Evaluating the Diagenetic Alteration and Structural Integrity of Volcanic Ash Beds within the Eagle Ford Shale

Abstract

Ash deposits are found interbedded within organic mudstones such as the Vaca Muerta, Niobrara and Eagle Ford shales. Alteration of ash deposits interbedded within mudstones and shales have presented challenges for completion and production including pinching of fractures and swelling upon contact with drilling and completion fluids. This paper will review the mineralogic and petrologic variations in altered ash beds found within the Eagle Ford and examine the impact these chemical and physical properties can have on completion. The paper will also present a methodology for detection of ash beds using high-resolution measurements. Understanding and detecting the mineralogic variation between the altered ash beds will enable better completion and minimize risk during production.

Impact of Ash Beds on Production in Eagle Ford Shale

“…The thickness of ash beds in the Eagle Ford shale is between 0.5 and 3 in. The ash beds fluoresce yellow under UV light, which helps detect thin beds that are difficult to see in the visible spectrum (Calvin et al 2015). The presence of ash beds provides a unique mechanism for creating a conductivity pinch point.…”

“…Spectral GR relies on the appearance of radioactive elements (like thorium) in the ash bed for detection (Schwalbach and Bohacs 1992). However, the ability to find thin ash beds or distinguish closely spaced ash beds is questionable given that GR has a vertical resolution of 1 to 2 ft. Calvin et al (2015) introduced a detection methodology that utilized high-resolution logs including borehole images (electrical and/or ultrasonic), dielectric dispersion, microresistivity, density, and photoelectric factor (PEF). Microresistivity processing (2-in.…”

“…Therefore, accurate characterization of these beds is critical. Calvin et al (2015) use whole core analysis to summarize the variations in observed grain size, mineralogy, bed thickness, and depositional and diagenetic features of altered ash beds in the Eagle Ford.…”

“…In this paper, the Calvin et al (2015) terminology will be used, and volcanic ash deposits in the Eagle Ford are referred to as altered ash beds.…”