Abstract

The coalbed methane industry has become an important source of natural gas production. Proper dewatering of coalbed methane (CBM) wells is the key to efficient gas production from these reservoirs. This paper presents the Hydraulic Gas Pump as a new alternative dewatering system for CBM wells.

The Hydraulic Gas Pump (HGP) concept offers several operational advantages for CBM wells. Gas interference does not affect its operation. It resists solids damage by eliminating the lift mechanism and reducing the number of moving parts. The HGP has a flexible production rate and is suitable for all production phases of CBM wells. It can also be designed as a wireline retrievable system. We conclude that the Hydraulic Gas Pump is a suitable dewatering system for coalbed methane wells.

Introduction

Coalbed methane has emerged as a significant source of natural gas. The original perception of coal-associated methane as a hazard in mining operations is changing. Today a coalbed is considered a reservoir from which large quantities of methane can be extracted.

Between 1983 and 1993, industry drilled an estimated 6,600 coalbed methane wells in the United States that account for nearly 5% of domestic natural gas production.

Production of coalbed methane is widespread in the US; however, production is primarily centered in the San Juan and Black Warrior Basins. The CBM gas is now estimated to account for some 17% of total recoverable gas reserves in the US. Early studies by the Gas Research Institute (GRI) and others estimate the size of the CBM resource base at approximately 400 TCF of gas-in-place for the continental United States.

Vast CBM resource potential also exists in many other countries. In some of these countries these resources dwarf the conventional gas potential and may be the only significant hydrocarbons present. Ref. 7 estimates a potential world coalbed gas resource of 4,000 to 7,000 TCF.

Coalbeds are naturally fractured, low pressure, water saturated gas reservoirs. The mechanism by which gas is stored and produced in coalbed reservoirs is quite different from sandstone reservoirs. In a conventional sandstone reservoir, gas is stored in the pore space and flows through the pores into the fractures and/or the wellbore In a coal-seam reservoir, while some free gas may exist in the coal deposits, the majority of the gas is adsorbed on the surface of the coal matrix. To produce this gas, the reservoir pressure must be reduced so that the gas can be desorbed and released from the matrix into the fractures. The gas can then migrate through the fractures and coal cleat system and flow into the wellbore.

Initially the natural fractures of the coal are typically water saturated. This water has to be removed in order to achieve any significant gas production. Dewatering of the coal seam reduces the hydrostatic pressure of the reservoir, which allows the gas to be desorbed from the coal matrix. At the same time, lowering the water saturation level of the reservoir increases the relative permeability of gas, thereby permitting the desorbed gas to flow to the wellbore. The maximum gas production is achieved when the BHP is minimized.

A proper dewatering system is essential for the production of methane from most coalbeds The most common systems for dewatering coalbed methane wells include sucker rod, electric submersible, progressive cavity, and gas lift. These systems have all been used in the field. Each has its own unique advantages for some situations and its own limitations. The advantages and limitations of these methods will be further discussed throughout this paper.

This paper presents the theoretical application of the Hydraulic Gas Pump as a new method for dewatering of CBM wells. The operation and the advantages of this method will be discussed. It concludes that the Hydraulic Gas Pump concept can be a suitable dewatering system for coalbed methane wells.

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