Leo Eisner

Uncertainties in passive seismic monitoring

The use of passive seismic techniques to monitor oil- field completion and production processes is on the rise. Stress changes induced by such reservoir activities as hydraulic fracturing, water injection, or fluid extraction will often result in failure of the rocks with a concurrent release of seismic energy in the form of compressional (P) and shear (S) waves. Passive seismic monitoring is based on recording these emitted waves and then using their arrival times to estimate the location of the failure events.

Felt seismicity associated with shale gas hydraulic fracturing: The first documented example in Europe

We describe the origin of felt seismicity during the hydraulic fracturing of the Carboniferous Bowland Shale at the Preese Hall 1 exploration well near Blackpool in the UK during 2011. The seismicity resulted from the interaction of hydraulic fracturing and a fault, the location of which was unknown at the time but has subsequently been located and does not intersect the well borehole. Waveform cross correlation is used to detect 50 events in the sequence. A representative hypocenter and strike-slip focal mechanism is calculated using the best recorded seismic event. The hypocenter is calculated to lie 300–400u2009m east, and 330–360u2009m below the injection point and shown to lie on a fault imaged using 3-D seismic at a depth of about 2930u2009m. The 3-D survey shows that not only the event hypocenter but also the focal mechanism correlates strongly with a subsequently identifiable transpressional fault formed during the Late Carboniferous (Variscan) basin inversion.

A technique for identifying microseismic multiplets and application to the Valhall field, North Sea

A fast, fully automatic technique to identify microseismic multiplets in borehole seismic data is developed. The technique may be applied in real time to either continuous data or detected-event data for a number of three-component receivers and does not require prior information such as P- or S-wave time picks. Peak crosscorrelation coefficients, evaluated in the frequency domain, are used as the basis for identifying microseismic doublets. The peak crosscorrelation coefficient at each receiver is evaluated with a weighted arithmetic average of the normalized correlation coefficients of each component. Each component is weighted by the maximum amplitude of the signal for that component to reduce the effect of noise on the calculations. The weighted average correlations are averaged over all receivers in a time window centered on a fixed lag time. The size of the time window is determined from the dominant period in the signal, and the lag time is the time that maximizes the average correlation coefficient. The technique is applied to a three-component passive seismic data set recorded at the Valhall field, North Sea. A large number of microseismic doublets are identified that can be grouped into multiplets, reducing the total number of absolute event locations by a factor of two. Seven large multiplets reflect the repeated multiple rerupturing (up to 30 times on a single fault) and significant stress release. Two major faults dominate the seismic activity, causing at least one-fourth of the observed events.

Prediction of magnitude of the largest potentially induced seismic event

We propose a method for determining the possible magnitude of a potentially largest induced seismic event derived from the Gutenberg–Richter law and an estimate of total released seismic moment. We emphasize that the presented relationship is valid for induced (not triggered) seismicity, as the total seismic moment of triggered seismicity is not bound by the injection. The ratio of the moment released by the largest event and weaker events is determined by the constants a and b of the Gutenberg–Richter law. We show that for a total released seismic moment, it is possible to estimate number of events greater than a given magnitude. We determine the formula for the moment magnitude of a probable largest seismic event with one occurrence within the recurrence interval (given by one volumetric change caused by mining or injecting). Finally, we compare theoretical and measured values of the moment magnitudes of the largest induced seismic events for selected geothermal and hydraulic fracturing projects.

Detection of repeated hydraulic fracturing (out-of-zone growth) by microseismic monitoring

Hydraulic fracture treatments are commonly carried out in several space- and time-staggered stages which are designed to stimulate isolated segments of a reservoir (i.e., multistage fracturing). However, repeated hydraulic fracturing of reservoir segments fractured in previous stages is commonly observed. In multistage fracturing this problem is also known as out-of-zone or out-of-pay growth, and we shall call it cross-stage fracturing in this article. Cross-stage fracturing can be detected by automated identification of multiplets, i.e., microseismic events with similar source mechanisms and nearly identical location. We applied multiplet identification to detect cross-stage fracturing on two hydraulic fracture monitoring data sets (Canyon Sand and Barnett Shale formations) one of which is described in this paper. We verified this detection method with the initial microseismic events locations. Cross-stage fracturing was detected only a few minutes after the first microseismic events had been detected an...

Effective anisotropic velocity model from surface monitoring of microseismic events

We develop a methodology to obtain a consistent velocity model from calibration nshots or microseismicity observed on a buried array. Using a layered 1D isotropic model derived from checkshots as an initial velocity model, we invert P-wave arrival times to obtain effective anisotropic parameters with a vertical axis of symmetry (VTI). The nonlinear inversion uses iteration between linearized inversion for anisotropic parameters and origin times or depths, which is specific to microseismic monitoring. We apply this technique to multiple microseismic events from several treatments within a buried array. The joint inversion of selected events shows a largely reduced RMS error indicating that we can obtain robust estimates of anisotropic parameters, however we do not show improved source locations. For joint inversion of multiple microseismic events we obtained Thomsen anisotropic parameters x02 of 0.15 nand δ of 0.05, which are consistent with values observed in active seismic surveys. These values allow us to locate microseismic events from multiple hydraulic fracture treatments separated across thousands of metres with a single velocity model. As a result, we invert the effective anisotropy for the buried array region and are able to provide a more consistent microseismicity mapping for past and future hydraulic fracture stimulations.

