Kouqi Liu

Date of Award

January 2018

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Petroleum Engineering

First Advisor

Mehdi Ostadhassan

Abstract

The focus of this thesis is to study the pore structures along with mechanical properties of the shale rocks from the Bakken Formation. The pore structures that are obtained from the SEM image analysis method showed that total surface porosity of the studied samples is less than 12% and that organic porosity is not the main contributor to total porosity for the samples analyzed. Clay minerals and feldspar have a positive influence on porosity while quartz, pyrite, and TOC has a negative impact. The results from the multifractal theory and lacunarity methods based on the segmented SEM images indicated that pores distribution and size in Bakken shale are heterogeneous.

Regarding gas adsorption analysis, the results showed that all range of pore sizes: micro (<2 nm), meso (2-50 nm) and macro-pores (>50 nm) exist in the Bakken shale samples. Meso-pores and macro-pores are the main contributors to the porosity for these samples. In comparison with the Middle Bakken, samples from the Upper and Lower Bakken exhibited more micro pore volumes. The deconvolution of the pore distribution function from the combination of N2 and CO2 adsorption results proved that five typical pore size families exist in the Bakken shale samples: one micro-pore, one macro-pore and three meso-pore size clusters. In order to analyze the heterogeneity of the pore structures from gas adsorption, multifractal method was applied to analyze adsorption isotherms (CO2 and N2).

The results explained that the generalized dimensions derived from CO2 and the N2 adsorption isotherms decrease as q increases, demonstrating a multifractal behavior. Samples from the Middle Bakken demonstrated the smallest average H value and largest average α10-- α10+ for micropores

while samples from the Upper Bakken depicted the highest average α10-- α10+ for the meso-macropores. This indicated that the Middle Bakken and the Upper Bakken have the largest micropore and meso-macropore heterogeneity, respectively. The impact of rock composition on pore structures showed that organic matter could increase the micropore connectivity and reduce micropore heterogeneity.

This study was followed by mechanical analysis of shale samples from the Bakken. Statistical grid nanoindentation method was applied to analyze mechanical properties of the Bakken. Then the Mori-Tanaka scheme was carried out to homogenize the elastic properties of the samples and upscale the nanoindentation data to the macroscale. The discrepancy between the macro-mechanical modulus from the homogenization and unconfined compression test was less than 15% which was found acceptable. The creep analysis of the samples describes that minerals with various mechanical properties exhibit different creep behavior. Under the same constant load and time conditions, the creep displacement of hard minerals would be smaller than the soft ones. On the contrary, the changes in mechanical properties (storage modulus, loss modulus, complex modulus, and hardness) of hard minerals are larger than soft minerals. The results from curve fitting led us to conclude that the changes in creep displacement, storage modulus, complex modulus and hardness with respect to the creep time would follow a logarithmic function.

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