NS44A-01 INVITED
Dealing With Shallow-Water Flow in the Deepwater Gulf of Mexico
Some of the Shell experience in dealing with the shallow-water flow problem in the Deepwater Gulf of Mexico (GOM) will be presented. The nature of the problem, including areal extent and over-pressuring mechanisms, will be discussed. Methods for sand prediction and shallow sediment and flow characterization will be reviewed. These include seismic techniques, the use of geo-technical wells, regional trends, and various MWD methods. Some examples of flow incidents with pertinent drilling issues, including well failures and abandonment, will be described. To address the shallow-water flow problem, Shell created a multi-disciplinary team of specialists in geology, geophysics, petrophysics, drilling, and civil engineering. The team developed several methodologies to deal with various aspects of the problem. These include regional trends and data bases, shallow seismic interpretation and sand prediction, well site and casing point selection, geo-technical well design and data interpretation, logging program design and interpretation, cementing design and fluids formulation, methods for remediation and mitigation of lost circulation, and so on. Shell's extensive Deepwater GOM drilling experience has lead to new understanding of the problem. Examples include delineation of trends in shallow water flow occurrence and severity, trends and departures in PP/FG, rock properties pertaining to seismic identification of sands, and so on. New knowledge has also been acquired through the use of geo-technical wells. One example is the observed rapid onset and growth of over-pressures below the mudline. Total trouble costs due to shallow water flow for all GOM operators almost certainly runs into the several hundred million dollars. Though the problem remains a concern, advances in our knowledge and understanding make it a problem that is manageable and not the "show stopper" once feared.
NS44A-02 INVITED
On estimating the impulse response between receivers in a controlled ultrasonic experiment
Seismology may have taken up a completely new way of imaging the Earth's interior by crosscorrelating what used to be considered noise, recorded at different receiver locations. However, to obtain the full Green's function, we need either strong scattering (and little intrinsic absorption), or sources distributed everywhere throughout the medium. The exploration seismologists are attempting to implement this technique under the nomen of seismic interferometry or the virtual source method. Near-surface geophysics could benefit from these developments as well. In order to understand the possibilities and limitations of these ideas, a controlled ultrasonic laboratory experiment provides a detailed analysis of retrieving a band-limited estimate of the Green's function between receivers in an elastic medium. Instead of producing a formal derivation, this presentation appeals to a series of intuitive operations, common to geophysical data processing, to understand the practicality of seismic interferometry. Whereas the retrieval of the full Green's function is based on the crosscorrelation of receivers in the presence of equipartitioned signal, an estimate of the impulse response is successfully recovered with 40~sources in a line covering six wavelengths at the surface.
NS44A-03
Gas Hydrate Estimation Using Rock Physics Modeling and Seismic Inversion
ABSTRACT We conducted a theoretical study of the effects of gas hydrate saturation on the acoustic properties (P- and S- wave velocities, and bulk density) of host rocks, using wireline log data from the Mallik wells in the Mackenzie Delta in Northern Canada. We evaluated a number of gas hydrate rock physics models that correspond to different rock textures. Our study shows that, among the existing rock physics models, the one that treats gas hydrate as part of the solid matrix best fits the measured data. This model was also tested on gas hydrate hole 995B of ODP leg 164 drilling at Blake Ridge, which shows adequate match. Based on the understanding of rock models of gas hydrates and properties of shallow sediments, we define a procedure that quantifies gas hydrate using rock physics modeling and seismic inversion. The method allows us to estimate gas hydrate directly from seismic information only. This paper will show examples of gas hydrates quantification from both 1D profile and 3D volume in the deepwater of Gulf of Mexico.
