Underbalanced drilling method and system for coalbed methane

Abstract

The invention discloses an underbalance drilling method and system for coal bed gas. The method comprises the following steps that an intermediate casing is lowered in, and a gas injecting pipe and a liquid injecting pipe which are communicated with the interior of the intermediate casing are arranged on the outer portion of the intermediate casing; after a destination layer is entered and after underbalance is established through the gas injecting pipe, a drilling pump and a gas pump set working continually; when one single drill pipe or drilling stand column is drilled and the preparation work of drilling a next single drill pipe or drilling stand column is carried out, the drilling pump is used for injecting and switching drilling liquid into the liquid injecting pipe; the preparation work of drilling the next single drill pipe or drilling stand column is completed, and the drilling pump is used for injecting and switching the drilling liquid into a drilling column. According to the underbalance drilling method and system, borehole purification is maintained in the drilling and non-drilling states, the occurrence of sand setting and drilling tool jamming accidents is effectively prevented, and even underbalance well bottom pressure can be maintained continually.

Description

technical field [0001] The invention relates to the technical field of coal bed gas exploitation, in particular to an underbalanced drilling method and system for coal bed gas. Background technique [0002] At present, in the field of coalbed methane exploitation, the drilling method adopted is usually underbalanced drilling. Underbalanced drilling means that the pressure of the drilling fluid column on the bottom of the well during the drilling process (including the hydrostatic pressure of the drilling fluid column, circulation pressure drop and wellhead back pressure) is lower than the formation void pressure at the bottom of the well. [0003] Due to the special composition and diagenetic properties of coal reservoirs, poor mechanical stability, low permeability, adsorptive storage, and low gas saturation, the physical properties of the reservoirs are weak. Therefore, the underbalanced drilling technology of coalbed methane needs to consider factors such as the stabilit...

AI Technical Summary

Problems solved by technology

When the next single drill pipe or drilling column is connected, a period of preparation work is required. During the preparation work, the drilling pump injecting drilling fluid must be stopped, because the drilling pump fluid injection and the gas injection of the parasitic gas injection pipe have reached dynamic Balance, in order to maintain the dynamic equilibrium state, the gas injection cannot be continued after the pump is stopped for liquid injection. Therefore, during the pump stop period, there is no gas-liquid two-phase flow in the vertical section and the build-up section of the horizontal well, and impurities such as cuttings cannot be driven. Cause debris such as debris to sink, if during the stop of the pump, too much debris is deposited and the drilling tool is buried, the deposited debris and other impurities cannot be washed away when the pump is started again, resulting in a stuck drill accident

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