Microseismic data interpretation — what do we need to measure first?

Abstract

Currently, there are four widely discussed theories used to describe how microseismicity interacts with hydraulic fractur¬ing. Each theory has a different implication for the interpreta¬tion of microseismicity used for reservoir modelling. Therefore, better understanding of the relationship between microseismic¬ity and hydraulic fracture stimulation is needed before further reservoir models are developed and applied. This would lead to a more precise estimation of hydrocarbon production and give greater value to microseismic data. We may use either seismic or non-seismic methods. While non-seismic methods provide an independent view of hydraulic fracturing they only provide a limited amount of information on the relationship between hydraulic fracturing and microseismicity. We propose micro¬seismic monitoring of directivity as the most promising way to determine the orientation of fault planes and associated slip vectors. Although this is a suitable method it requires sensors in multiple azimuths that are well coupled to obtain reliable high frequency signals. We suggest using Distributed Acoustic Sensing (DAS) sensors which are capable of sampling high frequencies and may provide continuous data along long offsets at reasonable costs. Hydraulic fracturing stimulation is accompanied by induced microseismic events resulting from the reactivation of pre-exist¬ing fractures or the creation of new fractures (e.g., Grechka and Heigl, 2017). Locations of microseismic events are then used to map the fracture geometry: the direction of fracture propagation, fracture length and height. Numerous authors and companies try to convert the measured microseismic information into estima¬tions of reservoir production. These approaches use microseis-micity to constrain linear and non-linear diffusion (e.g., Grechka et al., 2010), discrete fracture networks (Williams-Stroud et al., 2013), tensile opening of hydraulic fracture (Baig and Urbancic, 2010), or bedding plane slip (Rutledge et al., 2013; Stanek and Eisner, 2013). Many of these approaches aim to directly map microseismicity to production prediction and other highly valuable information but the reality of the current state of the art is that we do not know the exact nature of microseismicity and hydraulic fracture interaction with microseismicity. Therefore, many of the reservoir simulators based on microseismicity are subject to significant uncertainty.