Comparison of migration‐based location and detection methods for microseismic events

ABSTRACT Microseismic monitoring in the oil and gas industry commonly uses migration‐based methods to locate very weak microseismic events. The objective of this study is to compare the most popular migration‐based methods on a synthetic dataset that simulates a strike‐slip source mechanism event with a low signal‐to‐noise ratio recorded by surface receivers (vertical components). The results show the significance of accounting for the known source mechanism in the event detection and location procedures. For detection and location without such a correction, the ability to detect weak events is reduced. We show both numerically and theoretically that neglecting the source mechanism by using only absolute values of the amplitudes reduces noise suppression during stacking and, consequently, limits the possibility to retrieve weak microseismic events. On the other hand, even a simple correction to the data polarization used with otherwise ineffective methods can significantly improve detections and locations. A simple stacking of the data with a polarization correction provided clear event detection and location, but even better results were obtained for those data combined with methods that are based on semblance and cross‐correlation.

Microseismic monitoring — Introduction

Most papers the readers find in this special section were presented at the First International Workshop on Microseismic Technology held in Asheville, North Carolina, USA, on 17–22 August 2014. The workshop’s organizing and technical program committee consisted of Werner Heigl (Apache Corporation), Vladimir Grechka (Marathon Oil), Leo Eisner (IRSM Academy of Sciences of the Czech Republic), Christophe Maisons (Magnitude), Serge Shapiro (Freie University, Berlin), Julie Shemeta (MEQ Geo), and Stephen Wilson (Seismogenic). The workshop, designed to facilitate sharing of microseismic expertise and the key observations and findings, was organized with special attention to balancing the needs of industry and academia. To encourage participation from the industry, only brief abstracts of papers were requested and no recording was permitted at the workshop; to attract academic contributions, the special section in Geophysics was offered to authors who would be willing to develop their workshop presentations into full-scale papers. The workshop, attended by 80 geophysicists and engineers from 11 countries, was enthusiastically supported by operating and service companies, with sponsorship from ITASCA, Magnitude, Marathon Oil, Pinnacle, Read, Sigma 3 , and …

Advances in time-lapse geophysics — Introduction

Time-lapse geophysics is becoming an increasingly important and powerful method to measure, monitor, verify, and predict complex time-varying processes in the earth. Applications include resource management (hydrocarbons, groundwater, geothermal…), geohazard risk assessment (natural and induced seismicity, overpressure zones…), environmental issues ( CO 2 sequestration, groundwater contamination and remediation…), geotechnical engineering (dams, roads, bridges…), and fundamental science questions (subsurface flow of fluids, stress and heat, time-variant rock properties, fault dynamics and fracturing, geophysical source mechanisms, near-surface variations in vadose and permafrost zones…).nnAdvances in time-lapse geophysics are being driven by spectacular innovations in theory, data acquisition, and quantitative data analysis. Theoretical innovations include new math and physics developments to properly incorporate 4D space-time variations in modeling, imaging, and inversion methods, and to correctly account for full wavefield or potential field representations of time-lapse geophysical phenomena. Data acquisition innovations include new developments to improve 4D signal/noise levels, and repeat surveys more frequently or continuously, with new source and receiver instrumentation, (semi) permanent arrays, and others. New developments in time-lapse imaging and inversion are allowing us to extract more detailed (and often surprising) information, increasingly in near real-time, to help better understand time-varying processes in the earth’s subsurface. Recent developments in quantitative time-lapse data analysis and interpretation are providing new knowledge to help improve our …

Influence of the Double-couple Source Model on the Sensitivity of Microseismic Monitoring Networks

Microseismic monitoring has significant interest of researchers as well as operators of the hydraulic stimulations or material extractions. It is necessary for the successful monitoring to design the monitoring network in order to be able to locate seismic events above certain threshold of magnitude with sufficient accuracy. The detection threshold becomes more complicated if one considers double-couple (shear) source mechanism therefore the average values of the radiation pattern of the source are widely used. In this study we are illustrating how the double-couple source radiation can be included in network design. We discuss influence of the three main classes of mechanisms: strike-slip source mechanisms, normal (reverse) faulting mechanisms and unknown orientation of the source mechanisms. The specific sensitivity distributions are computed by the empirical relationships for the surface network of 4 stations and for single borehole array. These simulations showed, that the radiation pattern of the seismic sources can have large influence on the sensitivity of the monitoring networks (especially limited networks like surface network of 4 stations or single borehole array). We showed also the influence of the unknown orientation of the source mechanisms sensitivities compared with the sensitivities computed by using average values of the radiation pattern.