Clustered Wells, Skiddable Rigs Allow Drillers Access To Heavy-Oil Deposits

Dean E. Gaddy
Drilling Editor

The final touches are being put on Santa Fe Rig 178 before being shipped to Venezuela from Houston. Engineers from Mobil and Santa Fe worked together to custom design this 2,000-hp, skiddable drilling rig for batch drilling operations in the Orinoco (Fig. 1).

Skiddable drilling rigs, clustered well locations, 3D well configurations, and economic integration with upstream activities have furnished the means and incentive to develop the heavy-oil deposits of the "Faja del Orinoco."

Drilling plans for three of the seven major partnerships-Cerro Negro, Zuata, and Sincor-provide a glimpse into some of the innovative strategies being implemented by these companies.

Cerro Negro

Drilling operations in the Cerro Negro project, located on the eastern wing of the Orinoco Belt, will utilize specially configured well paths and skiddable drilling rigs to optimize reservoir drainage and to batch drill horizontal wells.

Over the 35-year life of the project, the joint venture involving Mobil, Pdvsa, and Veba Oel plans to cluster drill 18 horizontal wells per pad, for a total of 20 pads.

Initial appraisal work began in 1997 when the joint venture drilled 4 wells. Currently, batch drilling operations for the first two pads are in progress, and it is expected that 46 wells will be drilled by August 1999. This initial program will be followed by an additional 53 wells to be drilled by mid-2000.

In early September, a total of three Santa Fe International Inc. rigs were committed to the project. Two of the rigs, No. 178 (Fig. 1) and 179, are new builds, specially designed for the project, while rig No. 166 is an older unit.

Contract terms indicate the joint venture is serious about developing the Orinoco heavy-oil sands, despite depressed oil prices. Rig 178, which spudded July 16, has a 2-year contract. Rig 179, which spudded Sept. 9, has a 1-year contract. The third and older rig is a 900-hp rig that has been working in Venezuela for several years. It began a 15-month contract with the joint venture last July.

Well design, double doglegs

According to Mobil, which operates the project, the drilling program for a typical well begins by drilling a 171/2-in. hole section down to about 500 ft, followed by the setting of 133/8-in. surface casing (Fig. 2 [81,092 bytes]). A smaller drilling rig is used for this work.

Next, a larger rig is brought in to drill the remaining hole sections. First, a pilot hole is drilled through the pay zones immediately followed by logging runs used in stratigraphic identification (Fig. 3 [47,305 bytes]) and seismic calibration.

This vertical portion of the hole is then plugged back, and sidetracking operations begin above the 1,200-ft kick-off point. The subsequent build section entails drilling a 121/4-in. hole to about 3,150 ft true vertical depth (TVD). At this point, 95/8-in. casing is set at about 3,500-ft measured depth (MD). Next, a 81/2-in. horizontal hole section is drilled through the pay zones to a TD of about 8,500 ft MD (5,000 ft of horizontal displacement) upon which a 7-in. liner is run.

Water-based muds will be used throughout the surface and build sections, while bentonite-based muds will be used in the pay zones. Mobil does not expect any problems with over-pressured zones, lost circulation, sloughing shales, or H₂S. The company predicts each well will take 12 days to drill and complete.

Because the wells are closely clustered with a linear surface spacing of 25 ft, the joint-venture partners have designed the well configurations so the producing sandstone reservoirs are optimally drained. From a drainage standpoint, engineers plan to keep the horizontal well sections 600 m apart.

In many cases, the well bores will comprise a series of stacked horizontals. According to Steve Butt, operations manager for Mobil, the partners will use complicated 3D well configurations with "double dogleg patterns designed to avoid problems with well bore interference."

Rig-skidding system

In order to reduce the time it takes to rig-down, move, and rig-up on each of the "clustered" locations, the Cerro Negro partners will utilize two custom-designed rigs (Nos. 178 and 179) that incorporate an innovative rig-skidding system.

According to Greg Shanks, senior project engineer for Santa Fe, the lift-and-roll system was developed from the success of offshore batch-drilling operations used by jack up rigs to accomplish transverse drill-floor movements.

