Scribing Up.

Another fabulous day on the rig, another BHA(bottom hole assembly). This means we have changed out all the downhole drilling equipment for another set-up. Why? Well we were trying to kick off from the present well path after leaving part of another BHA in the hole. So we have to be able to steer hole away from the “fish” or other items lost in hole.
There are, as always, more than one way to skin a cat. With this BHA we are using a motor with a bend in it. Which is exactly that, a mud motor, which turns the bit and nothing else as you pump mud through it. Above this spinning bit is a bend of 1.4º that can be oriented when we get down to the bottom to point the bit in the direction you want to go. So all you have to do is be able to tell which way the bit is pointing when it is 10,000ft away.
Imagine you have a 1,000ft long drinking straw and you turned one end half a turn, how much would the other end turn? Not one bit. It would just twist up. No imagine a piece of drill pipe, same problem. There is no way you can mark up a pipe at surface and know where the end is pointing. Enter the MWD engineer. We have a tool in the hole that will tell us which way is up, using magnetometers measuring the magnetic field of the earth and its just behind the motor.
Now we get to the fun part. When everything is screwed together the bend in the motor will never line up with the reference line on the MWD tool. So we have to measure the angle between the “highside” of the MWD tool and the direction the bit is pointing in. This is the Scribeline Offset. It is a very very very important measurement. If it is wrong wells have been drilled in entirely the wrong direction, which might lead to some very probing questions.
The Directional Driller marks the highside of the motor. And then transfers this highside line up the tool to the MWD scribeline. Then he marks that line, with a torch strapped to the MWD tool or a polystyrene cup and the whole thing is lifted into the air and the line on the motor is checked with the cup or torch. Any adjustment is made an the tool lowered down again. The angle from the MWD scribeline to the Motor highside is then measured, from us to them, clockwise looking downhole. The scope for error at this point is huge, as you have just introduced a human element. At this point we are like airline pilots one wrong move could cost lives! Well, not really, but you could cut the tension with a knife. The tool is run into the well. Highside is pointed where we want to go and mud pumped through the string to make the bit, and the bit only, turn. This is Sliding, drilling without rotating, and the bit will slip slide away from the old hole into a whole new place of its own.