Susanne Rentsch

Characterization of hydraulic properties of rocks using probability of fluid-induced microearthquakes

The use of borehole fluid injections is typical for exploration and development of hydrocarbon or geothermal reservoirs. Such injections often induce small-magnitude earthquakes. The nature of processes leading to triggering of such microseismicity is still not completely understood. Here, we consider induced microseismicity, using as examples two case studies of geothermal reservoirs in crystalline rocks and one case study of a tight-gas sandstone reservoir. In all three cases, we found that the probability of induced earthquakes occurring is very well described by the relaxation law of pressure perturbation in fluids filling the pore space in rocks. This strongly supports the hypothesis of seismicity triggered by pore pressure. Moreover, this opens additional possibilities of using passive seismic monitoring to characterize hydraulic properties of rocks on the reservoir scale with high precision.

A passive seismic survey over a gas field: Analysis of low-frequency anomalies

Passive seismic low-frequency from approximately 1–6 Hz data have been acquired at several locations around the world. Spectra calculated from these data, acquired over fields with known hydrocarbon accumulations, show common spectral anomalies. Verification of whether these anomalies are common to only a few, many, or all hydrocarbon reservoirs can be provided only if more and detailed results are reported. An extensive survey was carried out above a tight gas reservoir and an adjacent exploration area in Mexico. Data from several hundred stations with three-component broadband seismometers distributed over approximately 200 km 2 were used for the analysis. Several hydrocarbon reservoir-related microtremor attributes were calculated, and mapped attributes were compared with known gas intervals, with good agreement. Wells drilled after the survey confirm a predicted high hydrocarbon potential in the exploration area. A preliminary model was developed to explain the source mechanism of those microtremors. Poroelastic effects caused by wave-induced fluid flow and oscillations of different fluid phases are significant processes in the low-frequency range that can modify the omnipresent seismic background spectrum. These processes only occur in partially saturated rocks. We assume that hydrocarbon reservoirs are partially saturated, whereas the surrounding rocks are fully saturated. Our real data observations are consistent with this conceptual model.

Fast location of seismicity: A migration-type approach with application to hydraulic-fracturing data

We propose a new approach for the location of seismic sources using a technique inspired by Gaussian-beam migration of three-component data. This approach requires only the preliminary picking of time intervals around a detected event and is much less sensitive to the picking precision than standard location procedures. Furthermore, this approach is characterized by a high degree of automation. The polarization information of three-component data is estimated and used to perform initial-value ray tracing. By weighting the energy of the signal using Gaussian beams around these rays, the stacking is restricted to physically relevant regions only. Event locations correspond to regions of maximum energy in the resulting image. We have successfully applied the method to synthetic data examples with 20%–30% white noise and to real data of a hydraulic-fracturing experiment, where events with comparatively small magnitudes (<0) were recorded.

Characterization of fluid transport properties of the Hot Dry Rock reservoir Soultz-2000 using induced microseismicity

Hydraulic stimulation is a procedure for increasing the permeability of a reservoir. At the geothermal site of Soultz, France, such experiments have been carried out since 1993 at different depths. During the Soultz-2000 hydraulic stimulation, about 7200 seismic events were located using a borehole and free surface seismic network. We analyse the spatio-temporal distribution and density of the events to estimate the large-scale permeability of the medium. We assume that the main triggering mechanism is a pore-pressure diffusion process. Based on this idea, we apply different, already developed, methods for the Soultz-2000 hydraulic stimulation. We obtain two independent scalar permeability estimations, a permeability tensor and a heterogeneous reconstruction of the hydraulic diffusivity. The results agree very well with independent in situ tests.

A hydrocarbon microtremor survey over a gas field: Identification of seismic attributes

