Page 212 - Well Logging and Formation Evaluation

202 Well Logging and Formation Evaluation

a
f Well Well q

q f
a
Case B
Case A

Well q
f Well a
f
q
a
Case D
Case C
Figure 13.3.3 Four Scenarios of Wellbore Leaving a Formation

If there is an angle between the borehole azimuth and formation dip
direction, the true formation dip will be greater than a. In order to correct
for this, the following formula should be used:

tan D) = tan( ) cos( ) g (13.3.6)
(
a
where:
D= the true formation dip
a= the apparent formation dip
g= the angle between the azimuth of the wellbore and the maximum dip
direction.

Note that in the event that one is drilling along the strike of the formation
(i.e., g= 90 degrees), it is not possible to say what the true formation dip
is.

Exercise 13.1 Formation Dip from Up/Down Logs

1
One is drilling an 8 / 2-in.-diameter hole at a deviation of 95 degrees when
the reservoir is exited. The offset between the up and down readings is
2m, with the up reading responding first.