Gas Hydrate Characterization Using Marine EM Operationss

Introduction

Gas hydrate is important issue as an energy resource and a potential hazard to offshore drilling and development. It could be a factor in seafloor slope stability and source of over pressured free gas during well spud and drilling operations.

For all of these reasons, understanding  a distribution and quantity of hydrate in marine sediments is important.

Naturally, methane hydrates widely occur in deep water areas but direct evident is limited because existing geophysical Operationss are not sensitive to bulk concentration of hydrate in the seafloor sediments. Seismic Operations is able to detect  the seismic bottom simulating reflector (BSR), that is an indicator of the base of the hydrate stability field, but it is ineffective at predicting the amount of hydrate between the seafloor and the BSR.

Electromagnetic (EM) Operationss, are sensitive to the concentration and geometric distribution of hydrocarbons and gas hydrate as one of them.

Black sea project

The main aim of the project was to investigate the capacities of marine electromagnetic (EM) surveying for detection and assessment the occurrence of natural gas hydrate in the sub-bottom sediments. The study based on IP field data acquisition and quantifying the inducted polarization parameters of natural gas hydrate deposit proved by previous seismic survey.

This project carried out the EM survey in the Tuapse trough of the Western  Black sea over areas where hydrate is known due to seismic and seabed sampling.

Study area locates in water depths ranging from 500m to 2000m.

The project employed patented EM  techniques that incorporated conventional offshore hydrocarbon exploration Operationss and newly developed Operationss designed by SOLITON ltd specifically for the purpose of hydrate mapping.

Also, laboratory measurements of natural methane hydrate features were used to calibrate the EM data obtained from the field surveys in terms of hydrate concentrations.

Results

The results of this study successfully demonstrating the ability of marine EM Operationss to detect an occurrence and quantify hydrate content in marine sediments.

Hydrate deposits are not stratigraphic and even extensive drilling provides only point measurements with no reliable Operationsology for interpolation or extrapolation to regions beyond the wells.

An attribute analysis of seismic data has recently been used to indicate areas where hydrate occurs, but not the concentrations. EM technique advantages are obvious if an area can be comprehensively surveyed using only a few days of ship time, or if continuous profiling can be carried out, this would represent the first steps toward the possibility of a direct assessment of hydrate and could contribute to the discovery of economically viable hydrate concentrations.

Induces Inducted Polarization (IIP) and Secondary Seismo-Electric Effect (SSEF) sections were designed for survey area.

The main study was carried out in the Kerch- Taman area that is rather typical for deep water basin. Four lines independently give a resistive body that associated with a mud volcano at the western slope assures that this is an attribute of the data due to geology.  The acoustic blanking  zones, at seismic lines are attributed more to free gas and gas hydrate than autogenic carbonate because of several hydrate crystals samples collected in sub-bottom sediments in this area. In TDEM- IIP sections several anomalies of increased polarization were detected  at the similar depth with seismic indicators of gas hydrates.

The study results show an advantages of integration of EM and seismic data for exploration of non massive gas hydrates accumulations when traditional seismic indicator (BSR) is not stable or absent. Three promising areas with predicted gas hydrate accumulation were mapped as a result of this study.

The South China sea project

The study was carried out with High Resolution Electromagnetic (HREM) technique application designed by Soliton Ltd for marine deep-water water areas. The dipole array with electrode offset  up to 1300м was applied.

As a result of HREM data processing and interpretations an obvious indicators of gas hydrates were detected in two areas on the continental slope. A results of integration of HREM and seismic data prove that in some areas the gas hydrates do not show any evident in seismic sections, but at the same place, EM data clearly indicate a gas hydrate occurrence in the sub- bottom sediments. Also, gas accumulations below hydrate seal were detected in HREM sections. It was concluded that presence of large accumulations of free gas introduces practical concern at prediction of geo-hazards to deep-water wells drilling and operations.

HREM Operations shows an obvious sensitivity to the concentration and geometric distribution of hydrocarbons and gas hydrate.

Case study, Black Sea

Case study, China 2019

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