Patsy Jorgensen

Virtual source applications to imaging and reservoir monitoring

The virtual source method is a breakthrough that allows us to image and monitor the subsurface in cases where surface seismic or VSP fail to deliver. In settings where the overburden is complex or changing, traditional time-lapse signals are weak or nonrepeatable, leading to ineffective seismic input to reservoir modeling and little of value generated by repeated seismic surveys. On the other hand, the virtual source method allows us to image under complex overburden, yields repeatable data for reservoir monitoring, enables shear seismic, and may help us “look ahead” as we drill.

Virtual shear source makes shear waves with air guns

We demonstrate a novel application of the virtual source method to create shear-wave sources at the location of buried geophones. These virtual downhole sources excite shear waves with a different radiation pattern than known sources. They can be useful in various shear-wave applications. Here we focus on the virtual shear check shot to generate accurate shear-velocity profiles in offshore environments using typical acquisition for marine walkaway vertical seismic profiling (VSP). The virtual source method is applied to walkaway VSP data to obtain new traces resembling seismograms acquired with downhole seismic sources at geophone locations, thus bypassing any overburden complexity. The virtual sources can be synthesized to radiate predominantly shear waves by collecting converted-wave energy scattered throughout the overburden. We illustrate the concept in a synthetic layered model and demonstrate the method by estimating accurate P- and S-wave velocity profiles below salt using a walkaway VSP from the deepwater Gulf of Mexico.

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.

Steering Virtual Sources For Salt And Subsalt Imaging

Using walk-away or 3D VSP data to create Virtual Sources in salt is a new useful tool for imaging and assessing drilling hazards in salt and immediately below the base of salt. It can be also used for accurate positioning of steep salt flanks. Creating Virtual Sources inside homogeneous salt allows us to eliminate any distorting effects caused by rugged top salt and complex overburden. The Virtual Source radiation pattern can be controlled and steered in the direction of desired illumination by selecting appropriate shot ranges at the surface. These new Virtual Source applications can have significant business impact for de-risking drilling through salt and improving well targeting in areas with salt.

Estimation of seismic anisotropy from P-wave VSP data

Many rock formations are anisotropic. Their anisotropy, or dependence of seismic velocities on the direction of wave propagation, has been extensively studied and documented throughout the world. It is now widely accepted that understanding subsurface anisotropy yields numerous benefits for both exploration and development of hydrocarbon reservoirs. For example, seismic images produced with ani-sotropic velocity fields are usually crisper and geologically more meaningful than those created with traditional isotropic velocity models. In addition, relating anisotropy to its physical causes (such as lithology, fractures, stresses) enables one to obtain valuable information about them, something that cannot be done if the rocks are deemed isotropic. Making accurate measurements of anisotropy is an obvious prerequisite for all these potential improvements. To be most useful, such measurements should be done in-situ and in the seismic frequency range. Here we discuss a recently developed technique for estimatin...

Accurate Estimation of Subsalt Velocities Using Virtual Checkshots

Virtual Checkshot This paper was prepared for presentation at the 2006 Offshore Technology Conference held in Houston, Texas, U.S.A., 1–4 May 2006. This paper was selected for presentation by an OTC Program Committee following review of information contained in an abstract submitted by the author(s). Contents of the paper, as presented, have not been reviewed by the Offshore Technology Conference and are subject to correction by the author(s). The material, as presented, does not necessarily reflect any position of the Offshore Technology Conference, its officers, or members. Papers presented at OTC are subject to publication review by Sponsor Society Committees of the Offshore Technology Conference. Electronic reproduction, distribution, or storage of any part of this paper for commercial purposes without the written consent of the Offshore Technology Conference is prohibited. Permission to reproduce in print is restricted to an abstract of not more than 300 words; illustrations may not be copied. The abstract must contain conspicuous acknowledgment of where and by whom the paper was presented. Write Librarian, OTC, P.O. Box 833836, Richardson, TX 75083-3836, U.S.A., fax 01-972-952-9435. Abstract

Application of virtual-source technology to the Zuidwending gas storage project

The Gasunie-Nuon Underground Gas Storage Consortium plans to drill and leach caverns in the Zuidwending salt dome in the Netherlands. According to safety regulations, gas storage caverns must be at least 150 m from the salt flank. The location of the Zuidwending salt flank, however, was not well known; its uncertainty based on surface seismic is some ±200 m (Figure 1). A more precise estimate of the salt-flank location was needed for the project to proceed.

2007 Arctic On-ice Seismic Experiment: Operations And Results From a 2D/3D Seismic Program to Investigate Mitigation of the Ice Flexural Wave

In the Spring of 2007, Shell, Eni, & Repsol conducted a research project offshore of Alaska’s North Slope on landfast ice in the Beaufort sea. Two principle goals of the program were to investigate mitigation strategies for noise contamination caused by the ice flexural wave and establish that 3D seismic operations could be safely and efficiently conducted on landfast, ungrounded ice. We assembled a large array of modern sources and receivers to investigate attenuation or removal of the flex wave from seismic data through acquisition methods and provide a 2D and 3D dataset for investigation of data processing approaches.

Virtual Shear Checkshot With Airguns

real data and effectively create a downhole shear source at one of the geophone locations using conventional acquisition with airguns. This process is completely datadriven and does not require knowledge of overburden velocity. In fact, the more complex the overburden, the better the quality of the Virtual Shear Source. In contrast to actual downhole sources, Virtual Shear Source does not radiate P-waves and thus the shear wave of interest becomes the first arrival. First, we illustrate the concept on a synthetic example using layered model from a North Sea field. Then we create Virtual Shear Checkshot using deepwater VSP dataset from the Gulf of Mexico and obtain shear-velocity profile that is in very good agreement with the dipole sonic log both in salt and below salt. Summary

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.