With the increase in production from shale oil and shale gas in North America during the last decade, many studies have been conducted in order to improve our knowledge from organic rich shale plays. Kerogen as one of their main constituents, is a complex macromolecule which generates hydrocarbons under adequate pressure and temperature. However, kerogen is still not fully known in terms of geochemistry and geomechanics. In addition, complexity of unconventional shale plays has led to employment new methods and analytical equipment for characterizing organic matter. In this study, samples from the Bakken Formation are collected and analyzed for thermal maturity and geochemical characteristics by Vitrinite reflectance (%VRo) and Rock-Eval (RE) pyrolysis. Then, Raman spectroscopy was performed on the samples as an analytical tool to provide us with deeper insight about molecular structure. To do so, organic matter properties in terms of thermal maturity were correlated with Raman signals. Furthermore, mechanical properties of organic matter that was acquired with Peak Force Tapping mode in atomic force microscopy (AFM) in-situ, was related to its Raman responses. Results showed a very good correlation between Raman signals and geochemical and geomechanical properties of organic matter reflecting molecular characteristics of this component of shale plays.
Kerogen as one the main constituents of mud rocks is not thoroughly understood in terms of its mechanical characteristics. Kerogen is not as stiff as inorganic minerals within the rock matrix, but it can have a significant impact on the propagation of fractures. This becomes more important in organic rich shale reservoirs. In this study, first we proposed a fast method to predict mechanical properties of kerogen using Raman spectroscopy, which is a function of its molecular structure and chemical compounds, and then incorporated the results in hydraulic fracturing simulation. FLAC software was used to simulate opening of a fracture in the shaly and kerogen rich members of the Bakken formation. Then results were compared with a shale simple model missing organic matter properties. Results showed organic matter mechanical characteristics affects the hydraulic fracturing simulation by having faster fracture closure and requiring more fracing volume to achieve a specified fracture length compared to the simple model. The proposed method of using Raman spectroscopy can be used to make a profile of mechanical properties of organic matter and then be used in hydraulic fracturing simulation to optimize the operation.
Creating a mechanical earth model (MEM) during planning the well and real-time revision has proven to be extremely valuable to reach the total depth of well safely with least instability problems. One of the major components of MEM is determining horizontal stresses with reasonable accuracy. Leak-off and minifrac tests are commonly used for calibrating horizontal stresses. However, these tests are not performed in many oil and gas wellbores since the execution of such tests is expensive, time-consuming and may adversely impact the integrity of the wellbore. In this study, we presented a methodology to accurately estimate the magnitudes and directions of horizontal stresses without using any leak-off test data. In this methodology, full waveform acoustic data is acquired after drilling and utilized in order to calibrate maximum horizontal stress. The presented methodology was applied to develop an MEM in a wellbore with no leakoff test data. Processing of full waveform acoustic data resulted in three far-field shear moduli. Then based on the acoustoelastic effect maximum horizontal stress was calibrated. Moreover, maximum horizontal stress direction was detected using this methodology through the whole wellbore path. The application of this methodology resulted in constraining the MEM and increasing the accuracy of the calculated horizontal stresses, accordingly a more reliable safe mud weight window was predicted. This demonstrates that the presented methodology is a reliable approach to analyze wellbore stability in the absence of leak-off test.
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