Detection of free gas and gas hydrate based on 3D seismic data and cone penetration testing: An example from the Nigerian Continental Slope

Abstract

We present a new method to characterize free gas, gas hydrates and carbonate concretions occurrence which are considered as high-risk factors for sub-sea developments in the Niger delta. This method is based on the combination of 3D seismic data to the geotechnical site characterizations using piezocone CPTU tests (Cone Penetration Test with additional measurement of the pore water pressure). A special processing of the 3D seismic data has enabled the determination of the interval compressional velocity. Using the effective-medium theory, velocity anomalies (negative and positive) within the first 15 m were translated in gas hydrate and free gas distribution. The calibration of the P wave velocity anomalies was done thanks to in-situ geotechnical testing carried out during two oceanographic surveys (2003 and 2004). Comparison between in-situ testing, recovered cores and the prediction of the gas and the gas hydrate distribution based on the compressional wave velocity have shown that 3D seismic data is a valuable tool to identify heterogeneous areas but the use of the piezocone was essential to discriminate between gas hydrate occurrences and carbonate concretions' presence. Furthermore, in-situ compressional wave velocity (V p ) measurements have clearly demonstrated what it was suspected from the 3D seismic data, the co-existence in the study area between gas hydrate and free gas.

Hydromechanical Properties of Gas Hydrate‐Bearing Fine Sediments From In Situ Testing

“…Compression indices were estimated from piezocone results using the following equation: Where α M is the constrained modulus cone factor. Based on correlations between piezocone data and results of oedometer tests reported by Sultan et al (), a site‐specific value of α M = 1 and a value of void ratio, e 0 = 6.15 (at ) were used to calculate compression indices for both hydrate‐free and hydrate‐bearing sediments.…”

The Fourth James K. Mitchell Lecture: The CPT in offshore soil investigations - a historic perspective

“…Over the years there have been many attempts to develop cone penetrometers that can measure differential cone resistance and pore pressure, but apparently none of them were successful. However, IFREMER (France) now operates a cone penetrometer that can measure differential cone resistance and pore pressure (Meunier et al 2004 andSultan et al 2007). Also, a new cone design has been developed and patented by Gregg Drilling and Testing Inc. USA, that is based on a hydrostatically compensated cone using specially designed load cells.…”

Geometry and chronology of late Quaternary depositional sequences in the Eastern Niger Submarine Delta

“…In industry seismic records (Jermannaud et al, 2010), Quaternary units display cyclic patterns, suggesting a link with sea-level changes as observed in other continental margins, but their architecture is complicated by the presence of widespread sediment instabilities (e.g., Garziglia et al, 2010), syndepositional growth faults (e.g., Hooper et al, 2002) and fluid escape features (e.g., Bayon et al, 2007;Sultan et al, 2007).…”