Kouqi Liu

Nano-dynamic mechanical analysis (nano-DMA) of creep behavior of shales: Bakken case study

Understanding the time-dependent mechanical behavior of rocks is important from various aspects and different scales such as predicting reservoir subsidence due to depletion or proppant embedment. Instead of using the conventional creep tests, nano-dynamic mechanical analysis (nano-DMA) was applied in this study to quantify the displacement and mechanical changes in shale samples over its creep time at a very fine scale. The results showed that the minerals with various mechanical properties exhibit different creep behavior. It was found that under the same constant load and time conditions, the creep displacement of hard minerals would be smaller than those that are softer. 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 showed that the changes in creep displacement, storage modulus, complex modulus and hardness over creep time follow a logarithmic function. We further analyzed the mechanical changes in every single phase during the creep time based on the deconvolution method to realize each phase’s response independently. Two distinct mechanical phases can be derived from the deconvolution histograms. As the creep time increases, the volume percentage of the hard mechanical phase decreases, while this shows an increase for soft phases. The results suggest that nano-DMA can be a strong advocate to study the creep behavior of rocks with complex mineralogy.

Multifractal Characteristics of MIP-Based Pore Size Distribution of 3D-Printed Powder-Based Rocks: A Study of Post-Processing Effect

Abstract3D printing technology offers an innovative approach to manufacture rock samples with controlled properties. However, in this process, pore structure is one of the major concerns when printing similar specimens to natural rocks. The purpose of this study was to lay out an optimal post-processing of 3D-printed samples that can facilitate replicating natural rocks with similar microstructure characteristics. In this study, four cylindrical rocks were manufactured without designed porosity by 3D printing using gypsum powder as the main component. Various types of infiltrants (Colorbond® and Surehold®) and coating conditions (SmoothOn® and WBAE®) were used after completing the printing process of binder jetting. Mercury injection porosimetry was then used to investigate their petrophysical properties including porosity and pore throat size distribution. Multifractal theory was applied to understand the heterogeneity of pore throat distribution within the 3D-printed samples on different pore size intervals. The results showed that 3D-printed rocks have a clustered and negative skewness of pore throat size distributions. The majority of pore sizes are micropores, while a small portion can be categorized under nanopore size category. Multifractal analysis results found a homogeneous distribution of micropores but a heterogeneous distribution of nanopores. Comparing four different samples, it was found that infiltrants could mainly affect the heterogeneous distribution of nanopores more than the micropores, whereas coating does not impact pore structure significantly. In comparison with pore multifractal characteristics of common types of natural rocks, 3D-printed rocks exhibited a higher heterogeneity of pore size distribution.

Nano-mechanical Properties

With the development of production from shale oil and shale gas in North America during the last decade, more studies are being conducted in order to improve our knowledge of the shale characteristics. In this chapter, we talk about mechanical properties of shale samples in micro- and nano-scale. Nanoindentation and Atomic Force Microscopy were newly used advanced techniques in petroleum engineering to investigate the mechanical properties of shales. X-ray diffraction and energy diffusive spectroscopy were used to study the mineral compositions. Based on nanoindentation experiments, elastic modulus and hardness can be extracted from the force-displacement curve. AFM Peakforce quantitively nanomechanical mode is a relatively new mode which can produce maps of surface height and DMT modulus at the same time. In this chapter, we report the application of these two techniques on shale samples taken from Bakken Formation in Williston Basin, North Dakota.

Characterization and Quantification of the Pore Structures of the Shale Oil Reservoir Formations in Multiscale

Pore structures can significantly impact the mechanical and physical properties of the rock such as permeability, strength and durability. Understanding the microstructures of the rocks accurately and quantitatively is essential to petroleum engineering for evaluating and development of oil and gas, especially for the unconventional reserves with abundant interior nanoscale pores such as shale. In this paper, we studied the pore structures of rock samples from Middle Bakken Formation which is a typical unconventional reservoir in North America. High resolution SEM images of five samples were derived after sample preparation. After determining the threshold of the images, we extracted the pore spaces at various magnifications and determined the representative elementary area (REA). Then we analyzed the pore structures properties such as pore size distributions and pore shape distributions of the five samples at their representative elementary area and applied statistics analysis method to compare their distributions. After that, we analyzed their heterogeneity and isotropy properties which have been identified as an important factor affecting reservoir productivity.