The new well needs to be able to control pressure on the new well it drilled and control pressure on the well it is tapping into.
While drilling a well they control pressure on by using what they call mud. To control the pressure the weight of the mud needs to meet or exceed the pressure of the well at the depth they are drilling. The formula is quite easy- Depth in feet x pounds per gallon of fluid x .052. This will give you the force in psi that the column of fluid is exerting at a certain depth. At 18,000 ft that mud weight is going to have a lower ppg than at 18,000 ft.
Also, when they tap into the well they are going to be losing mud to the well they are taping into and are going to need to have the ability to replace the mud at a rate as high as the other well is going to be taking on that mud. If the other well takes on the mud at a higher rate then the new well will lose pressure control of the new well they drilled.
It should read at 18,000 ft. the ppg of mud weight the rig needs will be lower than at 8,000ft.
The other thing I'm interested in is how do they guide and control the drill bit. Your trying to tap into a small drill hole at 18000 feet or so and at an angle to boot. Some tricky vectoring control ...no?
thanks - i do get the gist of what you are saying and appreciate the explanation