Identifying shallow gas zones by using seismic attributes, offshore Vietnam

Abstract

Shallow gas zones are a major concern in offshore drilling because of their potential to quickly cause kicks or blowouts. Shallow gas hazards are identified by using a series of seismic attributes. We have combined seismic data analysis and well-log analysis to identify the location and distribution of shallow gas layers. These shallow gas zones are formed during a large influx of gas due to a blowout well. The blowout well is located in the Nam Con Son Basin, offshore Vietnam. The seismic data acquired before and after the blowout record the changes in the shallow gas location. We compare seismic data without gas effects to data affected by gas after the blowout. The changes of reflectors between 2D seismic data (preblowout well) and 3D seismic data (postblowout well) are analyzed by using seismic attributes. The shallow gas is recognized in seismic data based on several criteria such as the push-down effect that demonstrated the delay in traveltime throughout the slower zone, high amplitude with negative phase reflection at the top of shallow gas layers, and acoustic blanking from wave scattering and amplitude attenuation. Geobodies mapped are associated with shallow gas zones by merging seismic attributes to identify zones that are a combination of strong amplitudes and low frequencies. The attributes that identify known shallow gas anomalies are also applied to the entire seismic volume for identification of shallow gas hazards.

Characterization of gas‐bearing sediments in the coastal environment using geophysical and geotechnical data

“…Four major gas areas (areas 1 to 3 in Figure 2 and area 4 in Figure 3) were identified in the data and characterized by typical acoustic behaviours (Thi Van Ngo & Ferguson, 2020): strong acoustic turbidity (gas blanking on seismic profiles, observed when the acoustic energy is scattered by gas bubbles) and high‐amplitude reflection. The occurrence of free gas strongly reduced the penetration of the acoustic signal, which is then limited to the gas caps 5 m to 20 m BSF (areas 1 to 3 in Figure 2b and area 4 in Figure 3b).…”

“…Gas-bearing marine sediments are very common and widely found across the world (Fleischer et al, 2001). They have been largely studied as they are known to cause major accidents, such as well blow-outs (Thi Van Ngo & Ferguson, 2020), possible sinking of ships (Deming, 2004) or drilling rigs (Prince, 1990).…”

“…Gas‐bearing marine sediments are very common and widely found across the world (Fleischer et al., 2001). They have been largely studied as they are known to cause major accidents, such as well blow‐outs (Thi Van Ngo & Ferguson, 2020), possible sinking of ships (Deming, 2004) or drilling rigs (Prince, 1990). The predominant biogenic gas in marine sediments is methane, a well‐known and powerful greenhouse gas (Hovland & Judd, 1992), much studied in the context of climate change.…”

“…Gas‐bearing marine sediments have practical (related to imaging and engineering issues), environmental and safety significance (Fleischer et al., 2001). Thus, the identification and avoidance of shallow gas is a principal objective during site survey procedures (Thi Van Ngo & Ferguson, 2020).…”