Questions and answers: fractures and hemispherical projection 1 1 1(3) Using the normal to set 2, sketch those parts of the circle at
an angular distance of 45" from this normal which intersect the circle
around normal 1. Label the two intersection points of these circles BHx
and BHy, rotate the tracing paper so that each is brought to the east-west
line in turn, and read off the orientation of the lines. These points are the
orientations of the two boreholes which will subtend an angle of 45" to
both fracture sets, and are 166/63 and 134/22.
(4) Rotate the tracing paper so that normal 1 and normal 2 are on the
same great circle. Measure the angle between the normals, SO0, divide by
2, and plot the bisector. Rotate the tracing paper so that the bisector is
on the east-west line, and then read off its orientation: 145/46. This is
the orientation of the borehole that makes the minimum angle to each
fracture set, and this angle is 40".47.8 A petroleum reservoir is known to contain numerous fractures
which are highly conductive. These fractures dip almost vertically in
a northeasterly direction. To maximize production from the reservoir,
the production wells are to be deviated to run sub-horizontally, so
that they intersect the fractures as close to perpendicular as possible.
In order to determine the optimal orientation of the production
holes, two test holes have been drilled to orientate the fractures.
The orientations of these test holes within the reservoir are 01 1 /09
and 087/03, and impression packers which were run down these
holes show that in both holes the angle between the normal to the
fractures and the hole axis is 40°. Determine the orientation of the
fractures, and the required trend of the production holes.A7.8 To find the orientation of the normal to the fractures, we draw
circles at an angular distance of 40" away from each of the two borehole
orientations, and locate the intersections of these circles. Only one of
these two intersections will be at the appropriate orientation, but this
will be apparent once we have found them.
(1) Draw tick marks for the two
boreholes, rotate each tick mark
to the east-west line in turn, and
mark the position of the bore-
holes on the tracing paper.
(2) For each borehole in turn,
sketch as much as possible of
a circle at an angular distance
of 40"; the intersection of these
two circles is in the northeast
quadrant of the projection. Ro-
tate the tracing paper so that the
intersection is on the east-west
line, draw a tick mark on the periphery and write next to it the plunge of
the intersection, 24".
(3) Rotate the tracing paper back to north and read off the trend of the
intersection, 051".