Erik H. Saenger*, ETH Zurich and Spectraseis, Arnaud Torres, Spectraseis, Susanne Rentsch, FU Berlin, Marc Lambert and Stefan M. Schmalholz, ETH Zurich, Efrain Mendez-Hernandez, Pemex Exploracion y Produccion Summary Narrow-band, low-frequency (from ~1H to ~6Hz) microtremor signals have been observed worldwide at the surface over hydrocarbon reservoirs (oil, gas and water multiphase fluid systems in porous media). These lowfrequency ëhydrocarbon microtremorsí possess remarkably similar spectral and signal structure characteristics, pointing to a common source mechanism, even though the depth (some hundreds to several thousands of meters), specific fluid content (oil, gas, gas condensate of different compositions and combinations) and reservoir rock type (such as sandstone, carbonates, etc.) for each of those sites are quite different. In this extended abstract we describe some results of an extensive low-frequency hydrocarbon microtremor survey carried out in Mexico. We describe how we extract a hydrocarbon potential map from spectral anomalies and compare this map with available reservoir location data. In a detailed analysis of two representative stations of the survey we extract different seismic attributes. Those attributes are compared with a theoretical model which can explain the source mechanism of hydrocarbon microtremors Introduction A growing number of surveys at different oil and gas field locations throughout the world have established the presence of ëhydrocarbon microtremorsí with a high degree of correlation to the location and geometry of hydrocarbon reservoirs (Dangel et al., 2003; Holzner et al., 2005; Graf et al., 2007 and references therein). These tremors can be used as a direct hydrocarbon indicator for the optimization of borehole placement during exploration, appraisal and production. The ever-present seismic background noise of the earth (e.g., Berger et al., 2004) acts as the driving force for the generation of hydrocarbon indicating signals. In contrast to conventional 2D and 3D seismic technologies, the investigation of ëhydrocarbon microtremorsí is entirely passive and does not require artificial seismic excitation sources. In this paper we describe some results of a specific survey carried out by Spectraseis over a gas field in Mexico. By using ultra-sensitive, portable 3C broadband seismometers (sensor type: G ̧ ralp) more than 500 measurements of the ever-present seismic wavefield at the surface were acquired over an area of approximately 200 km. We will briefly describe the main processing steps and the resulting hydrocarbon potential map. In a second step we analyse the recorded data at two locations in detail. We extract several key seismic attributes to characterize the hydrocarbon microtremor signal. The goal of this characterization is twofold: On the one hand this will solidify or falsify theoretical explanations of the origin of hydrocarbon microtremors. On the other hand the knowledge of those attributes will improve the processing and interpretation for future surveys. The survey: Acquisition, Processing and Interpretation The selected survey area lies in the Burgos Basin area in the north-western part of Mexico. The origin of this basin is associated to the opening of the Gulf of Mexico, during the Jurassic, starting the sedimentation in the Callovian with evaporitic deposits. The sedimentation conditions changed in the Cenozoic when a great regression occurred and a sedimentary sequence of at least 8000 meters thickness, with a great gas potential (represented by rocks with Type III Kerogen) was accumulated. The complex fault system in the surveyed area is made up of horst and graben structures, and is part of a large half-graben system (fault style is mainly listric). Minor faults compartmentalize the whole system in smaller blocks. A sensor grid layout is used with node spacing ranging from 250 to 1000 m. Several monitoring stations are installed for the duration of the entire survey. After each of the measurements (duration from 4 hours to 24 hours) the raw 3C sensor data (surface velocities) was stored with an individual identification number (id). The raw data may include strong perturbations (noises, artefacts) and discontinuities (data gaps). In order to obtain a clean signal in the time domain we cut out those time intervals with obvious strong artificial signals. From the cleaned data we calculate the power spectral density (PSD). An applied standard procedure is to determine the PSD for 40 sec. time intervals and to calculate the arithmetic average of each PSD for the whole measurement time. This leads to a stable and reproducible result in the frequency domain. In Figure 1 we show two spectra for the vertical component.

Location of Seismicity Using Gaussian Beam Type Migration

We propose a new approach for precise location of seismic sources using a Gaussian beam type migration of multicomponent data. This approach requires only the preliminary picking of event time intervals and is much less sensitive to the picking precision than standard location procedures. Furthermore, this approach is characterised by a high degree of automation. The polarisation information of multicomponent data is used to perform initial-value ray tracing. By weighting the amplitudes using Gaussian beams around these rays the stacking of amplitudes is restricted to physically relevant regions only. Event locations correspond to regions of maximum amplitudes in the resulting image. We show a successful application of the method to a synthetic data example with a noise level of 20 per cent. Currently, the method is applied to real data from the German Continental Deep Drilling (KTB) Program.

Multidomain Noise Attenuation for Multimeasurement Towed Streamer Data

Recordings of a multimeasurement towed streamer are subject to several different sources of noise that can negatively impact the useful signal content. Most of the conventional noise attenuation is performed in the shot domain. In this paper, we analyze the behavior of low frequency noise in domains other than the shot domain and exploit the fact that some noise that is coherent in the shot domain turns incoherent in other domains, e.g., the common-offset domain or the common-receiver-point domain. This noise randomization is used to introduce filters that can give a noise reduction of several dB in the low frequencies (< 30 Hz) while preserving signal fidelity. We demonstrate the noise attenuation capabilities on data from a survey carried out in the North Sea.