Diana Sava

SV-P: An ignored seismic mode that has great value for interpreters

ABSTRACTWe show that SV-P reflectivity closely matches P-SV reflectivity; thus, in concept, an SV-P image should be as informative and as valuable as a P-SV image for seismic interpretation purposes. If the dip of rock layering is not severe, the length of the SV raypath involved in SV-P imaging is approximately the same as the length of the SV raypath in P-SV imaging; thus, the important lithology-sensitive VP/VS velocity ratio determined with SV-P data should be approximately the same as the VP/VS velocity ratio determined with P-SV data. We compare velocities used in P-SV imaging and SV-P imaging to emphasize the equivalence of P-SV and SV-P stacking velocities, and therefore seismic-derived VP/VS velocity ratios, obtained with both converted-wave modes. We compare images of P-SV and SV-P data to illustrate the high-quality images that can be made with a SV-P mode. The SV-P data used in these comparisons are recorded by vertical geophones, whereas the P-SV data are recorded by horizontal geophones. In ...

Evaluation of deepwater gas-hydrate systems

The worlds offshore continental margins contain vast reserves of gas hydrate, a frozen form of natural gas that is embedded in cold, near-seafloor strata. Published estimates suggest that the energy represented by gas hydrate may exceed the energy available from conventional fossil fuel by a factor of 2 or more. Understanding marine hydrate systems has become critical for long-term worldwide energy planning. Groups in several nations are attempting to evaluate the resource and to define seafloor stability problems across hydrate accumulations.

Rock physics characterization of hydrate-bearing deepwater sediments

Estimates of worldwide hydrate resources are large, but they are also uncertain because of inherent difficulties in determining the amount of gas hydrate present in ocean sediments. Estimates of gas-hydrate concentrations across a deepwater site can vary widely. For example, estimates of the volume of gas existing in gas hydrates and as free gas on Blake Ridge offshore South Carolina (USA) range from about 70 trillion m3 over an area of 26 000 km2 (Dickens et al., 1997) to about 80 trillion m3 for a larger area of 100 000 km2 (Holbrook et al., 1996). Discrepancies between some estimates of hydrate concentrations can partly be attributed to poor understanding of how gas hydrates are distributed in their host sediments. In particular, estimates based on seismic measurements, if not supported by reliable rock physics models and by in-situ observations, can be inaccurate.

Rock physics-based integration of geologic and geophysical data for fracture characterization

Fracture characterization is of great practical importance in hydrocarbon recovery, but also in mine and well stability, earthquake studies, CO2 sequestration, and nuclear waste isolation. In particular, fracture characterization is important for hydrocarbon exploration and production, because significant amounts of hydrocarbons are trapped in tight reservoirs, where natural fractures are the main factors controlling fluid flow. The key requirement for efficient management of such reservoirs is the ability to locate fractures and to describe their parameters, such as fracture density, orientation, and type of fluid saturating the fractures.

Rock-physics Models for Gas-hydrate Systems Associated with Unconsolidated Marine Sediments

Rock-physics models are presented describing gas-hydrate systems associated with unconsolidated marine sediments. The goals are to predict gas-hydrate concentration from seismic attributes, such as primary (P)- and secondary (S)-wave velocities, and to analyze compressional-wave (PP) and converted-shear-wave (PS) reflectivity at the base of hydrate stability zones. Elastic properties of gas-hydrate systems depend on elastic properties of the host sediments, elastic properties of gas hydrates, concentration of hydrates in the sediments, and geometrical details of hydrate morphology within the host sediments. We consider various scenarios for hydrate occurrence, including load-bearing gas hydrate, pore-filling gas hydrate, and two different thin-layered models of gas hydrate intercalated with unconsolidated sediments. We show that the geometrical details of how gas hydrates are distributed within sediments have a significant impact on relationships between gas-hydrate concentrations and seismic attributes. Therefore, to accurately estimate gas-hydrate concentrations from seismic data, we need to understand how hydrates are formed and distributed within marine sediments. The modeling results for thin-layered hydrated morphologies show significant S-wave anisotropy, which may be used to infer gas-hydrate distributions and concentrations in alternating thin layers of hydrate-bearing sediments if multicomponent seismic data are available. We compare the theoretical predictions of the isotropic rock-physics models with published laboratory measurements on synthetic gas-hydrate formed in unconsolidated sands. We find good agreement between the rock-physics model of disseminated, load-bearing gas hydrate and laboratory measurements, which suggests that, in this case, gas hydrates may act as part of the mineral frame of the unconsolidated sediments.