The lift-and-roll equipment consists of four sets of hydraulic rig jacks, four sets of Hillman rollers, four sets of hydraulic rams, and a hydraulic power unit. The rig jacks, one in each corner of the substructure, are capable of lifting 250 metric tons each. These jacks use large hydraulic cylinders with short vertical strokes to lift the rig off the mats by about 2 in. The vertical rig jacks transfer the weight of the substructure and mast onto the Hillman rollers.

The rollers, which rest on a skidding plate consisting of interlocking mat sections, are mounted to the hydraulic rams. To skid the rig across the location, the rams are horizontally extended, rolling the upright mast and substructure over the top of the plate in the desired direction (Fig. 4 [144,892 bytes]).

To cycle the system, the rig jacks are retracted, which then removes the rig weight from the roller assemblies. The horizontal jacks are then retracted, leaving the system ready to repeat the jacking cycle (Table 1 [67,395 bytes]).

An additional 400,000 lb of drill pipe and tubulars, set back in the 142-ft derrick, can also be skidded along with the rig move. According to Shanks, the horizontal jacking speed is about 1.5 ft/min. He estimates that once the rig crews become experienced with the moving operations, it should only take about 45 min to move. Rig 178's first rig move in the Orinoco took 1.5 hr.

To further reduce costs associated with equipment mobilization, the joint-venture partners chose to keep the power package and mud pumps on one stationary location while drilling the entire 18-well series. According to Shanks, concerns of mud-pump hydraulic losses as the rig is moved from location to location are negligible. "As compared to hydraulic losses in the drillstring, the effective loss is only two more stands of drill pipe."

To further reduce the costs associated with location construction, the reserve and active pits will also remain on one location during the drilling of the 18-well series. The shale shakers are mounted at the rear of the substructure and the mud liquid phase will be directed to the shaker tank, also located in the substructure. The drilling mud will then be pumped to the centrally located process pits by two centrifugal transfer pumps.

Rig specifications

Not only do the Santa Fe rigs incorporate technology developed from offshore applications, the new builds are also outfitted with state-of-the-art drilling packages similar to that used on offshore drilling rigs.

Some of the drilling components include:

2,000 hp National 1320 drawworks
International rotating mousehole capable of assembling the drillstring in stands outside of the drilling operation
National PS350/500 dc top drive
Four 1,215 hp (1,200 rpm), Caterpillar D399s that drive four 1,050 kw generators
National 27.5-in. rotary table
Two National A-1700 triplex pumps
Three Brandt cascading shale shakers that incorporate elliptical motion screens on top of linear motion screens.

Zuata

The Sincor project, a strategic association involving Total, Pdvsa, and Statoil, is another 35-year-long program involving an aggressive drilling program of about 1,700 horizontal wells in the Zuata region.

Located in the central part of the "Faja," this project will incorporate cluster wells that will also be drilled along a regularly distributed, linear surface-location pattern.

A typical pad will encompass six batch-drilled wells, spaced 25 ft apart. However, as many as 24 wells may be "clustered" from a single pad. The number of wells to be drilled will depend on the geologic development of the sandstone seams in the lower Oficina formation (Fig. 5 [91,992 bytes]).

For example, if there are numerous sweet spots positioned throughout the lower and upper seams, then a larger number of horizontal penetrations, dependent on the number of surface locations, will be required.

As of September, no drilling contractors had been chosen for the work, although some equipment specifications have been decided upon, including the use of top-drive technology. In 1997, an appraisal program encompassing three vertical wells, two horizontal wells, and a 3D seismic pilot was carried out.

This year, the joint venture will complete a full-field 3D seismic survey while carrying out tenders for drilling contracts. The start of the development program is set for the second quarter of 1999. From this date until the beginning of 2002, about 200 wells will be drilled, followed by an average of 30-40 wells/year.

Drilling program

The drilling program for a typical well will begin by drilling a 171/2-in. hole section down to about 400 ft, followed by the setting of 133/8-in. surface casing at this point (Fig. 6 [61,147 bytes]). This string will be preset by a smaller rig.

From underneath surface casing, a 121/4-in. hole will be vertically drilled to 950 ft upon which the build section will be initiated.

Next, a tangential section from 2,235 to 2,385 ft will be drilled at an inclination of 75°, after which hole angle will be increased to 90° by 2,800 ft MD (1,900 ft TVD). At this point, 95/8-in. casing will be run.

