Unocal Targets Coalbed Methane Resource North of Anchorage

Sept. 6, 1999
In a new search for gas in Alaska, Unocal Corp. has turned its attention to coalbed methane (CBM), in the Matanuska Valley about 40 miles north of Anchorage.

In a new search for gas in Alaska, Unocal Corp. has turned its attention to coalbed methane (CBM), in the Matanuska Valley about 40 miles north of Anchorage.

The company has embarked on the Pioneer Coal Bed Methane Project, targeting high-BTU gas.

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The Pioneer Unit encompasses a substantial CBM prospect in the lower Matanuska Valley on the northern end of Cook Intlet (Fig. 1). While the possibility of conventional sandstone and conglomerate hydrocarbon reservoirs exists in the prospect area, the testing of CBM potential is the project's primary objective. Abundant coal has been recognized in the stratigraphic section throughout the Cook Inlet basin, with estimates of up to 245 tcf of total gas in place (Fig. 2). Success in the project could lead to expanding the play into the rest of the Cook Inlet area.

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The 72,000-acre Pioneer Unit is located between the communities of Wasilla and Houston in the Matanuska-Susitna Valley, an area of sporadic to moderate residential use. Proposed wells will be drilled in relatively sparsely developed wooded and previously cleared areas, avoiding wetlands.

Unocal will kick off the Pioneer project with the Vine Extension Pilot Project, in the south central portion of the unit.

Water disposal

Safe water disposal concerns are uppermost in the mind of any CBM well operator, because CBM wells commonly produce large quantities of water.

It is especially critical to identify the entire stratigraphic section containing all of the potential fresh water aquifers in order to design safe cementing and casing programs for both producing and disposal wells in order to protect the fresh water supplies. Alaska regulations require that application for a fresh water aquifer exemption is required if the potential disposal zones contain fluids with total dissolved solids of 3,000-10,000 ppm; most zones within the Tyonek section targeted by the project will contain fluids within those limits. Accordingly, Unocal applied for an exemption for its Class II waste disposal well.

All of the project's potential disposal zones are permeable zones within the Tyonek formation. Because it is a sand-rich system, it is anticipated that there will be no problem identifying the zones. Examinations of wireless logs from the exploratory deep wells within the unit, the Pittman No. 1 and the BLT No. 1, indicated that there are a number of potential disposal zones below 1,000 ft within the unit. Numerous confining layers existing above 1,000 ft in both wells will prevent upward migration of wastes into any fresh water aquifers, Unocal said.

However, because the Tyonek sands and conglomerates are lenticular and discontinuous, it is uncertain that specific potential disposal zones in those wells will exist at the proposed Class II waste disposal well location. So Unocal plans to identify disposal zones and overlying confining zones in the proposed wells from wireline and mud logs after drilling the well.

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Exploration plan

On Mar. 25, 1999, unit operator Unocal submitted a revised plan of exploration for Apr. 1, 1999-Apr. 1, 2000.

At the time, Unocal was negotiating with Ocean Energy Resources Inc., a Houston-based company with extensive experience in coalbed methane development, and Unocal then noted its earlier, approved plan of exploration might require modification to more accurately determine the economic viability of its CBM development approach.

Unocal and Ocean subsequently agreed to a modified exploration plan for the Pioneer Unit and are trying to hammer out a contract based on that new plan.

The present plan calls for three wells to be drilled and one well to be reentered prior to Sept. 1, 1999. The unit obligations also include five additional wells to be drilled prior to Jan. 1, 2003.

Ocean came up with a more effective, accurate way to validate the economic viability of Unocal's CBM development by clustering the wells into pods. This approach accelerates the dewatering process and thus the determination of commercial viability. The new plan and the pod's location will enable Unocal to start production of gas from the unit much sooner than under the earlier, approved plan, so Unocal modified its exploration scenario to provide for this more-effective initial pilot program.

Pilot

The Vine Extension pilot includes a drilling program comprising drilling and coring of at least two wells and possibly one Class II waste disposal well in Sections 14, 15, and 22, 17n-2w, in Area A.

The wells are programmed to minimum depths of 2,500 ft. At least two test wells would be drilled by Oct. 1, 1999, and production testing begun immediately upon completion of the drilling phase. One of the test wells will be drilled deep enough to test the feasibility of deep wastewater disposal. If the idea is not feasible, the Class II wastewater disposal well will be drilled. Also by Oct. 1, 1999, the potential of Area D, on the western edge of the unit, will be assessed by the reentry and testing of selected intervals within the ARCO BLT No. 1 well.

Within the proposed initial program, coal seams will be cored in at least two wells and measurements of gas yield, adsorption, cleat density, and permeabilities made. Fracture stimulations will be attempted on at least two wells, if necessary. If the reentry is not completed by Oct. 1, 1999, Area D will be relinquished from the unit. If the two wells are not drilled in Area A by that same date, Unocal said, the unit will be terminated.

If the project demonstrates economic viability of coalbed methane development, at least one exploration well would be drilled in each of Areas B and C to a minimum depth of 2,500 ft and testing begun by Dec. 31, 2002. Prior to Dec. 31, 2002, a total of at least eight wells will be drilled and tested as producers to depths greater than 2,000 ft, and the ARCO BLT No. 1 will be reentered.

If by Dec. 31, 2002, at least one well is not drilled in Areas B and C, each of the undrilled areas will be relinquished from the unit. If eight wells have not been drilled throughout the unit prior to Dec. 1, 2002, the unit will be reduced to include only the areas within approved participation areas at that time. The well locations encompass a 11/2 sq mile area 4 miles southwest of Wasilla.

The drillsite pad sizes will be 300 ft by 250 ft. Berming around the pad will prevent extraneous fluids, e.g., surface water, from entering or escaping the drillsite area. Flare pits will then be excavated on the production well pads. Drill pads will be constructed to promote drainage on the grave pad area toward, but not into, the flare pit.

One rig will be used to drill the wells. Drilling and completion of the wells is expected to require up to 30 days for each well.