries, well cost has been a function of the distance from the original well to the target sand.

A typical drilling curve is shown in Fig. 4 (54066 bytes). These wells have been individual in nature, with profiles varying in accordance with the status of the existing well bore and lateral target distance. Build rates have varied from 12 to 30/30 m.

There are several common features, however:

• In all cases, the initial preparation of the well bore for reentry has been performed with a service rig. This work has included the removal of production equipment, abandonment of existing production, and confirmation of well bore integrity as required (cement bond log, casing inspection, etc.).

• Drilling fluid selection has been the same as that for new wells. Gypsum mud was used through the build section to the point of Granite Wash entry and was then displaced with the polymer water system to drill the productive reservoir.

• For a casing exit method, section milling was chosen initially and has been used throughout the program.

  Section milling

  The choice of section milling, while less popular in Western Canada than the whipstock/window method, has proven to be particularly suitable to this application. The factors assessed included the well depth, rig time, and cost. The factors which contributed to the selection of the section milling approach included the following:

• Casing size

  With all of the reentries to date in either 140-mm or 178-mm casing, section milling was mechanically amenable to the removal of the casing material and the successful setting of kick-off plugs. In this program, setting densified cement plugs and allowing sufficient time for the plugs to set has resulted in 100% success in first-time sidetracks.

• Time of exposure

  A combination of long distances to target, low penetration rates, and relatively short bit life required many trips through the casing aperture. This favored the removal of all casing material, including any edges which may have caught the drillstring.

Favorable section milling on the first well (3-29/5-29) influenced continuing with the approach with subsequent reentries. One modification made after the first well was to increase the milled section length from 20 m to a minimum of 30 m. This increase eliminated the need for gyro surveys during kick off.

Geometric considerations

Uncertainty in seismic definition and target entry requirements have all contributed to build section design for this area. The basic circular profile has been modified to optimize drilling performance through the selection of KOP and build rate, reservoir entry angle, and the use of tangent sections. All profiles were constructed from the point of reservoir entry.

• Kick-off point

  The build section has been drilled beginning below the Beaverhill Lake top at 1,400 m TVD. This avoided a directional kick off in the Ireton shale and still allowed 200 m of working room above the Granite Wash target.

• Build rate

  Typically, medium-radius profiles have been used in both the new and reentry horizontal wells. In new-drill wells, build rates have ranged from 8/30 m, where there were no extraneous requirements, up to 13/30 m, where the distance from surface location to target entry was greater than 200 m and a tangent section was required. In reentries, build rates up to 30/30 m have been used where the target entry has been distant (300 m) from the existing well bore, and long tangent sections were necessary.

• Entry angle

  About 82 has been found a suitable entry angle into the Granite Wash. This angle provided a compromise between being able to level out quickly (in less than 2 m TVD) while still being able to drop in TVD at a reasonable rate (about 1 m TVD per 7 m drilled). This ability allowed searching for the reservoir target without sacrificing much lateral distance.

  Given the seismic uncertainty in target definition, it has been preferred to establish this entry at a reasonable vertical distance above the prognosed Granite Wash top, to reduce the geometric risks approaching the reservoir while still building angle. It was also found early in the program that greater approach angles (soft landing) created a tendency to "bounce" at the Granite Wash shale top.

• Tangent sections

  The earlier wells in the program essentially followed a circular build profile because the distance from surface location to reservoir entry was approximately the same as the vertical depth available for a build section (200 m). High angle (80) tangent sections were introduced when tangents became further displaced, while the TVD constraint of 200 m available for a build section remained.

Instances included new-drill wells with difficult surface access and reentries where surface coordinates were fixed. The advantages of tangent sections in this application have included a shorter distance drilled (straight line vs. curved path), greater penetration rates (drill in rotating vs. sliding mode), and a relaxation of precision requirements in the critical lower portion of the build section (all of the "circular" geometry is completed above target entry).

Liner

The initial design of horizontal wells in Otter was for open hole completions, both for new wells and reentries. The lithologies in the area were considered to be suitably competent for this approach.

Following the reentry at well 2-6/7-6-88-11 W5M, it became evident that a combination of higher water cuts and higher drawdown could cause production problems. The inflow to this well fell sharply in a short period, suggesting well bore bridging.

Redrilling of the horizontal section was required, and a liner was then run in this well bore. Subsequent reentries incorporated liner installation as part of the initial well program, as a preventive measure. Individual liner configurations varied from well to well, in accordance with specific geometry and geology.

