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circulation is resumed. The ECD at which ballooning occurs can be used as a lower bound for the magnitude of the least principal stress (if S3 was lower, lost circulation would have occurred). In fact, it has been argued by [Ward and Clark, 1998] that unless the ECD was close to S3, ballooning cannot occur. Modeling by [Ito et al., 2001] indicates that the most likely reason ballooning occurs is that en echelon tensile fractures form in the wall of a deviated well (see Chapter 8) that store fluid at the pressure corresponding to the ECD during drilling. When the pump is shut off and the pressure drops to the static mud weight, the mud comes out of the fractures and back into the wellbore.
Can Hydraulic Fracturing be Used to Estimate the Magnitude of SHmax?
Following the work of [Hubbert and Willis, 1957], [Haimson and Fairhurst, 1970] proposed open hole hydraulic fracturing in vertical wellbores as a technique for determination of the orientation and magnitude of SHmax. 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 SHmax. 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 limited application in the petroleum industry. In fact, we will conclude this section with a brief summary of the reasons why classical hydraulic fracturing is not particularly useful for determining the magnitude of SHmax 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 / = 0, parallel to SHmax), a hydraulic fracture will be induced when +//min = - To = 3Shmin - SHmax - 2Pp - $P - +$T Ignoring +$T, a tensile fracture will form at the wellbore wall when
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