When drilling the air layer, because the rocks are destroyed, the gap of the rock have a large of gas to immersion the drilling fluid reach the big crack or cave gas reservoir. It is maybe appear the large of gas of replacement immerse the drilling fluid.

1 Characteristics

(1)The density of the different of depth of the gas invasion is different .

(2)The density of the gas invasion drilling fluid will be very small when it is close to the ground, even if the drilling fluid gas invasion of the ground is very serious, the density is reduced, but the decrease of the pressure of the drilling fluid column is not very big, this time we can’t use the density of the ground gas invasion multiplied by the depth of the well to calculation the liquid column pressure.

(3)Because of long time to stop cycle (change the rig head, maintain the pump or electrical logging and so on), there is a large of gas column which is formed by nature gas under the well, when it rise to expansion, maybe led to the drilling fluid overflow.\

(4)After the drilling fluid gas invasion and closed, because the density difference, the nature gas will slip up, finally accumulate on the well month.

(5)In the case of the gas rise but it is not expansion , the pressure of the ground unequal to the pressure of the well month plus to the pressure of drilling fluid column, so we can’t use this pressure to calculate the required drilling fluid density.

2The damage of the gas invasion

(1)Reduce the density of the drilling fluid, after the drilling fluid density reduced, if the liquid column pressure is inferior to the ground pressure, it is maybe led to the well kick or blowout

(2)Make the absorption of the desander pump, priming pump and the drilling pump is bad, even can’t work normally, cause the device vibrate.

(3)Part of the toxic gas overflow from the drilling fluid, the operating personnel maybe poisoning.

(4)It can lead to fire.

(5)After the gas invasion, drilling fluid density will be increased, make the mechanical drilling rate decreased.

So how do we processing the gas invasion ?

The common methods of disposed the gas invasion

1 Circulation drilling fluid degassing method

It use the mechanical equipment continuously circulation drilling fluid on the ground, make the bubbles emersion the liquid level and overflow, the method only rid of the bid bubbles. The bubbles of diameter of less than 1mm, especially the bubbles of high viscosity, the foundation of cycle degassing is small. When use the methods to degassing, in order to reach the degassing effect, required long times, the drilling industrial is not allowed.

2 Chemical degassing method

Adding chemical gas degassing to the drilling fluid gas invasion, the method adopt to rid of the own bubbles. Use the chemical degassing method, need to mix and the time of reaction, so need long time, the high cost. The foundation is small to the gas of external invasion.

3mechanical degassing metood

Using all kinds of mechanical equipment, according the degasser clear the gas which immersing the drilling fluid. The methods apply to all kinds of the drilling fluid degassing. The mechanical degassing divided into ordinary pressure, vacuum type and the centrifugal.

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