Emmanuel Gaucher

Change of stress regime during geothermal reservoir stimulation

Earthquakes are induced by man-made changes of the stress field by injection or withdrawal of fluids in hydrocarbon production, geothermal exploitation, and wastewater disposal. However, the actual perturbation of the stress field and stress release by injection-induced seismicity remains largely unknown. We provide evidence for currently not understood hydromechanical processes after shut-in of the well. We invert earthquake focal mechanisms from a massive stimulation to invert for stress resolved in time and depth to obtain changes of the stress orientation and magnitude. Prior information about fracture orientations from well logs is taken into account. Comparison with independent stress measures reveals that stresses obtained from inversion of fluid-induced seismicity are highly perturbed and not representative of the initial stress field. The horizontal stresses change by tens of megapascals, turning the stress regime from transitional normal faulting/strike-slip faulting to pure normal faulting. The observed stress changes are attributed to large-scale aseismic deformation.

Inferring the in situ stress regime in deep sediments: an example from the Bruchsal geothermal site

BackgroundKnowledge of the ambient state of stress is of crucial importance for understanding tectonic processes and an important parameter in reservoir engineering. In the framework of the 2,500-m deep geothermal project of Bruchsal in the central part of the Upper Rhine Graben, new evidence is presented for the stress field in deep-seated sedimentary rocks.MethodsWith a sophisticated data analysis based on the concept of critical stress ratios, we evaluate the quality and uncertainty range of earlier stress field models in the Bruchsal area. New data from borehole logging and leak- off tests in deep sediments are used to propose an alternative stress profile for this part of the Upper Rhine Graben.ResultsThe revised stress field model for the Bruchsal area predicts a normal with transition to strike-slip faulting regime. Stress field perturbations and potential decoupling process within specific clay-, salt-, and anhydrite-bearing units of the Keuper can be observed.ConclusionBy comparison with other models, we can show a regional consistency of our stress field model that is reliable throughout the central Upper Rhine Graben extending from Bruchsal in the East to the Soultz-sous-Forêts EGS site in the West.

Multi-network Microseismic Monitoring of Fracturing Jobs – Neuquen TGR Application

Total has been developing different applications of microseismic monitoring in the E&P domain for the past several years, such as structural definition for exploration, reservoir fracturation (frac) monitoring, and field production monitoring mainly. A co

Comparison of picking-based and waveform-based location methods of microseismic events: Application to a fracturing job

In microseismic jobs, the ability to detect many events and correctly locate them despite their low signal–to-noise ratio is one of the main issues. Conventional locations, computed from the picking of the Pand S-wave time arrivals and the P-wave polarization, are not reliable enough when the Pwave has a low signal–to-noise ratio. Consequently, the number of useful events can be dramatically small. Therefore, it is important to propose alternative methodologies.

Effective Detection Capability of a Local Seismic Network

In applications such as geothermal energy, underground storage, mining, hydraulic fracturing, it becomes current practice to implement local seismic networks to monitor induced seismicity and to help mitigating the associated risks. In such contexts, it is crucial to guarantee that the network is able to detect a seismic event of predefined magnitude in a specific area. We propose a method to estimate this detection capability for existing kilometric-scale seismic networks which did not record any seismicity in the target zone yet. However, the network should be running for a time period long enough to record several local events listed in a reference catalogue. These earthquakes are used to calibrate an amplitude-magnitude relationship, knowing that the amplitudes are at the basis of the detection of seismic event candidates. This observation-based approach can take into account uncertainties in the magnitude estimate. The procedure was applied on the seismic network deployed over Bruchsal (Germany) geothermal field. Since mid-2010, no seismicity in the reservoir has been recorded by the network despite its good working order. The proposed technique suggests that there is 95% probability that no seismic event with ML ≥ 0.7 occurred below the network down to the reservoir depth at 2400 m.

A new probabilistic formulation to locate seismic events from P-wave polarization

Summary Deployment of networks at the surface or/and in wells to monitor the seismicity induced in underground reservoirs is common. The goals are to better image and characterize the reservoir and to mitigate the seismic hazard. With low coverage network, however, event location using only observed arrival times is either poorly constrained or not constrained at all, and may not bring any value. This is typical from borehole monitoring of hydraulic fracturing operations that requires using the seismic wave polarization to indicate the event direction of arrivals. Within a Bayesian framework, we propose a probabilistic formulation to integrate correctly the P-wave polarization for hypocentre determination. We take a single three-component sensor perspective and assume that the covariance matrix measured around the P-wave quantifies the polarization. This matrix contains all necessary axial information including uncertainties. Using directional statistics, we can define the so-called angular central Gaussian (ACG) likelihood that quantifies the probability density function associated with a modelled polarization vector given the observed covariance matrix. The ACG formulation eliminates physical issues existing in current formulations. It is simple, easy to implement and offers an “all-in-one” solution including spatial dependencies and uncertainties that lead to smaller location uncertainties.

Introduction to special section: Geothermal energy

Geothermal energy is heat generated and stored in the earth, a combination of residual heat from earth’s formation and heat generated by radioactive decay. Ninety-nine percent of earth’s mass has a temperature in excess of 1000°C, a source of low-pollution energy that potentially is available

Analysis of Uncertainty of Hypocenter Location Using the Combination of a Borehole Vertical Array And a Subsurface Array Network

A monitoring network composed of a vertical VSP array and a subsurface array can reliably locate microseismic events by using a traditional seismological location method. This distributed array method, which depends only on the first arrival times of P and/or S phases, avoids using timeconsuming, inconvenient and less-reliable measurements of signal azimuth and inclination angle to locate event hypocenters. Numerical analyses of hypocenter uncertainties demonstrate that events located by time-terms alone can achieve better location reliability than hypocenters located by range & bearing methods common to single VSP receiver configurations. The distributed array method is also applicable to those events identified with only S-phase arrivals.

How Microseismic Data Monitored from Treatment Well Can Lead to Fracture Imaging – Feedback from Field Experiments

For any oil company, the knowledge of fluid paths within its reservoir is key information for production optimizing. Hence the ever growing interest of understanding any seismic activity induced by stimulation operations such as fracturation jobs. Indeed, microseismic monitoring of such experiments can lead to fracture mapping – usually, it is carried out from a distant observation well. The originality of our field feedback relies on a monitoring from the treatment well itself. Over the last years, we have collected a significant catalogue of multiple frac jobs experiments (beyond 20 jobs, within various overburdens types, using different probes in various survey geometries). The variety of configurations yet leads to very consistent conclusions as to the fracture characterization, by taking advantage of both types of waves - and validates our treatment well approach.