Dear Members,
You are invited to a two-part technical with presentations being delivered by Dr. Roger Clarke, Senior Lecturer in Geophysics, MSc Exploration Geophysics Programme Manager, University of Leeds and Mr. Brendan - Student of UWI - St. Augustine

Presentation #1 by Mr. Brendan Figueira: "Gas Hydrate Indicators - Offshore Trinidad"

Abstract

Gas hydrate is a ‘hot topic’ in terms of unconventional resource plays. This solid form of natural gas is found in abundance in many permafrost and marine areas and is increasingly viewed as an energy resource of the future. Many governments, universities and oil and gas companies around the world are thus researching the methods of identifying this resource and exploiting it economically. While Brooks et al. (2004) observed hydrate in cores while conducting surface geochemical exploration coring programs offshore Trinidad; no previous study has attempted to firstly identify and then quantify the hydrate present offshore Trinidad. This study presents the preliminary results of an ongoing research project between the Petroleum Geoscience Unit of the University of the West Indies and the Ministry of Energy and Energy Industries (MOEEI), which aims to delineate and characterize natural gas hydrate deposits offshore Trinidad. In agreement with established methods of delineating potential hydrate deposits, 3D seismic data collected offshore Trinidad were examined for various gas hydrate indicators. Several positive indicators, which were identified and mapped, will be presented.

*Brendan Figueira graduated from the University of the West Indies in 2009 with a BSc. in Petroleum Geoscience. His research interests are varied and include gas hydrates and structural geology, particularly of the Galera Formation and by extension the evolution of the Northern Range. He also has more general interests in petroleum geology and geophysics. At present he is preparing to pursue a MSc. degree abroad in order to advance his career path and further his research interests.

Presentation #2 by Dr. Roger Clarke: “Seismic attenuation in hydrocarbon exploration: old pitfalls & new possibilities?”

Seismic attenuation (quantified by Q, the Seismic Quality Factor) is typically and reasonably viewed as a damaging feature of seismic wave propagation: it lowers amplitudes (so reduces detection capability and biases reflectivity measurements), suppresses high-frequency wavelet content (so reduces resolution) and induces dispersive velocities (so causes complex waveform changes frequency-dependent impedance). These detrimental effects can be corrected for, to varying degrees, if Q is known. In contrast, Q can be viewed positively: because it’s caused by fluid flow through pore spaces of all scales, and by friction between grains or crack faces, it’s an indicator of fluid saturation and of effective pressure.

This gives two important motives for measuring Q, done by comparing wavelet spectra at two or more points along a given raypath. Unfortunately, a number of factors make accurately and precise Q measurement elusive: spurious, ‘apparent attenuation’ arises from frequency-dependent source and receiver directivity and damage to wavelet spectra due to processing algorithms and interference from different seismic arrivals. In practice, we measure ‘effective attenuation’, not the intrinsic attenuation that is a petrophysical parameter. Typically, a VSP offers the easiest analysis but gives only a localised result. Surface seismic data offers cost-effective spatial coverage, but simple stacked-trace measurements smear together wavelets with different attenuation signatures – although results may be suitable for some purposes.

The Leeds group has been gradually evolving a measurement scheme using CMP gathers, that aspires to measure intrinsic Q sufficiently well that it can be used not just for remedial processing but for reservoir characterisation too. Early workflows had some success in lithology discrimination and defining reservoirs, validated by VSP data and by drilling – but in many other cases, apparent attenuation dominated results. However, these biases can cancel in time-lapse and multi-azimuth geometries, and they have yielded promising saturation-change and fracture orientation results respectively. Our current surface seismic workflow uses true-amplitude Radon-transformed CMP data, and estimates Q for a given event from variable-time-window spectral analysis of all frequencies and offsets simultaneously. Combined with spectral estimation using variable-time windows, this offers a way of making robust intrinsic attenuation measurements, and thereafter relating them to saturation and/or pressure.

Challenges remain, though. Q is itself frequency-dependent, recognised decades ago in global seismology but usually ignored in exploration work – except for disparities between Q from laboratory work at ultrasonic frequencies and field seismic data. It has been suggested that such results might, in the long term, allow measurement of permeability. The Leeds group has also been working, with some success, towards measuring frequency-dependent Q from surface seismic data, so far for near-surface geological materials.

The presentation will first review the causes of intrinsic attenuation, then show in principle and with field data how measured, or effective, attenuation can be biased and of no value. Then, our successes – and failures! – using pre-stack data will be used to illustrate the evolving efficacy of our workflows, with 3D, 4D, MAZ, and CO2 injection examples. Finally, some initial results will be shown from frequency-dependent studies, on glaciers and on a reservoir sandstone.

Date: Tuesday 1st June, 2010.
Venue: Kampo's Chinese Restaurant
Time: 11:30am - 1pm

Lunch will be provided.

Please RSVP via return email or tel no later than Thursday 27th May
This is necessary for catering purposes.

Comments or Questions?