Study of rock stress orientation from borehole breakouts and its correlation to drill parameters and geology : results from boreholes KFM01B AND KFM02A of Forsmark site investigation, Sweden
Abstract: Knowledge of in-situ rock stress orientation is crucial for the understanding of many processes in the Earth crust such as tectonic development, earthquake occurrence, and fluid transport along faults. In the Forsmark site investigation, the knowledge plays an important role in storage design and borehole stability. The borehole breakout method is an important indicator of stress orientations, particularly in aseismic regions and intermediate depths (< 5 km). Borehole breakouts are stress-induced ovalisations of the cross-sectional shape of the borehole wall. The ovalisation is caused by compressive shear failure on intersecting conjugate shear planes, resulting in piece of rock spalling off the borehole wall. This usually occurs when the borehole stress concentration exceeds that required to cause failure of the intact rock. This thesis presents data from boreholes KFM01B and KFM02A that are part of Forsmark investigation site of the Swedish Nuclear Fuel and Waste Management Co (SKB). Two types of borehole geometry and image tools, borehole televiewer (BHTV) and borehole image processing system (BIPS), have been used to reveal the stress orientation using borehole breakout methods. The objectives of this report have been to: (1) determine the downhole orientation of horizontal rock stresses: (2) identify the zones of rock continuum: (3) study the influence of geology on rock stress orientation: and (4) correlate borehole breakouts with measurement while drilling (MWD) parameters. The result from this study shows that borehole breakouts are common in both boreholes. In borehole KFM01B, borehole breakouts have been identified starting from 47 meters borehole length (mbl), continuing to 499 mbl. Borehole breakouts occupy 203 m of total 500 mbl, which corresponds to almost 41% of the entire logged borehole. Almost 85% of the identified borehole breakouts were identified in the borehole sections 113-270 mbl and 398-499 mbl, respectively. The majority of the identified borehole breakouts are shallow and have a limited failure depth. In addition, almost 50% of the identified borehole breakouts have their orientation of maximum horizontal stress ranging between 138°-142°N whereas 38% have theirs within the interval 154°-166°N. The length weighted average orientation of maximum horizontal stress is 146°±10°N, which corresponds to quality B according to the WSM ranking scheme, which is essentially assigned for a high quality borehole breakouts [Zoback, 1992].In borehole KFM02A, borehole breakouts are detected over almost 29% of the borehole length between 99 to 1002 mbl. They were identified more in abundance, and with uniform orientation of maximum horizontal stress within the interval 500-1002 mbl. The upper 500 mbl of borehole KFM02A is characterized by higher variability in the downhole distribution and orientation of borehole breakouts. The result also reveals that the orientation of maximum horizontal stress from borehole breakout is ranging between 126°-66°N with a dominating (89%) orientation of maximum horizontal stress ranging between 135°-159°N. They suggest a length weighted average maximum horizontal stress orientation of 146°±18°N. This also results in quality B according to the WSM ranking scheme, which is essentially assigned to high quality borehole breakouts. Hence, both boreholes yield the same average orientation with slightly higher variation in stress orientation in borehole KFM02A than in borehole KFM01B. Continuous rock stress orientations as well as decoupling zones were identified in both boreholes, which were strongly correlated to the existing geology and structures at the site. In borehole KFM01B, a heterogeneous interval existed between 47-200 mbl, which corresponds to almost 31% of the logged borehole whereas the homogeneous (a zone of rock stress continuity) section existed between 200-500 mbl. Likewise, in borehole KFM02A, the heterogeneous interval (zones of rock stress discontinuity) existed between 113-499 mbl and also corresponds to about 43% of the entire cored part of the borehole whereas a homogeneous interval existed between 499-1002 mbl. A remarkably good correlation is observed between the sections in both boreholes with scatter in orientation of borehole breakouts and deformation zones. The general observation indicated that deformation zone DZ2 (increased frequency of open fracture with several crushed zones) may have contributed to the scatter observed in the subsurface section 113 to 200 mbl in borehole KFM01B. Similarly, three deformation zones were suspected to have contributed to that of borehole KFM02A. The deformation zone DZ3 (increased frequency of both sealed and open fractures) was suspected to have contributed to the scatter in rock stress orientation observed from 174 to 190 mbl: whereas deformation zone DZ5 (increased frequency of sealed fractures but to a lesser extent of open fractures) may have contributed to the scatter from 296 to 310 mbl: and lastly: deformation zone DZ6 (a wide zone with increased frequency of both sealed and open fractures) may have contributed to the scatter from 419 to 499 mbl. Another plausible explanation for the scatter in orientation of borehole breakouts is the coexistence of several rock units in the boreholes and also the contact points where those rock units coexisted (coalesce). In such sections of two or several interlayered rock types, the compressive strength of the rock varies. Measurement while drilling (MWD) parameters were collected in borehole KFM02A. The general trend observed in the correlated section in the borehole suggest that the water flow rate is directly proportional to the water pressure and inversely related to both rotation speed and penetration rate, while the rotation speed is held constant. This is probably because the deformation zone DZ6 is penetrated in the upper interval. It is expected that in such a zone that the rock strength should be decreased. This is absolutely in order with MWD parameters, decreasing trend in water flow and water pressure, with increasing trend in both rotation pressure and penetration rate is an indication of deep fractures at those points, which serves as conduits by which water or drilling fluid injected into the borehole to cool down the drill-bit and at the same time flush the drill cuttings to the surface are lost. This interval could as well impact less resistance to the drill.
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