Summary
Natural fractures and drilling-induced wellbore failures provide critical constraints on the state of in-situ stress and the direct applicability to problems of reservoir production, hydrocarbon migration, and wellbore stability. Acoustic, electrical, and optical wellbore images provide the means to detect and characterize natural fracture systems and to distinguish them from induced wellbore failures. We present new techniques and criteria to measure and characterize attributes of natural and induced fractures in borehole image data. These techniques are applied to the characterization of fracture permeability in two case studies.
Introduction
Wellbore image logs are extremely useful for identifying and studying a variety of modes of stress-induced wellbore failures. We present examples of how these wellbore failures appear in different types of image data and how they can be discriminated from natural fractures that intersect the wellbore. We then present brief overviews of two studies, which illustrate how the techniques have been applied to address specific issues of fracture permeability.
Drilling-induced failures are ubiquitous in oil and gas and geothermal wells because the process of drilling a well causes a concentration of the far-field tectonic stress close to the wellbore, which often can exceed rock strength. Through the use of wellbore imaging and other logging techniques, stress-induced failures can be detected and categorized (compressive, tensile, or shear) and then used to estimate the unknown components of the stress field. We demonstrate how these modes of wellbore failures appear in different types of image data and the pitfalls in their interpretations.
The most valuable use of drilling-induced features is to constrain the orientations and magnitudes of the current stress field. The use of drilling-induced features as stress indicators has become routine in the oil and gas industry.1–8 The detection of these features at the wellbore wall has become a primary target for Logging While Drilling/Measurement While Drilling (LWD/ MWD) real-time operations.9
A strong correlation between critically stressed fractures (fractures optimally oriented to the stress field for frictional failure) and hydraulic conductivity has been documented in a variety of reservoirs worldwide.10–12 When faults are critically stressed, permeabilities are increased, and the movement of fluid along faults is possible. We present examples of how knowledge of the stress state and natural fracture population may be used to access reservoir permeability.
Drilling-Induced Tensile Wall Fractures
Compressive and tensile failure of a wellbore is a direct result of the stress concentration around the wellbore, which results from drilling a well into an already stressed rock mass.13 Compressive wellbore failures (wellbore breakouts), first identified with caliper data, are useful for determining stress orientation in vertical wells.14–16 The study of such features with acoustic and electrical imaging devices makes it possible to clearly identify such features and to use them to determine stress magnitude and stress orientation.15,17–19
It is well known that if a wellbore is pressurized, a hydraulic fracture will form at the azimuth of the maximum horizontal stress.20 The formation of drilling-induced tensile wall fractures is the result of the natural stress state, perhaps aided by drilling-related perturbations, that causes the wellbore wall to fail in tension.
The general case of tensile and compressive failure of arbitrarily inclined wellbores in different stress fields is described by Peska and Zoback,1 who demonstrate that there is a wide range of stress conditions under which drilling-induced tensile fractures occur in wellbores, even without a significant wellbore-fluid overpressure. We call these fractures tensile wall fractures because they occur only in the wellbore wall as a result of the stress concentration. These failures form in an orientation of the maximum principal horizontal stress in a vertical borehole (Fig. 1a) and as en echelon features in deviated wells (Fig. 1b). Because drilling-induced tensile wall fractures are very sensitive to the in-situ stress, they can be used to constrain the present state of stress.1,2,21–23
Pitfalls in Interpretation of Tensile Wall Fractures in Wellbore Image Data
In cases in which drilling-induced tensile fractures form at an angle to the wellbore axis, it can be difficult to distinguish them from natural fractures, especially in electrical image logs that do not sample the entire wellbore circumference. Because misinterpretation of such features could lead to serious errors in the characterization of a fractured (or possibly not fractured!) reservoir, as well as the assessment of in-situ stress orientation and magnitude, we present criteria that are useful for discriminating natural from induced tensile fractures when observed in wellbore image logs.
This is especially important because the wellbore stress concentration can have a significant effect on the appearance of natural fractures that intersect the wellbore. It is well known that fractures are mechanically weakened at their intersection with the borehole. This erosion causes the upper and lower peak and trough of the fracture sinusoid to be enlarged and subsequently enhanced in the standard 2D unwrapped view of wellbore image data (Fig. 2).
Where the borehole hoop stress is tensile, the intersection of a natural fracture or foliation plane with the tensile region of the borehole may be preferentially opened in tension (Fig. 3a). These drilling-enhanced natural fractures can be mistaken easily for inclined tensile wellbore failures (Fig. 1b), thus resulting in serious errors in geomechanical modeling.
Incipient wellbore breakouts are the early stages of wellbore breakout development, in which the borehole compressive stress concentration has exceeded the rock strength and initiated breakout development. The failed material within the breakout, however, has not yet spalled into the borehole (Fig. 3b). In a vertical borehole, these failures may appear as thin "fractures" that propagate vertically in the borehole and may be confused with drilling induced tensile wall cracks.
Evaluation of the Influence of In Situ Stress on the Stability of Mine Pit Walls: A Case Study of Songwe Mine