Page | 130
26. l fW ll
ll ng
It is now a requirement in many parts of the world to have a contingency plan in place for a potential relief well. This section of the manual sets out the main positioning considerations for such an operation and these have a very significant effect on well planning, especially for complex and crowded pads and platforms.26.1 Magnetic Ranging.
All steels are magnetic to some degree but the mild steel, used in casing and lining is very magnetic. When steel pipe mgn ssn u llynh h’smgn cf ld ndsomgn s whh oppos pol yoh E hF ld. This means that pipe that has been stored together tends to have a change of polarity at every thread joint and a strong detectable magnetic field along its length. A magnetometer placed close to the steel, will measure the vector sum of h h’s m gn c f ld nd h steel magnetic field. A magnetic influence increases with the inverse square of the distance from the source. If we hold a magnetometer at half the distance we see 4 times the field strength. As sul h s ng h of h h’s m gn c f ld c n be considered to be relatively constant due the fact that the source, however strong, is so far away, but the influence of nearby steel will vary considerably depending on our proximity to it. There are two broad types of magnetic ranging known as Passive and Active. Passive Magnetic Ranging uses a standard MWD sensor set, to detect changes in the background magnetic field. These changes are affected by the distance from the magnetic source as described above and the polarity of the influence which changes with every casing joint. It is possbl h n oobs h nflu nc foms lpos ons nd‘ ngul ’ h loc onof h c n ln of h other well from the results. This requires h h pol s ng h b suff c n o b ‘s n’ by h s nso s nd fo practical reasons this unlikely beyond about 15m or 50ft. Active Magnetic Ranging uses a power source downhole to inject current into the surrounding rock. When the current is passing through rock, the resistance is high but when the electrons find steel, they rush through the steel with very little resistance creating a very strong cylindrical magnetic field around the steel. This active magnetic induction can b ‘s n’fomup o100f w y nd s popul l n op ss ngng n l fw lls u ons. How , although it provides more accuracy, the ranging tool requires a separate trip in hole and for a deep relief well, this is very time consuming. More recently an Active Ranging While Drilling (ARWD) tool has been developed which will provide an active ranging facility within an otherwise normal drilling assembly which is very useful for the closer range operations and avoids the need for tripping. The magnetic field is detected using a set of three magnetometers but these are not arranged in the usual orthogonal geometry. They are arranged in a ring and set at 120 degrees to each other. The three readings will not exactly agree. The magnetometer closest to the source will show a stronger reading then the other two and the degradation in signal over the very short baseline provides an estimate of the distance. Ed note: the next two figs need to be fitted in the text
In this example, the magnetometers at A, B and C experience the magnetic field at different intensities as illustrated by the graphs. Comparing the values at A, B and C we can determine the direction of the field gradient which will point approximately to the centre of the well. The magnitude of the gradient can be determined from the degradation from A to B to C resolved along the vector of approach. From this an estimate of the distance to the magnetic source can be derived. The ‘direction’ is actually a toolface angle in a plane at right angles to the wellbore.
Figure 115: Estimating the range from a magnetic source.
Made with FlippingBook flipbook maker