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Ahtna completes its Tolsona well, plans flow tests for potential gas
On Dec. 5 Ahtna Inc. completed the drilling of its Tolsona No. 1 gas exploration well 11.5 miles west of the town of Glennallen, the Alaska Native regional corporation for the Copper River region has announced. The well reached a vertical depth of 5,500 feet; the drilling rig is being demobilized; and Ahtna plans to begin flow testing for gas in mid-December. The testing will involve perforating the well casing in a roughly 220-foot section identified from well logs at depths between 5,000 and 5,220 feet. The well was drilled to seek natural gas in the Nelchina sandstone in state land within the area of a state exploration license.
If an initial flow test shows positive results, Ahtna expects the subsequent data analysis to take about two months, to gain a better understanding of the potential of the prospect.
“We have learned so much throughout this drilling program and are thankful to have successfully reached our target and completed this important phase,” said Michelle Anderson, Ahtna president. “It would not have been possible without the relentless commitment of our drilling team, guidance from other owner/operators, and the support of the Ahtna board and leadership. We are very excited about the potential of this well and the impact it could have on our region if the testing proves positive.”
A gas prone region The region of the drilling is known to be gas prone. A previous gas exploration well, the Ahtna 1-19 well, drilled about two miles east of the Tolsona well between 2005 and 2007, encountered gas but eventually had to be abandoned because of exceptionally high formation pressures and problems with water encroachment.
Based on that experience, the drilling team for the Tolsona well took mitigation measures to guard against problems with high-pressure subsurface water. Those mitigation measures proved effective - Ahtna said that the drilling team successfully isolated the potential gas zone from the high-pressure water zone above it.
However, with complex subsurface geology, the well did prove challenging to drill. In fact, drilling continued to a depth 700 feet deeper than had originally been planned. Ahtna said that Schlumberger, the rig owner, and the next planned operator of the rig had both been cooperative in accommodating the resulting additional time required for the drilling - the well was finally completed in 70 days with no recordable safety incidents, Ahtna said.
Sparse data Dan Lee, Ahtna oil and gas development manager, told Petroleum News that the challenges for the drilling mainly emanated from sparse data about the subsurface geology.
“One of the biggest challenges is the lack of information,” Lee commented, reflecting on the sparse data from wells previously drilled in the region.
Ahtna does have 2-D seismic of the target area, but the geology is complex in detail. And, although the well is being drilled not far from the Ahtna 1-19 well and in a similar geologic setting, the two wells are in different fault blocks with subtly different geology.
“The thicknesses of the zones and the depths at which we encountered the zones are different in all the wells,” said Drilling Manager Marty Lemon. “We had to drill the well deeper than we had proposed because the formations were coming in deeper in the fault block that we were in.”
Apparently the high pressure subsurface water, which appears to be a feature of the region, permeates fractures in the rocks. The drilling team used especially large diameter casing strings to enable additional casing to be run and cemented when water was encountered, Lemon said. The drillers also used a technique called managed pressure drilling, to hold back the pressure in the well from the surface, he said.
A possible gas resource Although Ahtna does not yet know whether it has a gas find, log data obtained from the well does indicate the possibility of a gas resource. Lemon said that there are five distinct intervals of interest for the testing.
Ahtna’s prime interest in its gas exploration program is the possibility of finding a gas resource that could alleviate the high cost of energy in the Copper River region. Exploration in a region remote from specialized oil and gas support services and infrastructure is challenging. But a project such as the drilling of the Tolsona well brings valued employment and business for residents of the Copper River region, including Ahtna’s shareholders. In addition to direct work on preparing the drilling site and conducting the drilling operations, local businesses, including shops and lodges, have seen an uptick in business at a time of year when business is traditionally slack.
And the experience of involvement in the drilling of a well brings a sense of practical purpose and achievement in furthering the economic well being of the region, Ahtna thinks. Ahtna has also indicated that it is “open for business” and welcomes external investment in its exploration efforts. Further exploration opportunities exist in the region.
- ALAN BAILEY
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