New Technique: Multistage Hydra-jet Fracturing Technology for Effective Stimulation on the First U-shape Well in Chinese Coal Bed Methane and Case Study
Authors: Gokdemir Oktay Metin , Liu Ying , Qu Hai , Cheng Kun , Cheng Zhong 
Published: May 2013

Coalbed methane (CBM) is an important energy resource is a kind of unconventional gas stored in coal bed and adjacent formation. China is rich in CBM and has broad application prospective. However, the characteristics of CBM reservoir in china are low permeability, low pressure, low gas saturation, and the gas production from CBM wells is disappointment. In order to improve this situation, the U-shape well, which composed of a horizontal well and a vertical well, are firstly used in the exploitation of CBM. The purpose of horizontal well completed with screen pipe is to increase coal area and place multiple fractures. Then the vertical well is to drainages water to decrease bottom pressure for desorption of CBM.

Therefore, multistage hydrajet fracturing technology (MHJF), which depends on jet stream to achieve sealing for stimulation without mechanical devices, is the only method capable of performing the horizontal well with screen pipe. Because of the completion of screen pipe, several improved methods were used to overcome the serious difficulty of sanding in the fracturing string. For example, the jetting fluid with sand was discharged from the vertical well to decrease the number of settling sand in horizontal section. The sanding risk of screen pipe horizontal well has been precipitous declined.

A novel multistage hydra-jet fracturing job was performed successfully on the horizontal well in the U-shape well. Four fractures were placed along horizontal section from 1,680m~1,175m. During field operation, active 1,500m3 water was mixed with 95.8m3 proppants in total and pumped down the well. Finally, the fracturing string was pulled out of the well. The results showed there was no gas output from U-Shape well, but the production increased to the max 636 m3/d after hydraulic fracturing because of water-discharging and pressure-dropping, and the gas rate will gradually increase as the further job of water discharge.

The improved stimulation method has been proven by successful fracture treatment. Therefore, the well completed with openhole or preperforated liner can also be solved. In addition, it can stimulate the offshore well to place multiple fractures.

Introduction

As a result of rapid growth of economy, consumption of natural gas in China has grown dramatically in recent years, while domestic conventional natural gas cannot meet the demand. The coalbed methane resource is extremely large in China. According to a survey covering 45 coal bearing basins, coal gas areas with a buried depth less than 2,000 m is 41.5×104 km2. And the total CBM resource is around 36.81×1012 m3, which equals the traditional natural gas, making up about 13% of the world total CBM resource. The coal gas is mainly distributed in 9 basins, such as Ordos and Qinshui basins with geological resources of more than 10,000×108 m3. However, the coalbed methane reservoirs in China generally have the unfavorable characteristics of low reservoir pressure, low permeability, low gas saturation, strong anisotropy and the gas production from CBM wells is disappointing. Vertical wells with cement casing or cavity completion are normally used in Chinese coal gasfields. Cement casing wells are perforated and fractured in the coal seam for increasing near wellbore permeability; Cavity completion prevented the slurry pollution keeps the natural characteristics of coal formation. They are two relative low cost well types but the drainage areas are limited. Hence, it is necessary to improve gas production by optimization engineering technologies, such as drilling, completion, and hydraulic fracture.