Multicomponent seismic time-lapse cross-plot and its applications

A synthetic poorly consolidated sandstone reservoir of varying thickness, porosity, and clay content, embedded in shale is considered for this study. A reservoir segment which has constant properties equal to average reservoir properties is used for calculating seismic attributes including travel times and reflection coefficients (RCs). For a range of water saturation and pore pressure, Multicomponent (MC) seismic, i.e., P-P, P-SV, and SH-SH, travel times trough and RCs at top of the reservoir segment have been calculated. Then, we computed changes in travel times and RCs with respect to a reference travel time and RC calculated at a reference saturation and pressure. We plotted changes in RCs versus changes in travel times. The corresponding time-lapse cross-plot shows interesting patterns for saturation and pressure changes and has a potential for quantitatively discriminating pressure and saturation changes. Next, we deployed a statistical method to determine the efficacy of MC seismic in detecting production-induced time-lapse changes. The significant and representative data in time-lapse cross-plot allows us to statistically analyze the detectability of a known scenario of saturation and pressure changes using MC seismic attributes. Applying different thresholds for travel times and RCs, we constructed joint probability detectors (JPDs) that help us to compare the likelihood of detection of a known change in dynamic reservoir properties using different component of seismic data. Repeating the above mentioned procedures for reservoir segments with various porosities, thicknesses, and clay contents gives insight into a classification of various reservoir types or different parts of a laterally varying reservoir in terms of saturation and pressure discrimination and statistical detectability.

Rock-physics joint inversion of resistivity-log and seismic velocity for hydrate characterization

We present a method for joint inversion of electrical resistivity measurements and velocity data for estimating gas-hydrate concentration in deep-water environments. Our technique is based on a Bayesian approach and combines rock-physics elastic theories and empirical relations for electrical resistivity with stochastic simulations to account for the natural variability of the petrophysical parameters involved in the inversion.

Multicomponent Seismic Technology Assessment of Fluid-gas Expulsion Geology and Gas-hydrate Systems: Gulf of Mexico

Four-component ocean-bottom-cable (4-C OBC) seismic data acquired in deep water across the Gulf of Mexico were used to study near-sea-floor geologic characteristics of fluid-gas expulsion systems. Although these 4-C OBC data were acquired to evaluate oil and gas prospects far below the sea floor, the data have great value for studying near-sea-floor geology. The research results summarized here stress the importance of the converted-shear-wave (P-SV) mode extracted from 4-C OBC data. In deep water, the P-SV mode creates an image of near-sea-floor strata that has a spatial resolution an order of magnitude better than the resolution of compressional wave (P-P) data regardless of whether the P-P data are acquired with OBC technology or with conventional towed-cable seismic technology. This increased resolution allows the P-SV mode to define seismic sequences, seismic facies, small-throw faults, and small-scale structures that cannot be detected with P-P seismic data.

Sensitivity analysis of multicomponent seismic attributes to fluid content and pore pressure

We analyzed the sensitivity of multicomponent (MC) seismic (P-P, P-SV, and SH-SH) reflection coefficients (RCs) and traveltimes to water saturation and pore pressure by taking the appropriate partial derivatives. Applying this approach to a poorly consolidated sandstone reservoir partially saturated with light oil and brine, demonstrates that P-P traveltimes have the largest sensitivity to water saturation, but the least sensitivity to pore pressure. In contrast, SH-SH traveltimes have the least sensitivity to water saturation, and the most sensitivity to pressure. P-SV traveltimes have intermediate sensitivities to pressure and saturation, but are more affected by saturation than pressure. By analyzing the sensitivity of MC seismic RCs to water saturation at the reservoir top, i.e., shale over oilsaturated sandstone, and at the oil-water contact (OWC), i.e., oil-saturated sandstone over brine-saturated sandstone, we found that the absolute value of amplitudes at all angles is greatest for P-P, smallest for P-SV, and intermediate for SH-SH. In addition, the absolute values of AVO (amplitude variation vs. offset) gradients at the reservoir top and OWC can be organized in descending order as (P-SV, SH-SH, PP) and (P-P, P-SV, SH-SH), respectively. For the sensitivity of both interfaces to pressure (the reservoir top and OWC), angle-dependent relations are extracted.

Gas-hydrate Concentration And Uncertainty Estimation From Electrical Resistivity Logs: Examples From Green Canyon, Gulf of Mexico

Many gas-hydrate systems from deepwater, nearseafloor strata in the Gulf of Mexico have limited availability of data because the targeted logging intervals from these regions are deep reservoirs. The usual well-log information acquired over the gas-hydrate stability zone is restricted to gamma-ray and resistivity logs. Also sparse geotechnical data are available. Therefore, in estimating gas-hydrate concentration in these environments we should take into account the inherent uncertainty associated with our predictions.