Microseismic Moment Tensors: A Path to Understanding Growth of Hydraulic Fractures

Abstract

Abstract Event locations and magnitudes are the first-order quantities of interest in reporting the observations of a microseismic monitoring program. From this, interpretations can be made from the geometry of the microseismic cloud as to the efficiency of the treatment and the appropriateness of various fracture-mechanical models. Moment tensors add another layer of understanding to these processes. The moment tensor of a microseismic event is a representation of the mechanism responsible for the observed radiation of seismic energy. The moment tensors can help understand the states of stress and strain induced by the treatment, as well as giving insights into the geometry of the fracture network. Additionally, as these moment tensors can identify volumetric change, we have the opportunity try to correlate these mechanisms with the injection parameters and obtain a glimpse into how and where the medium is responding to the treatment. We present the results of a microseismic monitoring study where the moment tensors have been determined. This study involves recording microseismicity from multiple sensor arrays where a number of good quality events were recorded. The higher-order moment tensor analysis of these data show how the fracture is growing with the treatment. The distribution of moment tensors usually falls along a trend line between the mechanisms characteristic of crack-opening to crack-closure type events. The distribution of these crack-opening and closing events though time and space, and in relation to the parameters of the injection treatment, suggest how that the fractures grow with crack-opening events at the tip. The closure events occur behind the opening events, both temporally and spatially, and indicate that there is a flow of injectant throughout the volume. Based on these analyses, we suggest that moment tensor solutions for microseismic events can be used to calibrate models of hydraulic fractures and other injection programs. Exploitation of these data will add to the insight gained from microseismic monitoring.