Geomechanical data from the public domain for offset wells in the Horn River Basin, north–east British Columbia, Canada, were analyzed to obtain in–situ stress and rock mechanical properties in Devonian gas shale members that are under development. Static rock mechanical properties (e.g., Young’s modulus, unconfined compressive strength, peak cohesion) were calculated from wireline logs from offset wells for the shale members of interest. Derived static Young’s moduli for these shale members range from 18 to 31 GPa, and estimated unconfined compressive strength values range from 41 to 63 MPa. Assuming an average peak friction angle of 35°, the calculated peak cohesion values range from 11 to 16 MPa. From three offset wells, the average vertical stress gradient at the top of the Devonian shales was estimated to be 24.8 kPa/m. Borehole breakouts and anisotropy of the sonic log velocities from offset wells were analyzed to estimate the current orientations of the in–situ minimum and maximum horizontal stresses. The predicted orientations of the in–situ horizontal stress were compared to the well azimuths of drilled offset horizontal wells surrounding the property of interest. This regional data show that the dominant orientation of the in–situ minimum horizontal stress is NNW–SSE. Hence, hydraulic fractures will propagate perpendicular to this in an ENE–WSW orientation. The average orientation of the horizontal wells drilled by the other operators in the basin to date is NNW–SSE, with a wide scatter of other well orientations. There was insufficient data in the public domain (e.g., oriented caliper logs) to assess whether orientations or magnitudes of the horizontal stress vary over the basin, or whether they are affected by small or large faults. The average contrast of the magnitude of the horizontal in–situ stress between the shale members of interest was determined from the dynamic Poisson's ratio to understand the propensity for a hydraulic fracture to grow vertically out of zone. The result suggests relatively small contrasts may exist between the shale members. Recommendations were provided for obtaining site– specific in–situ stress data and rock mechanical properties.
It is well known that shale can be a problematic lithology that is capable of creating issues such as tight hole and pipe stuck during a drilling operation. Shale bedding is recognized as one of the key factors that directly contributes to drilling problems. This paper focuses on shale bedding's impact on wellbore stability, which is investigated with the comparison of wellbore stability results of the shale formation with shale bedding and without shale bedding. The geomechanical simulation results show that: (1) the characteristics of wellbore stability polar plot for all well trajectories can be dramatically changed by shale bedding; (2) both well azimuth and well inclination have significant impact on wellbore stability; (3) stress dominated and shale bedding dominated wellbore breakouts can be evaluated; (4) formation strength control and shale bedding strength control wellbore stability are indentified. Based on those simulated wellbore stability characteristics, optimizations for well azimuth, well inclination, well trajectory, and mud weights can be designed for drilling operation to mitigate the potential drilling hazards.
Successful underbalanced drilling (UBD) operations are strongly dependant on the understanding of in-situ stress condition in addition to rock mechanical properties. Wellbore integrity, which plays an imperative role in all the oil and gas operations, requires accurate geomechanical modeling and wellbore stability analysis. Borehole failure problems, which are very likly specially when drilling underbalanced, cost the petroleum industry several billions of dollars each year. Prevention of these problems requires clear understanding of the interaction between formation strength, in-situ stresses and drilling practice. Since in-situ stress and rock strength are not controllable parameters, adjusting the drilling practices, i.e. selecting optimal trajectory and bottom-hole pressure, is the common way to inhibit wellbore failure, which can be achieved by performing specialized geomechanical studies. Drilling through some challenging formations in Kurdistan, has been always associated with several wellbore stability problems such as mud losses, borehole washout, stuck pipe, extra cutting/caving, and tight holes, which caused numerous nonproductive time to the drilling program and required drilling sidetracks in some of the wells. Review of the drilling reports and dual-caliper logs from 11 offset wells in the area revealed a huge amount of washouts in these formations. These events put a question mark on the feasibility of UBD in these intervals and required conducting a geomechanical modeling and wellbore stability study. In this study, data from offset wells was analyzed to estimate the local in-situ stress magnitudes and orientations, in addition to pore pressure. The mechanical properties of the formations were evaluated using sonic, density, and gamma ray logs. A rock mechanical properties database and data management software was applied to correlate the calculated dynamic elastic properties to the most appropriate static stiffness parameters for a base case wellbore stability model and subsequent sensitivity analyses. 2D elastoplastic and 3D linear elastic models were used to back-analyze the borehole failures in the selected offset wells to verify and calibrate the geomechanical model. Finally, an operating mud weight window was defined, and the optimum profile of the mud weight was recommended for drilling through each formations. This study showed that underbalanced drilling is feasible through the carbonate intervals, however, will encouter severe wellbore stability problems in the shaly and silty formations.
The integrity of the wellbore plays an important role in petroleum operations (e.g., drilling, completion, production). Hole failure problems cost the petroleum industry several billions of dollars each year. Prevention of wellbore failure requires a strong understanding of the interaction between formation strength, in-situ stresses, and drilling practices. As in-situ stress and rock strength cannot be easily controlled, adjusting the drilling practices (i.e., selecting optimal trajectory and bottom-hole pressure) is the usual way to inhibit wellbore failure. Drilling in the problematic Kolosh formation in Kurdistan has always been associated with several wellbore stability problems (e.g., hole washout, stuck pipe, extra cuttings/cavings, tight holes). This has caused large amounts of non-productive time to drilling programs and the drilling of sidetracks in some of the wells in this field. A review of the drilling reports and dual-caliper logs from offset wells in the area revealed large amounts of washouts in the middle Kolosh section. These indicators demonstrated the requirement for performing a geomechanical modeling and wellbore stability study to mitigate such problems in future drilling operations. In this paper, local in-situ stress magnitudes, orientations, and formation pressures were characterized. For this purpose, data was analyzed from offset wells (e.g., borehole breakout data, bulk density logs, wireline formation tests, drillstem tests, pressure build-up tests, formation pressure data in this area). The mechanical properties of the formation (including dynamic and static Young's modulus, Poisson's ratio, and rock strength) were evaluated using sonic, density, and gamma ray logs. A rock mechanical properties database and data management software was applied to correlate the calculated dynamic elastic properties to the most appropriate static rock strength and stiffness parameters for a base case wellbore stability model and subsequent sensitivity analyses. 2D elastoplastic and 3D linear elastic models were used to back-analyze the hole collapse and enlargement in the selected offset wells to evaluate and calibrate the geomechanical model. Wellbore stability software was used for this purpose. Finally, a mud weight window was defined, and the optimum profile of the mud weight was recommended for drilling through the Kolosh formations. Due to a narrow mud weight window, additional potential problems were investigated including the possibility of fracturing at the top of Kolosh formation. Finally, relevant solutions were presented.
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