Ali Tura

Monitoring primary depletion reservoirs using amplitudes and time shifts from high-repeat seismic surveys

In the high-porosity, poorly consolidated turbidites of the deepwater Gulf of Mexico, production-induced compaction is the main production-drive mechanism when aquifer support is weak and prior to pressure support by secondary recovery water injection. Time-lapse (4D) seismic monitoring of this class of reservoirs has provided several new learning opportunities. The time-lapse amplitude response of these fields can be complicated due to saturation changes (water replacing oil) inside the reservoir, rock compaction causing density and velocity changes inside the reservoir, stress relief and associated deformation of the rock outside the reservoir, and changes in reservoir fluid pressures due to pore-pressure decrease. Methods that rely on time-lapse amplitude changes with offset to discriminate pressure and saturation changes can help separate and thus simplify the interpretation of some of these effects (Tura and Lumley, 1999; Landro, 2001).

3-D AVO migration/inversion of field data

This article describes the methodology for AVO migration/inversion and presents the results of its first field application. 3-D AVO migration/inversion can be applied in areas with lateral velocity variations and dipping events in which conventional AVO analysis is not valid. The method has been tested on both synthetic and field data in 1-D, 2-D, and 3-D cases and its accuracy verified.

Estimating and interpreting velocity uncertainty in migrated images and AVO attributes

Estimating a suitable velocity field for use in prestack depth migration is inherently uncertain because of limitations on the available data and estimation techniques. This uncertainty affects both the migrated depth of structures and their amplitudes in the inverted images. These effects can be estimated by performing multiple migrations with a set of velocity fields and colocating features in the migrated images. This lets us examine the imaging procedure’s sensitivity to changes in the velocity field so we can assess both structural and amplitude uncertainties in migrated images. These two types of uncertainties affect interpretation in different ways. For instance, with structural uncertainty interpretation we consider the change in migrated location of structures when deciding on drilling locations, optimizing well trajectories, or computing uncertainty in volumetric calculations. With amplitude uncertainty or amplitude versus offset (AVO) uncertainty interpretation, we consider (1) uncertainty in c...

Interpreting Uncertainty Measures For AVO Migration / Inversion

We examine the sources of uncertainty at various stages of an imaging procedure consisting of amplitude preserving migration followed by AVO inversion. Uncertainty arises from three sources: velocity estimation variability, migration quality and precision of the inversion. These give rise to various uncertainties on the scales of inverted amplitude and spatial location and also scale-free measures. We demonstrate methods to estimate and interpret these uncertainties on a dataset from the North Sea. Accurate assessment of uncertainty is found to be an important component when interpretating seismic attributes.

First dual-vessel high-repeat GoM 4D survey shows development options at Holstein Field

In the Gulf of Mexico (GoM), loop and eddy currents can cause large errors in 4D shot and receiver locations between baseline and repeat streamer surveys, which invariably lead to poor data quality. In a recent 4D acquisition, a dual-vessel 3D acquisition method addressed the repeatability problem and showed reliable time-lapse measurements over Holstein Field. The time-lapse seismic data show time shifts up to 6 ms over depleting sands and amplitude changes over swept and compacted sands. This 4D information has improved understanding of the field and can support optimal placement of injection and production wells.

Feasibility of monitoring compaction and compartmentalization using 4D time shifts and seafloor subsidence

Seismic monitoring of compacting reservoirs is known to be a difficult task and depends critically on the amount of compaction of the reservoir and velocity changes of the rocks as the effective stress changes (Tura et al., 2005; Hatchell and Bourne, 2005). The main question addressed in this study is whether a combination of surveillance methods can be used to help identify and manage the risks that are posed by an unknown degree of compartmentalization in a compacting offshore gas reservoir.

Introduction to this special section Rocks under stress

The objective of this special section is to show the state-of-the-art in assessing the sensitivity of rocks to stress and how this can be used to further our understanding in exploration and production. As we approach more and more quantitative use of seismic data for purposes of exploration, reservoir characterization, and reservoir surveillance, understanding how rocks react under stress and how this information can be used becomes more important. Changes in the stress field have an effect on layer thickness, seismic velocity, density, anisotropy, and fracture orientation as observed in reservoir sands, overburden/underburden shales, and in carbonates.

Time-lapse Seismic: Are We There Yet?

Time-lapse seismic can be seen as one of the major technology growth areas in recent years in the upstream oil and gas industry. Both our understanding of the subject and development of technology in this field have been substantial. Areas of technology development include data acquisition, data processing, data analysis, rock physics, borehole methods, passive monitoring, and geomechanics, amongst others.

An introduction to this special section Instrumented Oil Fields

The instrumented oil field consists of deploying permanent instrumentation to monitor an oil field and modify production continuously or on demand. This concept has evolved from recent advances in both downhole permanent instrumentation and time-lapse monitoring. Both of these technologies are new, and combining them, with permanent installations in mind, requires further steps in research and development. As a result, this special section on the instrumented oil field can be viewed as a “road ahead” section to help the reader to construct not only a vision of the instrumented oil field but also to become familiar with the many issues surrounding this new technology.

Estimating Pressure and Saturation Changes from Time Lapse AVO Data

Time-lapse (4D) seismic is a procedure where a reservoir is imaged with reflected seismic energy at several time steps while being depleted¹,².