August Lau

Hybrid inversion techniques used to derive key elastic parameters: A case study from the Nile Delta

This case study reviews the workflow and the results achieved in terms of updated development drilling plans from a 1500-km2 prestack inversion project over a West Mediterranean Deep Marine Concession in the Nile Delta. Four successful exploration and one appraisal well have been drilled to date on this concession. Figure 1 shows the location of the concession (red box) in the offshore Nile Delta, and indicates the locations and field names of the discoveries to date. The 3D survey over this concession was recorded using long offsets (6000 m). Information contained in these offsets has been used to try and obtain rock property estimates that may help distinguish lithology and fluid types in order to high grade drilling locations and help build more reliable reservoir models.

Imaging with complex decomposition: Numerical applications to seismic processing in difficult areas

Measured seismic wavefields have complexity that cannot be fully modeled by seismic processing, such as wave equation migration. Traditionally, a separation of data into signal and noise components is attempted, but it is difficult to isolate the noise component that cannot be modeled to be discarded. In this paper we use an alternative approach and assume that most energy in a measured wavefield is signal, and then we use variational methods to separate this signal into a simple part, which can be modeled by conventional seismic processing, and a complex part that is not amenable to investigation by these methods. In this approach, the complex part does have interpretation value and should not be discarded as noise. The simple part may be input to numerical processing such as depth migration, residual static after depth migration, and multiple attenuation, among others.

Poststack estimation of three‐dimensional crossline statics

In three‐dimensional land data acquisition, the crossline dimension of the receiver spread is often much smaller than the inline dimension, typically for reasons of economy. Because of fundamental wavenumber limitations in the estimation of residual statics by prestack surface consistent methods, unresolved static errors will persist through processing to the stack data, particularly in the crossline direction. The present method involves an analysis of poststack 3-D data through the creation of a correlation time surface from crosscorrelations of adjacent stack data traces. This time surface is decomposed in the wavenumber domain to isolate and correct some of the spectral components of residual static errors which are beyond the resolution of prestack approaches. Assumptions are implicit within the method regarding the expected 2-D wavenumber nature of the wave field response of true geologic structure. The method is proposed as an additional procedure to the standard 3-D processing sequence for land data.

Unconstrained near-surface velocity inversion of linear moveout velocities and its impact on structural mapping

Summary The near-surface velocity model in seismic exploration is the basis of static calculations, but its applicability goes beyond statics. These shallow velocities are also important for lithologic interpretation, for seismic processing including datuming, time and depth migration, and for structural interpretation. Most of the near-surface velocity information in seismic data resides in the traveltimes of the first arrivals. There are many algorithms that use them to estimate the velocity model of the near-surface. Of all methods, turning-ray tomography has recently gained wide acceptance, mainly by its ability to explain accurately first arrival traveltimes assuming that the energy propagates along continuously refracted rays. In this paper we describe a simple and efficient method to obtain nearsurface velocity information using a simplified version of turning-ray tomography. Our intention is to recover the long wavelength components of the velocity field without the need to run a full tomography based static solution. To accomplish this we use linear moveout velocities instead of first breaks in the inversion. Linear moveout velocities are easy to obtain and contain the long wavelength information that we are interested in recovering. Our inversion has two steps, an intercept-time to depth inversion using the Herglotz-Wiechert inversion, and a 3-D tomographic backpropagation of slowness anomalies. The method can be extended to include reflection data for a joint inversion of linear moveout and hyperbolic moveout.

