Analysis of microseismic events during a multistage hydraulic stimulation experiment at a shale gas reservoir

Abstract

Despite the current easing in demand for increased oil production linked to the global downturn in crude prices, energy demand continuously increases and the long-term demand will require maximizing the productivity of reservoirs and a search into the exploitation of new resources in increasingly challenging environments. In this study, we present the results from the monitoring of the very first multistage stimulation experiment at a shale gas reservoir in Saudi Arabia, presenting an analysis of the microseismicity induced during the treatment. Our aim was to analyse microseismic events to better understand fracture growth and the role of pre-existing fractures in these reservoirs. Microseismic (MS) event monitoring is used to track the creation of fractures during and after the stimulation, and therefore to evaluate the effect of the reservoir stimulation. The monitoring includes a downhole array of 12 3C-sensors that were deployed in a vertical well with a 30.5 m level spacing. A total of 415 MS events were located and analysed, with the results outlining induced fractures extending consistently with an average azimuth of N335° E, normal to the horizontal section of the treatment well. This implies that there are no changes in the local stress direction along the treatment well either in situ or induced along the treatment. There are significant changes in total length and aspect ratio (length/width) of the fractures induced in the different stages. These variations could be attributed to in situ fracturing, local rock heterogeneity or the influence of the treatment parameters. In general, early and late stages of stimulation show the longest fracture networks, with events induced further away from the initiation point. We found no immediate relationship between treatment parameters (peak pressure and pumping rates) and fracture extension. Sensitivity analysis using Monte Carlo simulation methods shows a higher location uncertainty for events located at the early stages, thus limiting the interpretation from monitored seismicity in the early stages. An analysis of magnitude distribution with distance shows a decrease in sensitivity of one degree of magnitude for every 375 m, and a maximum viewing distance of approximately 700 m for the current set-up. The low number of located events does not provide a complete enough dataset for a robust analysis of changes in b -value (slope in linear part of magnitude distribution) during the treatment: however, magnitude distributions, corrected for array sensitivity, provide a useful variable for the validation of geomechanical models currently being developed for the reservoir.