NS44A-04
Examples of submarine geo-hazard by high resolution swath bathymetry images
Research into the causes and effects of submarine geo-hazards has grown rapidly in recent years. The causes can be natural or man-made. The effects can range from minor sediment mobilization to devastating tsunamis. In the worse cases it is possible for coastal populations and economies to suffer serious damage. In order to recognize causes and predict likely outcomes, much technological effort has been spent developing bathymetric systems that image features on the sea floor with unprecedented clarity. The most successful are the so-called swath bathymetric systems that rely on multiple acoustic beams and precise satellite positioning to produce very accurate surveys of the sea floor. The resolving power of the resulting images depends largely on the height above the water bottom at which the acoustic sources and detectors are deployed. In relatively shallow water, deployment may be at or near the water surface, perhaps on the hull of a ship. In deep water, deployment on an autonomous underwater vehicle (AUV) operating a few meters above the bottom improves image resolution greatly. This presentation shows examples of both deep- and shallow-water images. Two are from AUV surveys of petroleum lease blocks in the northern Gulf of Mexico; one of extensive sea-floor slumping and the other of a mound that contains gas hydrates and vents from which hydrocarbon fluids enter the water column. Examples from the Mediterranean Sea, collected by a surface-deployed system, concern volcanoes slope stability and potential relationship with tsunami. Images of slumps and other sediment mobilization in shallow water are also shown. Further possible developments in the swath bathymetry technique and its use with other geophysical methods are discussed. It is concluded that a complete integration of various imaging methods promises to promote a better understanding of the processes that initiate geo-hazards on the sea floor.
NS44A-05
Detection of Shallow And Pressured Aquifer Sands Using Seismic Inversion Techniques
Shallow water flow (SWF) layers are frequently encountered in deepwater areas when drilling into poorly consolidated geopressured sands. These sands, when flowing, can cause extensive damage to a borehole. SWF sands are known to occur in water depths of 450 m or more and typically 300-600 m below the mudline. They are known to be present in almost all deepwater ocean basins where the rate of sedimentation is high. Recent studies in literature show that these nearly unconsolidated sands exhibit low bulk densities and anomalously low P- and S-wave velocities and high Vp/Vs ratio. Seismic data has long been recognized as key to SWF detection prior to drilling. In this paper, we present an integrated five-step approach for SWF detection, in which seismic inversion technique is the key to derive a quantitative measure of SWF probability. The first step requires large-offset 3D seismic data, reprocessed at a 2-ms sampling interval with particular emphasis to the shallow target areas. The second step involves stratigraphic interpretation of the reprocessed and stacked data to outline potentially hazardous sand bodies. The third step, which confirms the potential for SWF identification prior to prestack inversion, is AVO analysis of the prestack data. After AVO analysis and identification of potentially hazardous zones, prestack waveform inversion is carried out at selected locations. This is the fourth and most important step. For prestack inversion, we use the methodology developed by Mallick (1995, 1999) which allows accurate estimates of Vp, Vs, and bulk density as functions of time and depth at the selected locations. The fifth and final step deals with pore-pressure estimation using Vp and Vp/Vs values derived from the prestack inversion. The high-resolution Vp and Vp/Vs field, obtained from the prestack inversion, provides more accurate pore pressure than that predicted from a conventional velocity analysis. We will show Examples from real data in the Deepwater Gulf of Mexico.
NS44A-06
Dynamic Modeling of Speed Profiles of Extreme Dry Snow Avalanches
We present an improved method for calculating the speed of long-return-period, or extreme, dry snow avalanches. This is a response to the uncertainty surrounding the dynamics of frontal ploughing, basal erosion, and entrainment of existing snowcover by a flowing avalanche. This uncertainty is implicit when modeling an avalanche beginning from rest in the starting zone, and is often neglected entirely in dynamics calculations. To better manage this uncertainty, the new method restricts the domain of interest from the middle of the avalanche track down to the runout zone. An empirical relation for the maximum probable speed for an extreme avalanche defines an initial, non-zero flow speed for a calculation beginning in the middle of the path. As the flow decelerates, entrainment is assumed to be negligible compared to the acceleration phase in the upper reaches of the path. The flow mass is discretized longitudinally, and the path width is allowed to vary. A frontal stopping position is specified as a model input, based on available regional runout data. A one- parameter resistance law is assumed, with the friction coefficient determined by the stopping position. In comparisons with measured avalanche speed profiles, the new method better reproduces the observed sharp deceleration of frontal speed in the runout zone.