Geometric simplicity as a migration criterion: an application of computational topology to seismic imaging

Summary With more sophisticated algorithms like reverse-time migration and waveform inversion, it requires a closer examination as to how we do preprocessing of the input seismic data. Too much signal processing could remove signal which used to be called “noise” since previous noise was an incomplete understanding of signal based on the wave equation. Too little preprocessing could cause migration artifacts when the migration operator is presented with data which does not fit the assumptions. We introduce a new criterion for migration based on geometric simplicity. Most migration optimization or tomography optimization methods are based on amplitude like RMS measurements or semblance or correlations. Geometric constraints are usually difficult to define for migration or inversion since geometry is in general a global concept. In this paper, we will use the Betti numbers from computational topology to describe geometric simplicity. The larger the Betti numbers are in an area, the more complex the seismic response would be for migration. The B0 (Betti number of zero homology group) measures connectivity and B1 (Betti number of first homology group) measures one-dimensional holes and B2 (Betti number of second homology group) measures twodimensional holes. This gives a geometric criterion for migration or inversion methods like tomography.

Waveform gather inversion and attribute-guided interpolation: A two-step approach to log prediction

The prediction of log information from seismic data plays an important role in the characterization of oil and gas reservoirs because log information can be used in such applications as lithology discrimination, porosity estimation, fluid identification, and drilling hazard assessment.

Time-lapse simulator-to-seismic study - Forties field, North Sea.

Summary A Simulator-to-Seismic (S-to-S) study is conducted over the Forties field, North Sea, by combining petrophysical, engineering and geophysical data into an integrated workflow. Based on a calibrated rock physics model, static and dynamic properties from a reservoir model are converted into time-lapse synthetic seismic attributes that can be used in conjunction with conventional seismic attributes to get more insights into the interpreted 4D anomalies. The aim of the study is to better understand the drainage patterns of the reservoir and help validate the planned position of additional producer wells to ultimately reduce drilling risks.

Imaging Beneath Complex Layering: Processing Seismic Data With Complex Decomposition And Renormalization Group

Seismic Imaging beneath complex layering is challenging and problematic. Examples of complex layering are complex shale, carbonate, basalt and salt. Both the quality of the image and wavelet fidelity of the reservoir beneath the complex layering are usually not satisfactory below complex layering, largely because of the associated multiples and elastic waves. These effects interfere with the primary reflection. The problem is more acute for seismic reservoir analysis where agreement between observed logs data and seismic is important. Apache Corporation and WesternGeco undertook a Strategic Technology Alliance Project of a modeling study to investigate old and new seismic processing methodology. The purpose is to do an in-depth analysis of assumptions and approximations in the conventional seismic processes. The study identifies which of these programs are ineffective in the presence of interference due to strong interaction in the overburden. Then we can replace these programs with the new processing programs like complex decomposition and renormalization group.

Diffusion semigroups: A diffusion-map approach to nonlinear decomposition of seismic data without predetermined basis

Summary We present a nonlinear processing methodology designed to separate out various structures in the seismic data. This methodology builds on the strategy of decomposing seismic data into simple and complex parts (see Lau et al 2008) in which a variational minimization approach extracts the simple structure and complex structure. This paper presents an alternative construction using diffusion semigroups. It is a nonlinear method like the variational method. But it does have the advantage of controlling the components in the nonlinear decomposition . We show that by using various diffusion geometric tools we can build nonlinear filters which enable a decomposition of the data into various intrinsic substructures designed to facilitate interpretation tasks.

Repeat Imaging Using Grid Tomography And New Well Information: Application to Two Sub-salt Fields In the Gulf of Mexico

The goal of repeat imaging (detailed studies of prospects as new information is available) using PSDM is to generate new locations for both development and exploration targets. We present two case studies of repeat imaging and grid tomography in the Gulf of Mexico acreage. The first field (field H) consists of angular beddings truncating at the base of a thick salt. The second field (field T) consists of sediments under a salt of unknown thickness where well control shows only thin salt penetration. The up dip potential of the pay sands for both prospects had not been shown clearly in the previous depth processing. A careful model building approach with automated residual moveout estimation and 3-D grid tomography in the supra-salt area enabled us to estimate the sediment velocity properly. This helped us to reinterpret the top salt and base salt and enhance the sub-salt reflections. We provide two examples of the velocity model building and depth imaging in this area.