In-Situ Stress and Rock Strength in the GBRN/DOE Pathfinder Well, South Eugene Island, Gulf of Mexico
Authors: Zoback Mark D. , Peska Pavel 
Published: December 2007

Summary
We present a relatively simple technique to constrain in-situ stress and effective rock strength from observations of wellbore failure in inclined wells. Application of this technique in the Global Basins Research Network (GBRN)/DOE "Pathfinder" well demonstrated that (1) the azimuth of Shmin is N42 E, perpendicular to a major growth fault penetrated by the well; (2) the magnitude of SHmax is relatively close to the vertical stress; and (3) the effective in-situ compressive rock strength is 3,500 to 4,000 psi. We show that once we have estimated in-situ stress and rock strength, it is possible to compute the mud pressure required to inhibit failure for wells of any azimuth and inclination. Finally, we show how it is possible to estimate the magnitudes of both Shmin and SHmax in cases where independent knowledge of stress orientation is available (for example, from wellbore breakouts in nearby vertical boreholes).

Introduction
Improved knowledge of in-situ stress and effective rock strength in hydrocarbon reservoirs is important in a number of problems ranging from borehole stability and sand production to hydrocarbon migration and hydraulic fracturing. We have conducted a comprehensive series of calculations of the occurrence of compressive failures and drilling-induced, tensile wall failures in arbitrarily inclined boreholes1 and showed how such observations can be used to determine stress orientation and magnitude, effective rock strength and the optimal mud weight for borehole stability. In this paper, we present application of this theory to observations of compressive wellbore failures in the GBRN/DOE Pathfinder well, an inclined well drilled in conjunction with Pennzoil Co. in Block 330 of the South Eugene Island field of the Gulf of Mexico.
Stress-induced compressive failures of wellbores are commonly known as stress-induced wellbore breakouts and can be observed with either four-arm, magnetically oriented calipers (such as with dipmeter logs) or borehole televiewers. Drilling-induced, tensile wall failures in inclined boreholes also can be used to constrain in-situ stress magnitudes. In contrast to drilling-induced hydraulic fractures that propagate away from the borehole and are associated with lost circulation, tensile wall failures occur only in the wellbore wall and are detected only through careful inspection of the borehole wall with Formation MicroScanner/MicroImager logs (FMS/FMI).
In this study, we focus on stress-induced wellbore breakouts as they are commonly observed in oil and gas wells, and numerous studies have shown that breakouts in near-vertical wells accurately reflect in-situ stress orientations when care is taken to distinguish stress-induced wellbore breakouts from other processes that change the cross-sectional shape of a borehole.4 In this analysis, we use the following observations in inclined wellbores.The orientation at which breakouts occur around the wellbore.Leakoff data to constrain the magnitude of the least principal horizontal stress, Shmin, in the case study presented.Estimates of the vertical stress and pore pressure.
These observations are used to constrain (a) the values of the unknown components of the in-situ stress tensor (in the case study presented, these are the orientation of the horizontal principal stresses and the magnitude of SHmax), (b) the maximum effective strength of the rock in situ, and (c) the mud weight necessary to inhibit failure. We illustrate this technique with some relatively simple diagrams that show how breakout orientations depend on the in-situ stress state and borehole orientation and how the tendency for failure further depends on rock strength, pore pressure, and mud weight. We also demonstrate that in cases where the orientation of the horizontal principal stress is already known (for example, from breakouts in vertical wells), the magnitudes of both Shmin and SHmax can be estimated.
Mastin demonstrated that breakouts in inclined holes drilled at different azimuths are expected to form at various angles around a wellbore depending on both the stress state and exact hole orientation. In fact, for this very reason, observations of breakouts in inclined holes are not normally used to determine in-situ stress orientation. Qian and Pedersen and Aadnoy proposed complex nonlinear inversions of failures in multiple inclined boreholes to constrain the in-situ stress tensor. As illustrated later, the technique we propose can yield useful constraints on the stress field from observations in a single borehole. In addition, it does not depend on detailed knowledge of formation properties and basically assumes only that the formation behaves elastically up to the point of failure. Peska and Zoback described the mathematical basis for the technique in detail. In summary, to compute the likelihood of compressive failure around the wellbore, we need to estimate the maximum effective stress in the plane tangential to the borehole, stmax,
(1)
and the normal stress on the borehole,
(2)
In Eqs. 1 and 2, the stress state in a borehole coordinate system is given by
(3a)
(3b)
(3c)
where z is parallel to the borehole axis; r is radial distance; is the angle around the borehole wall measured from the bottom of the hole; is Poisson's ratio; and p is the difference between the borehole fluid pressure and the pore pressure in the rock, pp.15 In Eqs. 1 through 3, the effective stresses, ij, are given by
(4)
where Sij is a component of the "total" stress tensor defined in a local borehole coordinate system derived from the far-field stress state through a series of coordinate transformations and ij is the Kronecker delta.

Eugene Island 330 Field and the Pathfinder Well
The Pathfinder well is an extension (from 7,300 to 8,075 ft) of Production Well A-20ST in Eugene Island Block 330, offshore Louisiana. It was drilled in 1993 as part of a joint project between GBRN, the DOE, and private oil industry with the one objective of testing the hypothesis that an active growth fault can be conduit for oil and gas migrating to the reservoir.