Study on the Mechanism of a Multi-branch Horizontal Well in Enhancing the Production Performance of Coal-Bed Methane Reservoirs in the SY Block, Qinshui Basin

Abstract

In recent years, with the government’s continuous attention to coalbed methane (CBM) exploration and development and the scholars’ extensive research on CBM, China CBM industry has achieved rapid development. It has grown from an annual production of 0.3 billion cubic meters in 2005 to 4.96 billion cubic meters in 2017, forming two important CBM production bases in the Qinshui Basin and the eastern margin of the Ordos Basin. However, the rapid increase in annual CBM production mainly depended on the increase in the number of producing wells because of the low average gas production of CBM wells. There are many reasons to explain why the single well production is so low, but the principal one is that geological conditions of coal reservoirs are complex, such as low permeability, under-pressure, strong plasticity and so on. Besides, there is another important and crucial factor, affecting the production performance, that the exploitation technology is relatively simple and unsuitable for those geological conditions. This study takes the SY block located in the north of the Qinshui Basin as an example to analyze the production performance and technical advantages of a multi-lateral horizontal well as well as the production performance of the adjacent vertical wells. The results show that, for low permeability coal reservoirs, multi-lateral horizontal wells have great reservoir contact area compared with vertical wells with hydro-fracturing, which is beneficial to the rapid drainage and depressurization of coal reservoirs, causing the peak shifting forward and enhancing economic returns. The horizontal section, extending from the low part of the structure to the high, is conducive to change the gas-water flow characteristics with the gas-water gravity differentiation effect so that water flows to the lower part by gravity effect and gas flows to the high part by buoyancy effect, avoiding the presence of massive gas-water two-phase flow and Jamin effect to consuming reservoir energy, which is particularly advantageous for the development of under-pressured reservoirs. The special wellbore trajectory facilitates on-site workover and drainage management to ensure the continuity and stability of coal reservoir drainage. It is obvious that multi-branch horizontal wells have better technical advantages in developing low-permeability, under-pressure CBM reservoirs, which is the typical ones in China, than vertical well fracturing wells do.KeywordsCoalbed methaneMulti-lateral horizontal wellFlow characteristicQinshui basin