Abstract

In July 1979, a project was established to determine the feasibility of using vertical boreholes to degasify the Pocahontas No. 3 coalbed in advance of mining. To date, three methane gas drainage boreholes have been completed near the Virginia Pocahontas No. 5 mine in Buchanan County, Va. Various Pocahontas No. 5 mine in Buchanan County, Va. Various coalbed reservoir tests and in-situ stress determinations have been performed. Hydraulic stimulation treatments were designed and implemented at two boreholes, followed by preliminary gas and water production testing.

Introduction

In July 1979, a cost-sharing contract was established between the U.S. Bureau of Mines and Occidental Research Corporation to determine the feasibility of using vertical boreholes to degasify the Pocahontas No. 3 coalbed in advance of mining. The Pocahontas No. 3 coalbed in advance of mining. The test area is located in Buchanan County, Va. where four relatively new deep mines are operating. This geographic area was chosen for the test because the Pocahontas No. 3 coalbed in Buchanan County, Va. Pocahontas No. 3 coalbed in Buchanan County, Va. ranks among the gassiest coalbeds in the United States. Moreover, the most recent tabulation of mine gas emissions ranks Buchanan County, Va. as number three in total mine emissions.

TEST SITE LOCATION

Three gas drainage boreholes have been drilled, along a straight line, ahead of developing longwall sections of Island Creek Coal Company's Virginia Pocahontas No. 5 Mine (fig 1). Borehole VVH No. 90 is Pocahontas No. 5 Mine (fig 1). Borehole VVH No. 90 is located approximately 91 meters (300 feet) from the mine. Borehole VVH No. 91 is 564 meters (1,850 feet) from the mine and is furthest from mining operations. Borehole VVH No. 92 is positioned between No. 90 and No. 91, 335 meters (1,100 feet) from the mine. The distance between each borehole is approximately 236 meters (775 feet). The boreholes are located and engineered in such a manner as to directly interface with the coal company's gob ventilation program. Each borehole is numbered by order of chronological completion according to the coal company's system for vertical ventilation holes (VVH).

DRILLING AND CORING

The drilling phase of the project began on September 30, 1979, and was completed on October 27, 1979. Each borehole was rotary drilled 3 to 6 meters (10 to 20 feet) above the Pocahontas No. 3 coalbed, geophysically logged, cased, and cemented. Pertinent completion data are presented in table 1. The remaining portion of each borehole was then rotary drilled to just above the coalbed. At this point, all rock above the coalbed, the coalbed itself, and a small interval of the rock below the coalbed were cored for various testing procedures.

Conventional coring operations at VVH 90 were completed on October 7, 1979, and at VVH 91 on October 18, 1979. Samples of the coalbed and adjacent strata were recovered for rock mechanics testing. Coal samples were also collected by the Bureau of Mines for direct method gas content determination. The results of desorption tests conducted on coal samples from each borehole are presented in table 2. Gas content values generated from desorption testing show that the Pocahontas No. 3 coalbed in Buchanan County, Va., contains the highest amount of in-place gas observed to date for a bituminous coalbed in the United States.

A novel method of coal core recovery was attempted in VVH No. 92. On October 26–27, 1979, a specialized core barrel developed by Exxon and modified by DOWDCO was inserted into VVH No. 92 in an attempt to recover samples of the Pocahontas No. 3 coalbed under in-situ pressure conditions. Unfortunately, during rotary drilling operations, the top of the coalbed was inadvertently exposed. The borehole was immediately filled with water and refilled continuously as the drilling tools were removed.

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