The relief wells currently drilling for BP at the Macondo well blow out site will intersect the blow out well bore just above the pay zones to enable a bottoms up kill of the well.
The relief well bore hole is drilled to a specific target intersection with the blow out well by means of a combination of directional drilling tools and well parameter measurement tools.
The drill string used to drill the relief well consists of sections (joints) of drill pipe assembled together to extend into the well bore. A Bottom Hole Assembly (BHA) is used at the bottom of the drill string which includes the drill bit, drill collars, and Measurement While Drilling (MWD) tools.
During drilling operations the drill string must be periodically pulled out of the hole to change out the drilling assembly replace worn bits or enable a well logging operation. Pulling the drill string is referred to as a trip. A round trip (pulling the drill string and running it back in again) takes hours as the drill crew must pull and rerun up to 18000 feet of pipe.
During standard drilling operations, the entire drill string is rotated from the rig's rotary table while applying weight down on the bit to cut through the earth. Drilling mud is continually pumped down the drill string exiting the bit and flushing the drill cuttings up the well bore on the outside of the drill string. The mud circulates back up the hole to the rig where it is filtered, de-gassed and stored for re-circulation.
Periodically while drilling, the MWD tool signals data back to a receiver station on the rig by wireless transmission. This data includes the angle of deviation from vertical and the direction of travel of the drill assembly.
Using the well log data collected while drilling the blow out well, the directional drillers calculate the required kick off and deviation angle of the relief well to intersect the blow out well at the planned depth.
In most cases, to change the direction of the well bore, the directional drillers use a mud motor. The mud motor is an assembly that includes sensors, a bent section of pipe called a sub and a drill bit driven by mud flowing through it.
At the planned kick off point for a directional change in the well, the rig stops rotating the entire drill string. The mud motor is engaged and the bent sub pointed in the direction the well must deviate. Using the mud motor to drill and not rotating the drill string directs the bit in the direction the bent sub is oriented.
The deviation is built as the drill bit progresses until it is on its new track. The mud motor is then disengaged and the rig drills ahead by rotating the entire drill string.
Periodically to verify accuracy the entire drill string is pulled from the hole and a logging system rigged up on the well. The well loggers lower a probe into the well suspended from an electric cable. The logging tool collects data about the well as it passes up through the bore hole. This data is compared to the measurement data received from the MWD tools to verify the well's progress.
When approaching the blow out well, a magnetic ranging tool is run in the BHA. The magnetic ranging tool detects the magnetic signature of the casing in the blow out well, transmitting the distance and direction of the casing back to the rig.
The relief well is drilled to within range of the magnetic ranging tool, to verify the blow out well location. The relief well bore is then directed parallel to the blow out well until the target depth is reached.
The mud motor is then engaged and the final approach to the blow out well is drilled.
As the relief well penetrates the bore hole of the blow out well, the relief well will begin losing mud to the blow out well. This signals the rig to begin pumping the kill mud.
The rig then pumps mud furiously until a balance is achieved and the blow out well is no longer taking fluid from the relief well. At this point the well should be dead at the sea floor.
The well will be observed for a period of time to insure it is stable while the rig prepares to pump cement.
The well will then be permanently sealed by pumping a calculated volume of cement into the wellbore using a specialized remedial cementing tool referred to as a Squeeze tool.
Run on drill pipe, the squeeze tool locks into the well casing and seals the annulus between the drill pipe and the casing.
A slurry of cement is mixed and pumped down the drill string exiting ports on the squeeze tool. Pump pressure forces the cement to penetrate the blow out well bore and fill any spaces between the well and the surrounding strata.
Once the cement has set, the blow out well will be accessible for permanent plug and abandonment.