Page 239 - Reservoir Geomechanics

220 Reservoir geomechanics

Can hydraulic fracturing be used to estimate the magnitude of S Hmax ?

Following the work of Hubbert and Willis (1957), Haimson and Fairhurst (1970) pro-
posed open-hole hydraulic fracturing in vertical wellbores as a technique for determina-
tion of the orientation and magnitude of S Hmax . While a number of minor modifications
have been made to the techniques they proposed over the years, suffice it to say that
many successful stress measurements have been made around the world using the basic
technique they proposed over 30 years ago. Amadei and Stephansson (1997)offer a
fairly comprehensive review of the experimental techniques and analytical procedures
associated with using the hydraulic fracturing technique for in situ stress measurements.
We briefly review here the classical use of hydraulic fracturing for determination of
the magnitude of S Hmax .However, because these techniques are best suited for relatively
shallow holes where both stress and temperatures are low (generally about 2 km, or less)
and relatively strong rocks (so that breakouts are not present), this technique has very
limitedapplicationinthepetroleumindustry.Infact,wewillconcludethissectionwitha
briefsummaryofthereasonswhyclassicalhydraulicfracturingisnotparticularlyuseful
for determining the magnitude of S Hmax in the oil and gas (or geothermal) industries.
Following the discussions in Chapter 6,at the point of minimum compression around
the wellbore (i.e. at θ = 0, parallel to S Hmax ), a hydraulic fracture will be induced when
σ min =−T 0 = 3S hmin − S Hmax − 2P p −
P − σ
T (7.1)
θθ
Ignoring σ
T ,a tensile fracture will form at the wellbore wall when

(7.2)
P b = 3S hmin − S Hmax − P p + T 0
where P b is called the breakdown pressure, similar to the FBP (formation breakdown
pressure) referred to in Figure 7.2. Assuming that S hmin is measured from the pumping
pressure (FPP) or shut-in pressure (ISIP), and that P 0 and T 0 have either been measured
or estimated,

S Hmax = 3S hmin − P b − P p + T 0 (7.3)
Bredehoeft, Wolf et al. (1976) pointed out that to avoid the problem of determining
T 0 ,a secondary pressurization cycle can be used (i.e after a hydraulic fracture is initiated
at the wellbore wall), and this reduces to

S Hmax = 3S hmin − P b (T = 0) − P p (7.4)
where P b (T = 0) indicates the breakdown pressure after an initial hydrofrac has been
created at the wellbore wall.
Numerous papers have been written that both use this basic technique for measure-
ment of S Hmax or propose modifications to result in improvements. Two compilations of
papers related to hydrofrac stress measurements summarize much of the relevant expe-
rience (Zoback and Haimson 1983; Haimson 1989). Most of the successful application