Secondary well control technology 

When the level of well control expires, borehole fluid column pressure is less than the formation pressure, bottom hole in underbalanced condition, and then through the shut-in, the control of the throttle valve opening, implement throttling circulation, cause certain in wellhead back pressure, the use of the back pressure and well within the sum of the rest of the fluid column pressure to balance formation pressure, inhibit the formation fluid to the invasion of the borehole, and to exclude the formation fluid in well with heavy slurry pumping Wells, rebuild the balance relationship between borehole and formation, back to normal drilling well control technology. 

1. The cause of the overflow 

Bottom hole pressure is less than the formation pressure is the primary cause of overflow, and the causes of bottom hole pressure is less than the formation pressure is multifaceted. 

(1) pressure control is not accurate, design the drilling fluid density is low 

(1) the new exploration area, the new block formation pressure are not familiar with 

A well, for example, in the depth of 3000 m open the oil and gas layer, drilling fluid density of 1.25 g/cm3. Due to the formation pressure equivalent density is greater than in drilling fluid density of 1.25 g/cm3, and produce gas blowout. 

(2) development zone water injection caused by formation pressure (or high) 

With 1.20 g/cm3, for example, a well to open the equivalent density of drilling fluid density is 1.35 g/cm3 of formation pressure, the results in the 2300 m blowout. 

(2) the borehole drilling fluid height 

(1) the drilling failure to fill up when the well's mouth 

A well, for example, when the drill not timely into drilling fluid, gas cut, the results cause fire in the blowout. 

(2) well drilling fluid in well leakage caused by the liquid column height decreased 

Lost circulation, borehole hydrostatic column pressure drop, if the borehole hydrostatic column pressure of bottom hole pressure is not enough to balance formation pressure, easy cause overflow or blowout. So when the lost circulation to spray in a timely manner. 

(3) the drilling fluid density decreased 

(1) drilling and high pressure oil and gas layer, oil and gas invasion of drilling fluid and drilling fluid density decreased. 

(2) in the treatment of the downhole accident, to the borehole card agent into the crude oil, diesel oil, or other solution, caused by borehole hydrostatic column pressure drop. 

(3) a high Angle directional well and horizontal well, in order to reduce friction between tool and borehole wall, the borehole mix oil or friction improver, caused by borehole hydrostatic column pressure drop. 

 (4) the drilling pump drew overflow 

 Due to borehole and the annular clearance is too small, drilling fluid performance variation, such as viscosity, shear force is too large, the drilling speed is too large, the drill still cycle time is too long or borehole safety before additional quantity is small, big suction pressure, make the bottom hole pressure is less than the formation pressure, producing the overflow. Pull out before should check can balance the formation pressure, bottom hole pressure whether suction overflow has occurred. The inspection measures are as follows: 

Method: (1) short trip before the formal start working, since well within the first 5 ~ 10 column pipe, and then down into the well, open pump cycle at least one time late, the drilling fluid of return observed. Such as are found in the drilling fluid of oil or gas, then the suction overflow, then can not continue the drill operation, should be tested for throttling circulation or increasing the drilling fluid density, until the drill pump overflow won't happen again. 

(2) verify the amount of drilling fluid in the pumping well