Several unconventional reservoirs currently producing in the US contain volcanic ash beds and other lamina-scale trends that affect reservoir productivity. Though these thin beds can often be valuable for stratigraphic correlation and horizontal target interval separation, they are often avoided due to their ability to inhibit hydraulic facture height growth or serve as pinch points for fluid conductivity. The identification of volcanic ash beds has been linked to increased production, and perforation clusters that have little to no production have been shown to correlate with bentonite ash beds (Pirie et al., 2015; Kreimeier et al., 2016, Xu et al., 2016). The ability to differentiate and, if needed, avoid these thin beds is therefore critical to characterizing heterogeneity and optimizing completion strategies in unconventional reservoirs. Core acquisition and expensive logging tools are typically needed to identify these lamina-scale stratigraphic changes but are rarely acquired due to high costs and risk. This analysis uses vibration measurements recorded while drilling to identify mechanical properties of bentonite ash beds and other thin beds to assess their impact on completion processes. In multiple cases, these mechanical properties were then calibrated to core to generate a mechanical facies model. Examples are included within this extended abstract.
This study examines the changes in mechanical properties of thin (less than 1") ash beds and other fine-scale trends and their implications for well productivity and completion processes. It utilizes an approach that involves well-understood geophysical signal processing techniques commonly used by earthquake seismologists to parameterize microearthquake source mechanisms (Brune, 1970). For this abstract, volcanic ash beds will generally be referred to as bentonites based on their primarily marine depositional environment. Bentonite layers observed in this study varied in thickness from 0.03–4 inches and displayed a range of sedimentary structures, continuity, and diagenetic alteration. These layers contained a high percentage of additional material including skeletal fragments and displayed a wide range of mixed-layer clays often organized into fining-up sequences. This is primarily attributed to settling and transport processes within the water column and regional depositional setting. Mineralogic differences are also observed in cored bentonites due to differing volcanic sources, consistent with previous studies (Pierce, et al., 2014).