Next, an 81/2-in. hole will be horizontally drilled to TD at 6,600 ft MD after which a 7-in. slotted liner will be run. This will result in a MD/TVD ratio of 3.5. In comparison, the world record at Wytch Farm has an MD/TVD ratio of 6.6.

The upper section of the hole will be drilled with water, while bentonite-based muds will be used prior to setting the 95/8-in. casing. The type of drilling fluid to be used through the pay zones has not been decided upon yet.

Drilling technologies

During the learning stage of Sincor's drilling program, the well configurations will be kept simple. However, the alliance is considering the use of four advanced drilling technologies, including underbalanced drilling, needle coring, multilateral drains, and 3D well configurations. First, Sincor engineers and geologists are planning a well-bore stability study in the horizontal section to see if drilling fluids below water gradient can be used.

Second, a few stratigraphic wells might be cored with Security DBS Co.'s needle technology, specially designed for the Orinoco. This promising technology is intended to overcome problems with core protection while drilling very soft sediments-including unconsolidated Oficina sands.

The needle sleeve, positioned freely in the outer tube, penetrates and cores soft sediments ahead of the six-bladed corehead while protecting the core from fluid-flow contact. As harder formations are encountered, the needle sleeve is pushed upward until the corehead comes into direct contact with the formation. Thus, the needle sleeve cores the softer sediments and the corehead cores the harder sediments.

The sleeve also prevents the core from rotating. In addition, a full closure system has been incorporated into the design elements of the inner barrel to help ensure full recovery, utilizing a collapsing shoe system.

Third, a feasibility study concerning the use of multilateral drilling technology will be considered later this fall. Finally, more-complicated 3D well configurations will probably be used, especially if there are several pay zones that must be targeted and optimally drained while reducing well bore interference.

The alliance does not anticipate any problems associated with overpressured zones, lost circulation, sloughing shale, or differential sticking.

Petrozuato

The Petrozuato project, a joint venture between Conoco and Pdvsa, began drilling operations in August 1997. Although Conoco and Pdvsa declined to comment for this article, published reports indicate they plan to drill 500 wells over the life of the 35-year program.

Batch drilling of the horizontal sections began February 1998 and by September of this year, 31 horizontal wells were complete with an additional 45 more in various stages of development.

Precision Drilling de Venezuela is the drilling contractor with two rigs in operation. Rig PD 731 is a 500-hp, single-derrick rig rated to 5,000 ft. Rig PD 732 is a 1,000-hp, triple-derrick rated to 9,750 ft.

Hamaca

The drilling program for the Hamaca project, involving ARCO, Pdvsa, Texaco, and Phillips, is economically and operationally dependent on the project's downstream activities, as fixed by the oil-rate profile of the upgrader (see related story p. 58). This has led the partners to implement a "solid, routine, and low-cost drilling program," that strives to minimize life-cycle costs, said Guillermo Quintero, Hamaca develoment manager for ARCO.

The drilling program will target the prospects with the highest-potential production rates first, followed by the lowest potential prospects last. Because the surface facilities will be fully loaded throughout the life of the project, the partners intend to "optimize the cost stream" over the duration of the program.

Drilling operations will involve a 700-1,300 horizontal well program over a 35-37 year period. During the first development stage, the partners will drill 30-55 wells. At this time, there is no drilling activity as the partners are still finalizing the work program. No drilling contracts have as of yet been approved.

The generic well configuration consists of setting a 135/8-in. surface casing string at 550-600 ft followed by running a 95/8-in. casing string to 2,100 ft TVD. A typical well will be horizontal by 2,000-2,500 ft TVD followed by a 2,000-3,000 ft MD horizontal section.

The two main producing members within the Oficina formation occur at about 2,000-2,200 ft TVD in the southern edge of the field and 2,500-2,700 ft TVD in the northern edge. Thus, drilling plans will change according to the structural nature of the block. The gross sand thickness for the producing intervals is about 100-250 ft.

Quintero said the current pad design is similar to "putting together a jigsaw puzzle," with intertwining north-south and east-west well orientations that form a pattern similar to that of a rectangle. The well spacing will be 124 acres. Quintero pointed out that these plans are not definitive, and will probably change with time as the project gains momentum.