Abstract Reservoir rocks can be highly sensitive to fluids introduced through hydraulic fracturing, water disposal, or waterflood injection. The sensitivity of reservoir rocks to fluids can lead to reduced permeability and permanent formation damage resulting in reduced productivity or injectivity. It is generally assumed that clays are the primary culprit in formation damage caused by swelling, increased clay-bound water, water shock, or denigration. In this chapter, we present results from a two-year effort to understand the fluid sensitivity of tight sandstone reservoirs in the Greater Monument Butte Unit (GMBU) in the Uinta Basin, Utah, U.S.A. Newfield Exploration (NFX) drills and completes approximately 200 wells per year in the field, which is currently under waterflood with injection rates of ~90,000 barrels per day. When we initiated this study, NFX completed wells with fresh water. Pore-scale imaging was the key to designing new core flood experiments that led to optimized completions fluids for the field. Initially, we assumed that potential fluid sensitivity was caused by mixed-layer illite-smectite (I/S). XRD (x-ray diffraction) and SEM (scanning electron microscope) images indicated that some GMBU reservoir rocks contained pore-bridging I/S. We designed initial core flood experiments combined with core nuclear magnetic resonance to identify and quantify clay reactions. The results of these initial tests indicated that the reservoirs were sensitive to lower-salinity completions fluids and a reduction in permeability was observed. We utilized a new approach involving pore-scale imaging to identify the mechanism causing permeability reduction. Comparison of SEM images of minerals in pores before and after fluid placement identified calcite dissolution and fines migration as the cause of permeability reduction. Micro-CT (micro-computed tomography) scans combined with registered EDX (energy-dispersive x-ray spectroscopy) mineralogy provided the context for the severity of the problem, especially in the better reservoir rock. The results of this work challenge a number of commonly held assumptions of rock–fluid sensitivity and have implications on how to design effective fluid sensitivity studies using core. This work involved collaboration between petrophysicists, geologists, engineers, and facilities personnel to design and implement a completions fluid that does not damage multiple reservoirs while remaining cost effective and efficient. This work demonstrates the value of focused science within the context of cost and field operational constraints.

Abstract The Neoproterozoic Uinta Mountain Group is undergoing a new phase of stratigraphic and paleontologic research toward understanding the paleoenvironments, paleoecology, correlation across the range and the region, paleogeography, basin type, and tectonic setting. Mapping, measured sections, sedimentology, paleontology, U-Pb geochronology, and C-isotope geochemistry have resulted in the further characterization and genetic understanding of the western and eastern Uinta Mountain Group . The Red Pine Shale in the western Uinta Mountain Group and the undivided clastic strata in the eastern Uinta Mountain Group have been a focus of this research, as they are relatively unstudied. Reevaluation of the other units is also underway. The Red Pine Shale is a thick, organic-rich, fossiliferous unit that represents a restricted environment in a marine deltaic setting. The units below the Red Pine Shale are dominantly sandstone and orthoquartzite, and represent a fluviomarine setting. In the eastern Uinta Mountain Group, the undivided clastic strata are subdivided into three informal units due to a mappable 50–70-m-thick shale interval. These strata represent a braided fluvial system with flow to the southwest interrupted by a transgressing shoreline. Correlation between the eastern and western Uinta Mountain Group strata is not complete, yet distinctive shale units in the west and east may be correlative, and one of the latter has been dated (≤770 Ma). Regional correlation with the 770–742 Ma Chuar Group suggests the Red Pine Shale may also be ca. 740 Ma, and correlation with the undated Big Cottonwood Formation and the Pahrump Group are also likely based upon C-isotope, fossil, and provenance similarities. This field trip will examine these strata and consider the hypothesis of a ca. 770–740 Ma regional seaway, fed by large braided rivers, flooding intracratonic rift basins and recording the first of three phases of rifting prior to the development of the Cordilleran miogeocline .