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6.E.4.b
Sensitivity to the half fracture length
Keeping
f
f
f
D
kX
wk
Fc
constant the pressure change and the Bourdet derivative curves shift to
the right as the fracture length increases. The longer the fracture, the longer it takes to reach
IARF. The following figure illustrates this.
Fig. 6.E.5 – Constant Fc
D
; sensitivity to half fracture length, X
f
In practice, massive hydraulic fracturing is only common in very low permeability formations.
We typically encounter this sort of stimulation in Tight Gas, Shale Gas and Coal Bed Methane
(CBM), type of reservoirs. This topic is covered in the chapter on ‘Unconventional Reservoirs’.
It suffices to say here that these types of wells would not be economically viable or would not
produce at all without this sort of stimulation. It is now becoming popular not only to induce
one single fracture in the well, but horizontal wells are selectively ‘fracced’, and multiple
individual low conductivity fractures may exist. It goes without saying that the analyst’s task
can become more than just challenging.
It is easy to imagine that for a period subject to pressure transient analysis the time to reach
infinite acting radial flow will be prohibitively long, and in some cases the transient will never
reach this flow regime at all (well, maybe after thousands of years). Thus one can understand
that lacking some of the flow regimes the interpretation of massive hydraulically fracced wells
can be quite difficult.
It is therefore always recommended that a pre frac test be carried out to determine the
permeability thickness product of the formation (kh).
6.E.5
Specialized analysis
The data that matches the quarter unit straight line in both the pressure change and the
Bourdet derivative is in bi-linear flow. A plot of the pressure change versus the fourth root of
elapsed time,
4
t
will be on a straight line of slope:
4/1
11.44
kc wkh
qB
m
t
f
and
2
1
1945
hm
aB
kc
wk
t
f