Since the first commercial sale of Coalbed Methane (CBM), the process of drilling and producing methane with multilateral horizontal wells has been a "win-win" plan for the entire coal industry in North America. It creates a revenue stream by exploiting a valuable resource while de-gassing coal and lowering safety risks ahead of mining operations. Modern CBM wells are usually drilled up dip, in a tri-lateral "Turkey Track" pattern. Each lateral extends approximately 4000' beyond the horizontal build section.
Normal mining operations allow approximately ten years of production to produce and deplete the adsorbed gas contained in the coal. Recent increased energy demands with favorable market increases have accelerated the mining schedules and shortened the available gas production time. These factors have lead to hazardous conditions encountered when mining through the yet depleted lateral well bores where dangerous volumes of trapped methane can inundate the occupied mine work area. Currently, mine operators are forced to use water infusion and in-mine drilling and grouting operations to safely intersect the horizontal well bores. This results in costly mine production delays.
The best solution requires a stable medium that can be positively placed through the extent of the well laterals and effectively displace and fill void spaces where methane gas can accumulate. This paper will explore and evaluate the results of applying variety of products, equipment, and techniques solve this quandary.
Coalbed Methane (CBM) development originated with vertical wells patterned to most effectively drain or deplete the methane gas trapped and adsorbed in the cleats and matrix of the coal. Feasible drilling technology progressed from vertical to simple single lateral horizontal wells, then continued forward to include very complex multilateral, multi-well systems designed to de-water coal seams and free entrained and trapped gas for flow to the surface facilities. Thousands of these wells were drilled in the coal fields across North America designed for the purpose of effectively producing methane gas from the coalbeds with little regard given to safely plugging and abandoning the well bores when depleted or mined through. The huge demand for energy in recent years has accelerated mining schedules to the point that CBM wells are being encountered much sooner than originally anticipated. The safety issue facing the industry now is finding a method to properly plug the horizontal laterals without leaving voids of entrapped methane that can be a serious hazard when unexpectedly encountered during mine operations. The resulting condition can be more dangerous than the problem that CBM wells were supposed to eliminate in the first place.
The purpose of this paper is to identify and describe an unexpected safety issue that is directly related to horizontal CBM wells with lateral branches that have been drilled into coal seams for methane production and coal degassing prior to mining. The report will also describe the current experimental methods being applied in an attempt to solve this problem by plugging the wells. These approaches involve specially designed cement slurries, placement techniques, polymer gel plugs, water infusion, grouting and various combinations of each. The pros and cons of each method will also be presented with recommended future procedures that may be more effective.