Abstract:

Bakken Formation is a typical unconventional reservoir which is the primary source of oil production in the Williston Basin, ND. Due to its low permeability and porosity, hydraulic fracturing has been the leading and productive tool to increase the production in the past decade. Elastic properties of the rock such as Young's Modulus, Poisson's ratio, and Shear modulus is one of the most important parameters which can influence the success of hydraulic fracturing. Thus, accurate and intensive understanding of the elastic properties is essential and compulsory for hydraulic fracturing design. In this paper, we collected samples from three different members (Upper and Lower shale and Middle member) of Bakken Formation. We used 2D XRD to analyze the mineral compositions and FESEM to capture images from the microstructures of the rock samples. Then we used nanoindenter to evaluate the Young's Modulus and Hardness values of the rock, and compared the results. The results showed that the mineral composition varies with the samples through depth. Upper and Lower Bakken shows more content of clay minerals while Middle Bakken incorporates more quartz. From the SEM images, abundant natural fractures can be seen in the Middle Bakken Formation. Finally, it was found that Young's Modulus also varies with the depth and mineralogical and microstructural compositions.

Introduction

After the commercial boom of shale gas and oil in North America, shale reservoir exploration has been widely carried out by many countries. For the unconventional reservoirs, hydraulic fracturing has been a significant development to increase the oil and gas production (Li, 2016). Fracturing not only increases the production but also adds the total reserves 9 billion bbls of oil and more than 700 TSCF of gas has been added to US reserves since 1949 (Carl and Michael, 2010). Many factors can influence the performance of hydraulic fracturing, such as the in-situ stresses, heterogeneities, fractures and pore structures (Kissinger, 2013; Lamont, 1963; Johnson, 1998). Elastic properties such as Young's Modulus is one of the most important parameters that can influence the design of hydraulic fractures (Britt, 2009). Rocks with higher modulus are always easier to be fractured. Conventionally, the industry or researchers either use cores to carry out dynamic and/or static experiments on plugs or use the sonic log to obtain Young's Modulus. (Priyavrat, 2013). Nano-indentation, which measures the applied load with the depth of penetration of an indenter shows promising application to estimate the elastic properties. In petroleum and civil engineering, many researchers have already acquired the nanoindentation method to study the elastic properties of rock particularly those including organic matter (Kumar, 2012; Mason, 2014; Bathija, 2008). We employed nanoindentation analysis to study the samples from the Upper, Middle, and Lower Bakken Formation to better understand the mechanical behavior of this rock unit. Also, this data was integrated with the XRD and SEM results to correlate the elastic parameters with the rock fabric and composition.

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