Hydraulic Fracturing, Microseismic Magnitudes, and Stress Evolution in the Barnett Shale, Texas, USA

Abstract

AbstractWe present new techniques to analyze the effectiveness of hydraulic fracturing for stimulating production in shale gas reservoirs. The case study we have analyzed involves five parallel horizontal wells in the Barnett shale with 51 frac stages. To investigate the numbers, sizes and types of microearthquakes initiated during each frac stage, we created Gutenberg-Richter-type magnitude distribution plots to see if the size of events follows the characteristic scaling relationship found in natural earthquakes. We found that slickwater fracturing does generate a log-linear distribution of microearthquakes, but that it creates proportionally more small events than natural earthquake sources. Finding considerable variability in the generation of microearthquakes, we have used the magnitude analysis as a proxy for the "robustness" of the stimulation of a given stage. We have found that the conventionally fractured well and the two alternately fractured wells ("zipperfracs") were more effective than the simultaneously fractured wells ("simulfracs") in generating microearthquakes.We also found that the later stages of fracturing a given well were more successful in generating microearthquakes than the early stages. This increase in microearthquake activity in the latter frac stages corresponded with an increase in the instantaneous shut-in pressure (ISIP). The net ISIP increase was most pronounced in the simulfrac wells and least pronounced in the zipperfrac wells. We have attempted to model this increase as a cumulative "stress shadow" using an elastic crack-opening model. However, even the maximum reasonable propped fracture aperture causes a stress increase that is less than what was measured in the wells. Since the fracturing of the simulfrac wells took only half the time of the fracturing of the zipperfrac wells, we believe that poroelastic effects associated with time-dependent leak-off is controlling the rate of ISIP escalation and the increase in microseismic creation. We are incorporating poroelasticity in the model to fully integrate stress evolution and permeable volume creation.