A New Method of Neostress Determination from Passive Seismic Data

Abstract

This contribution describes an entirely new method for determining the neostress (present-day stress) in reservoirs using passive seismic data from surface arrays. Until now the only stress information available from passive seismic methods has been derived from the focal mechanism solutions of microearthquakes (MEQs). This method has several weaknesses:• The P and T axes of the solution do not correspond to the maximum and minimum compressive stress axes even though they are often used as proxies. • MEQs that are sufficiently strong and clear to determine focal mechanism solutions are relatively rare. • Both surface and downhole microseismic methods are limited in their ability to accurately determine focal mechanism solutions. • The method provides information on the orientations, but not magnitudes, of the three principal stresses. • Focal mechanism solutions yield two potential fault plane solutions which are not determined uniquely.The new method relies on analysis of the orientation, size, and seismic activity of surface segments of Tomographic Fracture Images SM (TFIs). TFIs are direct images of both induced fractures and natural fractures stimulated by fracking (Geiser et al, 2012; Doe et al, Lacazette et al URTeC 2013, Lacazette et al, Sicking et al, URTeC 2014). The method uses passive seismic data acquired with surface arrays.TFIs can be generated as tessellated surfaces. A tessellated surface is a continuous surface composed of triangles with common edges. Slip Tendency Analysis uses the orientations, areas, and cumulative seismic activities of individual triangular facets of tessellated TFIs to find the orientations and relative magnitudes of the principal stresses that best fit the properties of the population of facets. If independent stress magnitude information is available then quantitative estimates of the reservoir stresses can be determined. The method has the advantage of working with large data sets (tens of thousands to millions of facets) that are distributed across the region of the reservoir affected by a hydraulic fracture treatment. The large sample leads to robust solutions. Also, it may be possible to extend the method to map reservoir stress variations. Other important advantages of the method are that it is not dependent on determining first arrivals and is independent of fault plane orientation or MEQ location.

On the origin of exponential growth in induced earthquakes in Groningen

“…Independent of dimension, it has implications also to three dimensional microfracturing experiments (Scholz 1968) on induced fractures. This applies especially, since the latter are typically limited to a few hundred meters (Davies et al 2012, Lacazette andGeiser 2013), which is rather local relative to the scale depth of about 3 km of the Groningen reservoir. Given that subsidence in the Groningen field increases at an approximately constant rate in time (Fig.…”