Considerations for Water Disposal and Management in the Development of Coalbed Methane Resources in Rural Alaska

Abstract

The objective of this study is to evaluate the alternatives for water disposal and management during coalbed methane production in remote arctic conditions. In this paper, we evaluate five methods of water disposal in the arctic environment. These methods are (1) direct discharge onto the surface, (2) controlled discharge into streams, (3) underground injection using wells, (4) concentration of produced saline water in a multiple effect evaporator, and (5) conversion of wastewater into clean water by reverse osmosis, a submerged combustion evaporator, and a pulse combustion dryer. The methods were ranked based on a set of criteria including logistics and engineering challenges of water treatment and processing in the arctic, environmental regulations, costs of installation, operations and maintenance. We conclude that surface discharge by use of ground sprinklers is not acceptable in the arctic; use of injection wells and the freeze thaw/evaporation processes hold promise for dewatering coalbed methane wells in rural Alaska.

Introduction

Coalbed methane has become an important source of natural gas production in the Lower 48 States and other places around the world. One current estimate shows that coal basins in the State of Alaska contain over 28 to 43 trillion standard cubic metres (1,000 to 1,500 trillion standard cubic feet) of coalbed methane. Although the actual extent and magnitude of producible gas from Alaska's remote coal basins are unknown, the Alaska Department of Natural Resources postulates that if only 10% on the estimated 28 trillion cubic metres of methane gas is recoverable, this coalbed methane resource would triple the State's current proven conventional gas reserves(1). Exploratory drilling is the key to delineating the potential of the coalbed gas resources of Alaska.

Coalbeds are naturally fractured, low pressure, water saturated gas reservoirs. In a coalbed methane reservoir, even though some free gas may exist in the coal deposits, the majority of the gas is adsorbed on the surface of the coal matrix. Proper dewatering of the coal seams is a critical factor associated with gas production from coalbed methane reservoirs. The saline water that is trapped within the deeply buried coal seams must be removed in order to reduce the pressure of the system, and allow the gas to desorbs from the coals and flow into the wellbore. Large amounts of water production can be anticipated during exploration drilling and early life of the producing field.

There are three different mechanisms by which water can be stored in coal seams. First, adherent moisture or bulk moisture refers to the free water contained in the cleat system that has a normal vapor pressure(2,3). This is the volume of water that must be removed during the dewatering phase in the coalbed methane production operations. Second, inherent moisture or absorbed moisture is the water in the microspore system that decreases the adsorptive capacity of the coal for methane(3). Although this type of water storage is inconsequential when considering water disposal, it is extremely detrimental to the gas concentration of the coal. The other forms of water found in coal seams include chemically bound water and water of hydration.