Jorg V. Herwanger

Linking reservoir geomechanics and time-lapse seismics: Predicting anisotropic velocity changes and seismic attributes

Seismic technology has been used successfully to detect geomechanically induced signals in repeated seismic experiments from more than a dozen fields. To explain geomechanically induced time-lapse (4D) seismic signals, we use results from coupled reservoir and geomechanical modeling. The coupled simulation yields the 3D distribution, over time, of subsurface deformation and triaxial stress state in the reservoir and the surrounding rock. Predicted changes in triaxial stress state are then used to compute changes in anisotropic P- and S-wave velocities employing a stress sensitive rock-physics transform. We predict increasing vertical P-wave velocities inside the reservoir, accompanied by a negative change in P-wave anisotropy (Δe=Δδ 0) . A stress sensitive rock-physics transform that predicts a...

Predicting time-lapse stress effects in seismic data

Time-lapse changes in seismic data are commonly evaluated in terms of changes in reservoir properties such as pressure, saturation, or temperature. Traditionally, the evaluation of time-lapse seismic data has focused on changes of seismic signatures within the reservoir interval. Recent studies, however, have shown convincingly that time-lapse seismic changes occur not only in the reservoir, but also in the overburden and (generally) in the rock mass surrounding the reservoir. These time-lapse changes can be explained by production-induced stress changes in the rocks surrounding the reservoir.

Anisotropic Velocity Changes In Seismic Time-lapse Data

A fundamental challenge in the interpretation of time shifts observed in time-lapse data is the decomposition of the time delay into a spatial compaction component and a velocity change component. Several authors (Hatchell and Borne, 2005; Janssen et al., 2006) have published the application of pragmatic linear relationships between overburden stretching and velocity changes which, have proved applicable in a wide range of geological settings.

Integrated Modeling for 3D Geomechanics and Coupled Simulation of Fractured Carbonate Reservoir

Geomechanical models have multiple uses in reservoir management and field development planning. In this case study from a producing deepwater fractured carbonate reservoir (water depth approx. 1500m) we use the same geomechanical model to understand (i) why hydraulic stimulation failed to open a hydraulic fracture due to high minimum principal stress, (ii) what the limits of mud weights for drilling horizontal wells into the pressure depleted reservoir are, and (iii) investigate what the effect of pressure depletion and associated stress changes are on fault permeability, thereby providing an explanation for an observed early water-breakthrough. Multiple data sources, including structural seismic interpretation and Amplitude-versus-Offset (AVO) inversion, well-log measurements and rock-mechanics laboratory tests were combined in building a mechanical property model, and the computed stress state was calibrated with wellbore observations of breakouts directions, induced fractures and leak-off tests. Introduction Reservoir geomechanics has become an accepted technology for the petroleum industry during last decades. There are many benefits on improving geomechanical understanding, including: · A better characterization of reservoir volume deformation and impact on rock permeability (compaction and dilation); · Prediction of surface subsidence; · Reducing of risks of fault reactivation and out-of-zone hydraulic fracture propagation during waterflooding or other improved oil recovery process. In Petrobras, Reservoir Geomechanics is a fundamental tool for development of fields with large number of faults/fractures, which is the case of several offshore sandstone reservoirs in Campos Basin, as well as some carbonates fields. Here we discuss the development of a comprehensive reservoir geomechanics study in an actual field of the Campos Basin. The presented technology has been developed as part of the Technology Cooperation Agreement on Reservoir Geomechanics between Petrobras and Schlumberger. A similar study has been carried out for a sandstone field and presented at a SPE Conference (Souza et al, 2012). This multidisciplinary project has been developed with a team of geophysicists, geologists, reservoir and geomechanics engineers. An integrated approach involving seismic inversion, structural geology, rock mechanics and multiphase flow in porous media is used to build a 3D geomechanical model of an offshore naturally-fractured carbonate reservoir, Campos Basin, Eastern Brazilian Continental Margin. The first phase of the project involves auditing the data and building 1D Mechanical Earth Models (1D-MEM). These 1D-MEMs use data observations of stress directions and magnitudes (e.g. breakout directions, induced fractures, leak-off-tests, in combination with drilling experience and employed mudweights) for creating stress models along the well trajectory and are later used as calibration tool for the 3D geomechanical model. For the second phase a 3D Mechanical Earth Model (3D MEM) is built, by gridding the reservoir as well as over-, underand sideburdens, and populating the model with mechanical properties. Rock properties such as Young’s modulus, Poisson’s ratio, density, friction angle, unconfined compressive strength are distributed using a combination of well log observations, seismic AVO inversion models and rock mechanics test data (Herwanger and Koutsabeloulis, 2011). Pore pressure is also assigned based on seismic interval velocities calibrated with pressure observations (Dutta, 2002). The properties are developed based on

Applying time-lapse seismic methods to reservoir management and field development planning at South Arne, Danish North Sea

Abstract At South Arne a highly repeatable time-lapse seismic survey (normalized root-mean-square error or NRMS of less than 0.1) allowed us to reliably monitor reservoir production processes during five years of reservoir depletion. Time-lapse AVO (amplitude v. offset) inversion and rock-physics analysis enables accurate monitoring of fluid pathways. On the crest of the field, water injection results in a heterogeneous sweep of the reservoir, whereby the majority of the injected water intrudes into a highly porous body. This is in contrast to a pre-existing reservoir simulation model predicting a homogeneous sweep. On the SW flank, time-lapse AVO inversion to changes in water saturation Δ S w reveals that the drainage pattern is fault controlled. Time-lapse seismic data furthermore explain the lack of production from the far end of a horizontal producer (as observed by production logging), by showing that the injected water does not result in the expected pressure support. On the highly porous crest of the reservoir compaction occurs. Time-lapse time shifts in the overburden are used as a measure for compaction and are compared with predictions of reservoir compaction from reservoir geomechanical modelling. In areas where compaction observations and predictions disagree, time-lapse seismic data give the necessary insight to validate, calibrate and update the reservoir geomechanical model. The information contained in time-lapse seismic data can only be fully extracted and used when the reservoir simulation model, the reservoir geomechanical model and the time-lapse seismic inversion models are co-visualized and available in the same software application with one set of coordinates. This allows for easy and reliable investigation of reservoir depletion and gives deeper insight than using reservoir simulation or time-lapse seismic individually.

Stress induced seismic velocity anisotropy study

A method of calculating stress induced seismic velocity anisotropy is presented and studied. In a first step an effective stiffness tensor of a stressed rock is calculated using third order elasticity (TOE) theory. In a second step, anisotropic seismic Pand S-wave velocities are calculated from the effective stiffness tensor using Tsvankin’s notation, which is an extension of Thomsen’s weak anisotropy notation. This method is used to study anisotropic velocity changes due to changes in hydrostatic, uni-axial and tri-axial stress using stress-sensitivity (thirdorder) parameters given in the literature. The computations show, in agreement with experimental observations, that the strongest P-wave velocity increases are observed in the direction of the largest stress increase. For S-waves, the largest velocity increase is observed for S-waves polarized parallel to the direction of maximum stress-increase. Therefore, S-waves have the potential to be used as a tool to monitor horizontal stress changes.