SUBSALT DRILLING WOES DON'T DROWN ENCOURAGEMENT

G. Alan Petzet
Exploration Editor

High encouragement was the primary product of Phillips Petroleum Co. and Anadarko Petroleum Corp.'s latest subsalt oil discovery in the Gulf of Mexico.

Many key bits of information surrounding the Teak prospect discovery well, which is to be plugged, remain proprietary. Phillips, operator of the find on South Timbalier South Addition Block 260, said further evaluation is needed to determine whether the discovery is commercial.

Anadarko agreed but was more open about its optimistic view of the play's future in an extended conference call with analysts and reporters earlier this month.

Without giving depths, thicknesses, or areal extents of pay zones, Anadarko said it is highly encouraged by the results of the Teak well and the company's overall experience in to play to date.

Mechanical problems that will result in plugging relate to casing wear at the Teak well, drilled from a floating rig. The problems had nothing to do with penetration of the subsalt section, and Anadarko said it is adjusting its casing program design to compensate.

Anadarko said it has 20 subsalt prospects, all of which will be drilled. As of July 7, three were ready for drilling. They are Corundum near the play's west end, and Ebony and Birch near its center (see map, OGJ, June 20, p. 30). All three are held 50-50 bi, Phillips and Anadarko.

TEAK DISCOVERY

Phillips and Anadarko tested four zones below salt in the Teak discovery, known as 1 ST1 OCS-G-12037.

The intervals tested are at 12,000-16,000 ft measured depth, but the companies listed the zones as first, second, third, and fourth. Anadarko cautioned against drawing conclusions about the zones' relative vertical position in the wellbore.

This is because of the play's competitive nature. For instance, CNG Producing Co., New Orleans, is drilling another well on South Timbalier South Addition Block 289 within 5 miles of the Teak discovery. And Unocal Corp. and Conoco Inc. are drilling on Ship Shoal South Addition Block 360.

The first zone averaged 413 b/d of oil with 673 Mcfd of gas and 463 b/d of water flowing at 2,120 psi through a 16/64 in. choke.

The second averaged 276 b/d of oil with 1.096 MMcfd of gas at 4,200 psi on a 11/64 in. choke.

The third flowed as much as 3,742 b/d of oil with 5.988 MMcfd of gas through a 2-/64 in. choke with 7,220 psi flowing pressure. No water was encountered.

The fourth zone tested water with no hydrocarbons.

Total depth is 16,610 ft MD. Testing lasted 2 months, and the released flow rates are representative of performance of the zones tested, Anadarko said. A next well at Teak is likely in late 1994 or Early, 1995.

IS TEAK COMMERCIAL?

Anadarko engaged in considerable discussion about why commerciality is vet to be determined at Teak.

All zones indicated to be hydrocarbon bearing on logs were tested, but the highest flowing zone was not tested on a choke larger than "/64 in. Because of drill pipe wear and tear on the casing, the Teak wellbore could not be saved regardless of flow rate, Anadarko said.

The Teak discovery well evaluated part of a prospect that covers blocks 259, 260, and 283. Primary terms of the three leases end in 1995.

Phillips and Anadarko had 3D seismic data and depth migrated 2D data over the prospect, but the 3D data had not been pre-stack depth migrated by the time Teak was spudded in October 1993. No other subsurface well control exists in the prospect area. The 3D pre-stack data are in hand, and processing, interpretation, and engineering will take 2-3 months.

By comparison, the Phillips operated Mahogany wildcat on Ship Shoal South Addition Block 349 tested a structure that covers parts of two blocks, and another wellbore exists on the structure. That discovery's commerciality was immediately clear because of the additional data in hand.

Anadarko said it has mapped a very large prospective area at Teak but does not have enough information at this stage to know the true extent of the productive area.

Of three main Gulf of Mexico measures of productive potential-pay thickness, areal extent, and fault compartments-the latter two cannot be mapped at Teak at this time. Well test data can be extrapolated no further than 500-600 ft from a wellbore, Anadarko said.

DRILLING DIFFICULTIES

Casing wear and tear was the primary cause of cost and time overruns at the Teak discovery well.

The well cost approximately double Anadarko's initial $12 million estimate, and drilling took 8 months, not the 3-4 expected.

The well, in 295 ft of water, could have been drilled by a large jack-up, but rig rates for semisubmersibles were very competitive compared with rates for large jack-ups, Anadarko said.

Anadarko praised Phillips's geoscience and operational prowess on the Teak prospect. Anadarko also expressed confidence in its own ability to drill and set casing through horizontal salt.

Meanwhile, the 2 Mahogany confirmation well on Ship Shoal Block 349 is undergoing tests after being drilled "relatively trouble free" to 19,100 ft MD, Anadarko said. Amoco Production Co. is a 25% participant in both Mahogany wells.

The I Mahogany discovery, drilled to 16,500 ft in 370 ft of water, flowed 7,256 b/d of oil and 7.3 MMcfd of gas through a 32/64 in. choke in September 1993.

ENHANCING THE PLAY

Anadarko said the Teak discovery "greatly enhanced" the value of the Teak prospect and that the previously reported Mesquite dry hole on Vermilion Block 349 west of the play's midpoint "did not condemn any leased acreage."

Phillips and Anadarko plan to drill a wildcat late this year or early in 1995 on Ship Shoal South Addition Block 337. The block offsets the Mahogany discovery diagonally to the northeast and contains a structure covering about 3,500 acres that is about 500 ft updip to Mahogany field. This prospect is known as Alexandrite, and 3D pre-stack data have been acquired over it.

Allochthonous salt sheets as thick as 6,950 ft have been penetrated in the Gulf of Mexico.

Anadarko noted that Exxon Co., U.S.A. and Conoco Inc. drilled the Gulf of Mexico's first successful subsalt well in 1990 in 4,400 ft of water on Mississippi Canyon Block 211. They drilled through 3,000 ft of salt, below which then, encountered five hydrocarbon zones at 10,000-13,000 ft deep that could hold a combined 100200 million bbl of oil equivalent.