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1994). The productivity of a fractured horizontal well is many times higher than :.'Jat of a
non-fractured horizontal well, which in rum is many times higher than that of a 'ertical
well. The benefit of hydraulic fracturin; must be carefully evaluated against the gain in
productivity using simulators. The simulator is developed to predict the ultimate revenue
rerum.
Hydraulic fracturing will be used increasingly to make wells more economic and
increase productivity. TheJas Research Institute predicts that hydraulic fracturing and/or
horizontal wells will be" most attractive methods for developing marginally economic
reservoirs as well as increasing productiVity of tight gas sands and multizone reservoirs
(GR!, 1992).
In situ stress profiling
Strong emphasis over the past decade has been on evaluating in-situ rock stresses
before performing a hydraulic-fracturing job (Gidley, 1989). This is because the
hydraulic-fracturing pararneters obtained from a fracturing job are especially sensitive to
the minimum principal in-situ stress profIle. The design and simulation of a hydrauiic
fracture requires correct information about the fonnation u, situ stresses and rock
properties. Therefore, emphasis has been on obtaining an accurate minimum in-siru stress
profIle for the least amount of money.
It has teen shown over the past decade L1at the method with the least number of
problems in ootaining the correct minimum principal in-situ stress profIle is direct
measurement using small-volume hydraulic fractures. These are expensive and may not
be compatible with the well completion scheme, particularlv if measurements are made in
layers above the pay zone. Many techniques exist for eVL,:.ting in-situ stress ar.:epth.
All have been investigated in the past decade. but they suffer from various disaavantages.
Core-based methods, including anelastic strain recovery. differential strain-curve analysis,
shear acoustic anisotropy. acoustic emissions, etc.• all requite the taking of core followed
by detailed analysis. Furthermore, problems with core quality, rock fabric, etc., may
degrade the accuracy of the stress estimate. Methods of using sonic log or drilling data
may give correct results in some instances but are not always reliable (Harikrishhnan,
1995).
A recent study demonstrated that knowledge of the in-situ stress profile is essential
in designing and optimizing the hydraulic-fracturing design (GR!. 1992). The usage of
correct minimum principal in-situ streSS profile bas shown productivity to improve
markedly over jobs designed without detailed in-situ stress information.
In the future, knowledge of in-situ stresses will be essential for any economic
evaluation of a hydraulic fracturing job. Therefore. new. accurate, and inexpensive
methods will need to be investigated to obtain in-situ stresses using either logs. drilling
data, core analysis, or new tools.
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