The primary function of a liner in this type of application is to provide a conduit for oil production from the Granite Wash formation. As there was no requirement for hydraulic isolation from other formations in the build section of the reentry wells, the liners have not been cemented.

Typically, blank pipe was run to the top of the Granite Wash and ported pipe in the productive sand. Liners have been run to a depth short of TD by about 20-30 m to allow some discretion in liner positioning. This procedure has reduced the risk of having to pull a liner if hole problems occur while attempting to run in to TD.

Reentry of abandoned wells

The reentry of abandoned wells has been attempted twice to date in this project, at the 3-14/4-14 and 4-13/4-13 locations (Table 1 (55478 bytes)). This variation on the general reentry concept made use of an existing well bore and a reclaimed surface location.

There were additional risks and uncertainties, however, and further work was also required to refurbish both the well site and the well bore.

It is advisable to approach any abandoned well reentry candidate with caution. The decision to proceed with such an operation must be made with the understanding that the potential for problems can be higher than for reentering producing wells. In the end, a new abandonment liability must also be assumed.

The initial data review can be hindered by the age of the well, how long ago it was abandoned, and the clarity of records. One must pay particular attention to any "holes" in the available data, as making assumptions can lead to costly problems during field operations.

Examples of unscheduled events encountered in reentering the abandoned well bores have included a leaking casing annulus and junk in the hole.

Field operations

Field operations specific to reentry of abandoned well bores focuses on refurbishment of the cut and capped casing strings. The pipe stub has to be located first, and the area around it excavated. At this point, a check for obvious leaks at surface is required.

One has to assume that there is an unknown pressure on the inside of the pipe.

To maintain well control, a valve may be installed and the casing hot tapped through the valve. Any gas pressure in the annulus must be bled off, and the well top-killed if necessary.

When the well is dead, the collars are installed, and extensions welded onto the casing strings. A new casing bowl can then be welded onto the surface casing at ground level, and the slips reset on the production casing when the rig moves on location.

Completions

The completion needs for these wells were generally simple. Fundamentally, the completions required only running production tubing and swabbing to initiate flow. Depending on reservoir pressure, the wells may flow, but all eventually required artificial lift. A flow/swab test and static gradient were also included as part of a typical completion.

In new wells, the production tubing was run to Granite Wash TVD and anchored above the directional kick-off point.

In the reentry wells, the production tubing was run to above the milled section in the vertical well or above a liner top.

In the new-drill wells from surface, sand sections behind intermediate casing have been perforated to expose more productive reservoir to the well bore. Completion costs for reentries ($50,000) have been about half that for a new horizontal well completion.

Remedial operations

The term remedial is used in this context to describe a variety of operations which have been part of this program. These include stimulation treatments, well bore cleanouts, and liner installations.

Some wells have required stimulation following the initial completion. The methods included acidization, hydraulic fracturing, and nitrogen blow down with coiled tubing. The nitrogen cleanouts have had the greatest impact on production.

Liner installations have been included as part of this discussion because they were not initially included in the well programs. In instances where reduced production has indicated well bore bridging, liner installations have been performed.

In reentry wells at 2-6/7-6 and 5-30/5-30, the horizontal section and part of the build section required redrilling prior to liner installation. At the new-drill location 5-29/7-30, however, a successful cleanout of the original horizontal well bore was possible, and a liner was run in the horizontal section.

This liner incorporated aluminum knock-out plugs to provide ports through the productive sand. The plugs were required to allow for circulation of the liner while running and were milled off during completion.

Results

For the Granite Wash formation, horizontal well technology has been used in an "exploratory" exploitation program to produce attic oil reserves. The objective has not been to drill extensive lateral sections, but rather to access productive reservoir sand which would otherwise have been missed. Economics favored reentry horizontal wells where possible.

Drilling techniques have been adapted on a well-by-well basis to suit a particular play concept. Optimization of geometry with geology has improved drilling performance.

Section milling has been applied as a casing-exit method in the reentry wells. Correct procedures had to be followed to achieve "first-time" kick-off success.

Reentry of abandoned wells should be approached with caution, with respect to both preliminary data review and initial field operations.

Acknowledgment

The author would like to thank Numac Energy Inc. for permission to publish this article. The author also thanks the members of Team Otter, the Numac drilling department, and field personnel who have taken part in this project.

The Author