WORLD'S FIRST TRILATERAL HORIZONTAL WELLS ON STREAM

Unocal Corp. continues to advance the frontier of horizontal drilling technology off California.

The company has placed on production in the Santa Barbara Channel's aging Dos Cuadras oil field four trilateral horizontal wells it says are the world's first. All four were drilled and completed before a Union Pacific Resources Co. trilateral horizontal well was completed in Brazos County, Tex. (OGJ, Nov. 15, p. 32).

Unocal has claimed other horizontal drilling records off California as well.

PRODUCTION BOOSTED

The four trilateral horizontal wells Unocal drilled in Dos Cuadras field have boosted production from Platform B by about 3,100 b/d.

The platform, in 195 ft of water 51/2 miles off Carpinteria, was installed in 1968.

The trilaterals have produced at rates of 600-1,100 b/d. The 35 conventional wells drilled from the platform during Dos Cuadras development have produced about 50 b/d/well for an average total of 1,750 b/d. In recent years, Dos Cuadras production had been declining at about 8%/year. No new drilling had occurred in the field since the late 1970s until Unocal started its horizontal drilling program in 1990, reversing the decline.

Platform B now produces about 4,000 b/d of oil and 3 MMcfd of gas.

The four trilaterals and seven conventional horizontal wells Unocal has drilled in Dos Cuadras the past 2 years ultimately will add about 10 million bbl to field reserves. Since Dos Cuadras went on stream in 1968, Unocal production from the field has totaled about 170 million bbl of oil and 80 bcf of natural gas.

Unocal is operator of Dos Cuadras field, with other interests held by Chevron Corp., Mobil Oil Corp., and Texaco Inc.

DRILLING DETAILS

Target for development by the trilaterals and other horizontal wells in Dos Cuadras is Pliocene Repetto, which produces 22-24 gravity oil from depths too shallow to be fully developed commercially by conventional wells.

True vertical depths of the trilateral holes drilled from the fourth well, OCS P-0241 B-34 are 950-1,086 ft. The well was spudded Aug. 27 and went on stream late in September.

The trilateral mode basically stacks one horizontal lateral above another at an average angle of 75-76'. Drilled depth of the first B-34 horizontal lateral was 4,792 ft. Unocal cemented 95/s in. casing at 2,000 ft and ran 65/8 in. slotted liner from 1,980 ft to 4,790 ft.

Coming back up the hole, Unocal milled a window at 1,590 ft in the 9 5/8 in. string and set a retrievable whipstock to drill the second lateral to total displacement of 4,788 ft. It then ran 6 5/8 in. slotted liner from 1,610 ft to 4,786 ft.

Unocal moved back up the hole to mill another window at 1,302 ft, set a retrievable whipstock, and drilled the third lateral to a total displacement of 4,792 ft, running 65/s in. slotted liner from 1,322 ft to 4,790 ft.

On average, the B-34 well extends about 4 ft horizontally for every 1 ft vertically. It is producing about 800 b/d. Current production from the other three Dos Cuadras trilaterals is B-7 and B-39 600 b/d each and B-9 1,100 b/d.

Total cost for the B-34 well is about $2 million, compared with about $3 million for three conventional horizontal wells at $1 million apiece. That means a trilateral well can do the work of three regular horizontal wells at two thirds the cost.

SMALLER FOOTPRINT

Along with increased production, the Dos Cuadras trilateral wells offer another step forward with what Unocal Senior Drilling Engineer David Payne describes as a reduction in the footprint of offshore oil.

"If we had this technology during the original development of Dos Cuadras, we could have done it with only two platforms instead of the four that were used," Payne said.

Unocal plans to drill another trilateral well from Platform Hillhouse, 1 mile east of Platform B, before yearend.

The new technology also means that platforms not only could be fewer but smaller as well. Unocal currently is discussing development of a field off Netherlands with a four slot platform instead of a 12 slot platform.

OTHER HORIZONTAL RECORDS

Aside from drilling the first trilateral horizontal wells, Unocal claims other records in its Offshore California horizontal program.

Its B-9 well, drilled in May, also set what Unocal believes is a world record for longest completed section at 10,300 ft, of which 7,754 ft is net pay.

An earlier conventional Dos Cuadras horizontal well, C-30, drilled from Platform C, holds the U.S. record for the highest average angle from start to finish, nearly 79, or an extension of more than 5 ft horizontally for every 1 ft vertically.

In addition, well A-21, drilled from Platform Irene in Point Pedernales oil field 20 miles northwest of Point Conception, claims two U.S. records. They are for longest horizontal reach at 14,671 ft and longest pay zone section at 5,990 ft.