A Feasible Discussion on Exploitation Coalbed Methane Through Horizontal Network Drilling in China
Authors: Chi Weiguo , Yang Luwu 
Published: December 2007

Exploration coalbed methane through horizontal network drilling is a new technique, the principle of the method is to drill several horizontal wells to form a network or to drill several main horizontal wells and several branch horizontal wells to form a horizontal network like twigs, then coalbed methane is produced through pumping water. It has been succeeded using the technique in a area, west Virginia, USA, the average production of a single well is 28000m3/d, and comes into a commercial exploitation stage. There are low pressure and low permeability in most of the coal seams in China, the seepage condition of coalbed gas can be improved effectively using the horizontal network drilling and it's suited to the formation of low pressure, so it's recognized that it's feasible through this method for geology and engineering in drilling and producing, especially in such a areas which have a large volume of gas and a good coal texture or a high coal rank, and it will be contributed to the security of coal production and the environment protect of atmosphere extremely.

Foreword

Coalbed methane prospection and pilot exploitation with vertical well pumping water have been made in the north, northeast and south of China since 1990, up to now, more than 140 wells have been drilled and a significant breakthrough has been gained in some pilot areas such as Liulin and Jincheng, Shanxi province, Dacheng, Hebei province, and Tiefa, Liaoning province, etc., which gas production per well is 4000 m3/d from 3000 m3/d generally, the peak production reaches 16000 m3/d. the major progress has been obtained for coalbed exploration in China, but a large scale gas field hasn't been found with commercial development, and direct economic benefit hasn't been gotten. So some new methods and new techniques to be suited to geological condition of coalbed methane of China must be found as well as the method of vertical well exploration. In this paper we present and summarize the concept and the mechanism of a horizontal network exploitation and procedure in engineering based on the exploration other people did, and briefly introduce a successful example of development in USA. Considering the fact of coalbed methane exploration in China, we have discussed the feasibility of using the method through geology, drilling and producing in engineering in China.

The technique of coalbed methane horizontal network exploitation

The concept and mechanism and procedure in Engineering

The horizontal well network exploitation of coalbed methane means to drill several horizontal wells within a distance in a area to form a network (fig.1-a) or drill several main horizontal wells parallelling to each other in a direction in the area, meanwhile drill several branch horizontal wells on both sides of each main horizontal well with an angle to form a network like twig, then coalbed methane is produced through pumping water. General speaking, the type of fig. 1-a is suited to the coal formation with stable thickness and slope gently, and twig type is suited to the coal formation with a certain angle.

The mechanism of horizontal well network exploitation is similar to vertical well's which coalbed methane is desorped from the surface of coal grain through pumping water to drop the original formation pressure and flows to wellbore by diffusing and seeping, but the velocity of diffusion and seepage of coalbed methane in horizontal well network is more quickly than vertical well's during gas migration, This is because (1) coalbed methane diffusion to fissure from the surface of coal grain and microporosity follows Fics law, that isMJ=-DP×?C/?Y

The "negative sign" represents gas diffusion is to low concentration through high concentration.

The concept and mechanism and procedure in Engineering

The horizontal well network exploitation of coalbed methane means to drill several horizontal wells within a distance in a area to form a network (fig.1-a) or drill several main horizontal wells parallelling to each other in a direction in the area, meanwhile drill several branch horizontal wells on both sides of each main horizontal well with an angle to form a network like twig, then coalbed methane is produced through pumping water. General speaking, the type of fig. 1-a is suited to the coal formation with stable thickness and slope gently, and twig type is suited to the coal formation with a certain angle.

The mechanism of horizontal well network exploitation is similar to vertical well's which coalbed methane is desorped from the surface of coal grain through pumping water to drop the original formation pressure and flows to wellbore by diffusing and seeping, but the velocity of diffusion and seepage of coalbed methane in horizontal well network is more quickly than vertical well's during gas migration, This is because (1) coalbed methane diffusion to fissure from the surface of coal grain and microporosity follows Fics law, that isMJ=-DP×?C/?Y

The "negative sign" represents gas diffusion is to low concentration through high concentration.