Denis Kiyashchenko

Distributed acoustic sensing for reservoir monitoring with vertical seismic profiling

Distributed Acoustic Sensing is a novel technology for seismic data acquisition, particularly suitable for Vertical Seismic Profiling. It is a break-through for low-cost, on-demand, seismic monitoring of reservoirs, both onshore and offshore. In this article we explain how Distributed Acoustic Sensing works and demonstrate its usability for typical Vertical Seismic Profiling applications such as checkshots, imaging, and time-lapse monitoring. We show numerous data examples, and discuss Distributed Acoustic Sensing as an enabler of seismic monitoring with 3D Vertical Seismic Profiling. Key words: Borehole geophysics, Acquisition, Seismics, Time lapse, Monitoring.

Virtual source method applied to crosswell and horizontal well geometries

The virtual source method is a useful tool for redatuming a seismic survey below complicated overburden by creating virtual sources at downhole receiver locations, hence generating data independent of the overburden and the time-lapse changes therein. In this article we first apply this technique to crosswell geometries, whereby a virtual crosswell survey is simulated by shooting a line of surface shots passing through two boreholes instrumented with downhole sensors. Using this acquisition geometry, receivers in one of the two wells are turned into virtual sources by correlating the wavefield recorded by those receivers with the recording at receivers in the other well, and summing the correlated data over the surface shots.

Comparing Virtual Versus Real Crosswell Surveys

The virtual source method (VSM) is a useful tool for imaging below complex overburden and monitoring in the presence of time-varying overburden. This concept, when extended to crosswell geometry produces data comparable to real crosswell data. Using a field data example we demonstrate that virtual crosswell data is kinematically comparable to real crosswell data, but the virtual crosswell method possesses flexibilities, which are difficult to achieve in a real crosswell survey. Some of these flexibilities include the ability of the virtual source to radiate either horizontally or vertically and the possibility for the virtual source to radiate only Por only S-waves. It is also possible to create virtual crosswell data that contain only the direct arrivals or only the reflections. These features of the virtual crosswell method should make it useful for crosswell tomography, imaging and reservoir monitoring for moderate interwell distances.

Time-lapse 3D VSP using permanent receivers in a flowing well in the deepwater Gulf of Mexico

The vision of smart wells permanently instrumented with geophones has existed for some time. Such installations would enable opportunities for frequent reservoir monitoring, high-resolution 3D VSP imaging, and illumination of difficult areas. For many subsalt fields, 3D VSPs acquired in such wells may be the best option to illuminate and monitor reservoirs without the need to stop production or enter a well. However, this technology is not yet widely applied, nor perhaps proven.

Modified imaging principle: An alternative to preserved-amplitude least-squares wave-equation migration

Impedance contrast images can result from a least-squares migration or from a modified imaging principle. Theoretically, the two approaches should give similar results, but in practice they lead to different estimates of the impedance contrasts because of limited acquisition geometry, difficulty in computing exact weights for least-squares migration, and small contrast approximation. To analyze those differences, we compare the two approaches based on 2D synthetics. Forward modeling is either a finite-difference solver of the full acoustic wave equation or a one-way wave-equation solver that correctly models the amplitudes. The modified imaging principle provides better amplitude estimates of the impedance contrasts and does not suffer from the artifacts at-tributable to diving waves, which can be seen in two-way, least-squares migrated sections. However, because of the shot-based formulation, artifacts appear in the modified imaging principle results in shadow zones where energy is defocused. Those artif...

Wave Equation Vector Migration For Subsalt VSP Imaging And Interpretation

Interpretation of subsalt VSP data is a challenge due to low fold, uneven illumination, and the resulting presence of a large amount of migration artifacts. We propose a wave equation vector migration algoritm (VWEM) to exploit the potential of multi-component VSP surveys for imaging in complex subsalt environments. The algorithm naturally takes into account multi-pathing and is designed to enhance true events in VSP images. We also propose a noise imaging algorithm that enhances noise and suppresses true events to consistently characterize migration noise in VSP images. Joint interpretation of vector and noise images enables clear identification of true and false events. The approach was successfully applied to a 3D VSP acquired in the Gulf of Mexico in order to clarify the subsalt scenario.

Advancements in processing and imaging of downhole multi-component data

The virtual source method (Bakulin and Calvert, 2006) is a useful tool for imaging and monitoring the subsurface. This technique, when applied to crosswell geometries, uses the vertical and the horizontal component recording to steer the virtual source. In terms of offset coverage, benefits of the virtual source method are maximized by using downhole sensors in a horizontal well. For a horizontal well, it is useful to have 4-C sensors in order to separate the upgoing and the downgoing waves using the dual-sensor summation technique. The virtual source method can also be applied to downhole multi-component recording in vertical wells to generate accurate P-wave and S-wave checkshots and estimate shear-wave splitting, especially in the presence of complex overburden such as salt.

Multiple migration of VSP data for velocity analysis

Summary We investigate the new method that combines the migration of VSP data and updating of the velocity model. The method is based on the comparison of subsurface images obtained by using different types of waves: the primary reflections and surface-related multiples. As a measure of similarity of the images we use the functional based on cross-correlation. Estimation of the velocity model parameters is carried out by maximization of this functional. Therefore the resulting velocity model provides the maximal similarity between images obtained by using the primary reflections and surface-related multiples. We develop an iterative procedure to maximize the functional and demonstrate the efficiency of the method using the synthetic walk-away data. The method can be used for very short VSP receiver arrays and allows us to estimate the interval velocities below receivers, which is considered to be a challenge with VSP acquisition geometry.

Velocity analysis for VSP data using multiples

Summary The quality of the subsurface images obtained using VSP data strongly depends on the velocity model used for migration. The velocity model derived from surface seismic is often not accurate enough for VSP imaging and there is a need for its improvement. We propose the method for updating of velocities using VSP data. The main idea is to use the images of the subsurface obtained using different types of waves: primary reflections and surface-related multiples. If the background velocity is correct, then these images will be similar, and they will not coincide, if the velocity model is erroneous. We develop the algorithm of velocity updating based on this criterion. The proposed method allows us to retrieve the velocity below the borehole receivers. This is complementary to first break VSP travel time tomography, which helps to retrieve velocity only above the receivers.

Scattering objects location with cross-well data

Summary Most of conventional imaging techniques are designed to locate the reflecting interfaces in the subsurface. But the scattering objects (diffractors), such as faults or salt inclusions, are also of interest for exploration. Locating these objects may be useful for seismic data interpretation, production monitoring and reservoir characterization. In this paper we propose the technique of diffractor location using the cross-well data. We use Kirchhoff migration with special weights for the diffractor imaging. We demonstrate that the diffractors, invisible with more conventional processing, have been imaged with the proposed technique.