With funding provided by the State Energy Research Center (SERC), researchers at the Energy & Environmental Research Center (EERC) have developed a new method for imaging the subsurface of the Bakken. The quantitative image analysis (QIA) method is a useful tool for characterizing the porosity and microstructure of unconventional source and reservoir rock.
The method is an improvement on the EERC-developed shared-border analysis (SBA) method. The QIA method complements special core analysis techniques and is faster and less expensive than laboratory flow-through experiments. It will also provide investigators with a new tool with which to evaluate the CO2 capture and storage potential of conventional and unconventional reservoirs and provide valuable information about potential fluid flow pathways that could inform enhanced gas and oil recovery efforts. Enhanced understanding of such properties in unconventional systems like the Bakken could provide insight into the pore networks in terms of spatial material relationships to increase overall productivity within a reservoir.
The field emission scanning electron microscope (FESEM) at the EERC has been used to image tiny pores and minerals in Bakken samples for nearly 4 years. While some quantitative data are produced detailing mineral chemistry, any information about porosity is largely qualitative, making it impossible to reliably compare samples. The QIA method maximizes quantitative data by making specific changes to the way microscope images are collected and processed.
Using a brightness calibration, the QIA method first labels four categories of solid material in the sample (High-Z, Low-Z, Mixed Pixels, and Organic Matter). Porosity can then be described quantitatively according to its association with these categories. This method can estimate the specific surface area of each category and generate functions for describing the size, shape, and distribution for pores in the sample. This makes it possible to determine subtle differences between the pore structures in the sample.
To demonstrate the usefulness of the method, two samples from a previous flow-through experiment were studied with QIA. These data revealed important microstructural differences in the samples that helped explain the observation of methane capture in one of the samples. Twelve samples from the Middle Bakken reservoir and Upper and Lower Bakken shales have been processed with this new method, forming a set containing over 600 processed images and data. The more often the QIA method is used, the clearer the picture of the Bakken will become.