HORIZONTAL DRILLING IMPROVES RECOVERY IN ABU DHABI

Ahmed Al Muhairy
Abu Dhabi Marine Operating Co.
Abu Dhabi, U.A.E.

Essam Ahmed Farid
Abu Dhabi National Oil Co.
Abu Dhabi, U.A.E.

Both onshore and offshore Abu Dhabi, horizontal wells have increased productivity three to four times more than that from vertical and deviated wells in the same reservoirs. Horizontal drilling technology was first applied in Abu Dhabi in February 1988, and through March 1993, 48 wells have been horizontally drilled (19 wells onshore and 29 wells offshore).

During the 5 years of horizontal drilling, the experience gained by both operating company and service company personnel has contributed to a substantial improvement in drilling rate, and hence, a reduction in drilling costs. The improvements in drilling and completions resulted from the following:

The horizontal drilling and completion operations were analyzed daily, and these follow-up analyses helped optimize the planning of subsequent wells.
The bits and bottom hole assemblies were continuously analyzed for optimum selections. Steerable drilling assemblies were found very effective in the upper sections of the wells.

Short radius (8/100 ft), medium radius (6-8/100 ft), and long radius (26/100 ft) techniques were all used successfully.

ONSHORE

The 19 horizontal wells onshore were drilled in five different fields. The first wells were drilled using short radius technology, but in the later programs, five wells were drilled using medium or long radius technology. Only 4 of the 19 wells were drilled as new wells; the other 15 were drilled as re-entry horizontal sections from existing wells.

Horizontal drilling technology was used mainly to improve productivity or infectivity. For the short radius wells, the horizontal section lengths ranged from 175 ft to 1,000 ft. The longest horizontal section drilled onshore was 2 393 ft in a long radius well. Fig. 1 shows the respective lengths of the horizontal sections for all the onshore wells. (Twenty bars are shown because one well had two drain holes.)

In the new wells, the horizontal lengths averaged 1,820 ft, and the wells were drilled in an average of 57 days. In the re-entry wells, the horizontal section lengths averaged 820 ft, and the sections were drilled in an average of 44 days,

In the short radius wells, there were no attempts to run liners, logging tools, or coring tools in the horizontal sections. However, five of the medium and long radius wells were logged, and two wells were cored. The typical logging suites included resistivity, porosity, density, and neutron-gamma ray.

In the wells cored, three conventional cores were taken with 100% recovery; one oriented core was taken, but the operation was considered a failure because recovery was only 8%.

In Well Bb-108, two horizontal drain holes were drilled (Fig. 2). The upper lateral hole had a 484 ft horizontal section, and the lower lateral hole had an 883 ft horizontal section. The well was completed as a dual selective oil producer (Fig. 3).

OFFSHORE

The first offshore horizontal well was drilled in March 1989, and since then, 29 horizontal wells have been completed in three different fields. The offshore horizontal wells consist of 28 producers and I injector.

The offshore horizontal wells fit into two categories: wells sidetracked from existing wells (16 wells) and wells drilled conventionally from surface to the 9%-in. casing point and then horizontally to total depth (13 wells).

The offshore wells were drilled with either a medium radius (7-8/100 ft) or a long radius (2-3/100 ft). The longest horizontal section drilled was 4,500 ft (Fig. 4).

The 13 new wells had an average horizontal hole length of 2,530 ft, and the average drilling time was 69 days. The 16 re-entry wells had an average horizontal hole length of 2,460 ft, and the average drilling time was 53 days.

The drilling performance in the horizontal sections improved from an initial 280 ft/day to more than 1,300 ft/day during the horizontal program.

Seven wells were logged with resistivity, porosity, density, and neutron-gamma ray tools. Four wells were cored, with two conventional cores and four oriented cores taken. The coring operations were satisfactory, with 95% recovery for the conventional cores and 99% recovery for the oriented cores.

COMPLETION DESIGN

AU the short radius horizontal wells (re-entries) drilled onshore were left as open hole completions. The majority of onshore vertical re-entry wells with short radius horizontal holes were dual completions with the horizontal hole in the upper or lower zone (Fig. 5).

Slotted liners were run in the medium and long radius horizontal holes drilled both onshore and offshore, and all were single completions. Fig. 6 is a well schematic of a typical horizontal single zone completion. The majority of the offshore horizontal wells were drilled with multilevel profiles to penetrate more than one producing zone with the horizontal drain hole (Fig. 7). Recently, a dual completion was run with the upper zone in the deviated section and the lower zone drilled horizontally.

OUTLOOK

Abu Dhabi National Oil Co. plans to continue applying horizontal drilling in different fields and reservoirs. A total of 70 horizontal wells are planned through the end of 1994.

Onshore, 33 wells are planned, with 27 as new wells and 6 as re-entries. Offshore, 37 wells are planned, with 29 as new wells and 8 as re-entries. At least one offshore well will be drilled with two lateral holes.

Several of the wells will be injectors for an evaluation of a horizontal well injection system.

The completion designs will be modified to use totally cemented liners or slotted liners with external casing packers for better control of fluid influxes. Other plans include trials of new stimulation techniques in the horizontal zones.