Effect of CO2 on Mineralogy, Fluid, and Elastic Properties in Middle Bakken Formation Using Rock Physics Modeling

Abstract

ABSTRACT: The Bakken Petroleum System in the Williston Basin consists of three main members: Upper Bakken Shale (UB), Middle Bakken (MB), and Lower Bakken Shale (LB). The Middle Bakken is a calcareous siltstone and fine-grained sandstone which is a proven reservoir within the Williston Basin. Injection of Supercritical CO2 (ScCO2) may cause changes in the elastic behavior of the rock and therefore may reactivate the fault in the Bakken Formation. This research aims at studying the changes in the MB mechanical properties when flooded with ScCO2 for 30 days, and to quantify the physical and chemical effects of ScCO2 saturation on the mineralogy composition and fluid properties of the formation. This will be achieved using different rock physics models before and after ScCO2 injection. Two different samples were taken from MB clastic and carbonate portions of well 24123 in McKenzie County. X-Ray Diffraction (XRD) analysis was applied on two samples pre-and post-CO2 saturation. The results of post-CO2 modeling resulted in a decrease of densities and elastic properties of both lithologies at different rates. Only change in mineralogy and fluid properties were considered for the post-CO2 injection modeling. 1. INTRODUCTION The Bakken formation consists of three main members: Upper Bakken Shale (UB), Middle Bakken (MB), and Lower Bakken Shale (LB). The UB and LB shales are proven source rocks with a similar lithology and are composed of an organic-rich, siliceous, pyritic black shale. The UB covers a larger areal extent than the LB within the Williston Basin. The MB member is a calcareous siltstone and fine-grained sandstone which is aproven reservoir within the Williston Basin (Lefever & Helms, 2006; Nordeng et al., 2010; Pitman et al., 2012). Despite the enormous hydrocarbon reserves within the Bakken formation, the Bakken is classified as unconventional oil-bearing reservoir due to its ultra-low porosity and permeability characteristic (Sorensen et al., 2008). The variation of MB lithology across the Williston Basin from mainly clastic to carbonate led to divide it intoseven distinct lithofacies (Lefever & Helms, 2006; LeFever & LeFever, 2005; Ozotta et al., 2021c).