Engineering Rock Mechanics

Methods OF stress determination 47

pressures are measured at the test section, i.e. downhole rather than at
the surface, if possible. Third, it is necessary to use an impression packer
or equivalent system to establish the orientation and location of fracture
initiation. Finally, it should be remembered that, using the basic
technique, it has to be assumed that the borehole is parallel to a principal
stress direction.
A schematic representation of the test equipment (consisting of a straddle
packer and an impression packer) is shown in Fig. 4.5, together with the
interpretative calculations. In Fig. 4.6, an early stage in the hydraulic
fracturing stress measurement procedure is shown.
There are several problems inherent in the use of this equipment to
measure the stress state. With reference to the four points mentioned
earlier, it can often be difficult, if not impossible, to identify a 1 m length
of borehole which is fracture free. Furthermore, there can be difficulties in
measuring water pressures accurately, and in correctly identifying the
breakdown and shut-in pressures. There is the question of whether the
crack initiating at the borehole wall in fact propagates in the same direction
(e.g. it may curl into the plane normal to the borehole axis). Lastly, it is
often a completely unjustified assumption that the borehole is indeed
parallel to a principal stress. Against all these points, however, is the fact
that the hydraulic fracturing method is the only direct method available
for stress measurement at any significant distance from the observer (i.e.
distances greater than 100 m), and it has been used to depths of several
kilome tres.

Breakdown

Tn pressure transducers pump, flowmeter. I Pressure -P, pressure

(a1 (b)

Shut-in P --

P, = uh

P, = 3Uh - uH+ u,

a, = tensile

strength of

housing

f

-1 I m

-r +M Rnre v

lmprer

packer

a1 cture 4- -0 9 rn -1^2 ni

"F

J

Sti addle

i packer

Drillh

Drillhole

t t \OH

Figure 4.5 (a) The hydraulic fracturing system and (b) associated calculations (from

Suggested Methods for Rock Stvess Determinution, Kim and Franklin, 1987).