Sediment damage caused by gas exsolution: A key mechanism for mud volcano formation

Abstract

Gassy sediments are common in marine environments and are characterized by a specific mechanical behavior significantly different from that of water-saturated sediments. It is shown that gas causes damage and initiates fractures in sediments. To define the controlling parameters dominating the damage process during gas exsolution and its consequences in terms of hydro-mechanical behavior, we developed a specific consolidation apparatus to test sediments collected from the active Absheron Mud Volcano. Indeed, mud volcano formation is initiated by gas exsolution and subsequent mud generation at depth from stratified sediments. Gas was generated in the samples by circulating carbonated water through the fine-grained sediments, then decreasing the total pressure. Particular attention was given to the impact of gas saturation and associated damage and fractures on P-wave velocity, sediment compressibility, permeability and preconsolidation pressure. Results show that fracture geometry is mainly controlled by the degree of gas saturation and preconsolidation pressure. The damage level increases with the degree of gas saturation while the elastic modulus of sediments is degraded. Experimental data show that sediments do not entirely recover their original mechanical behavior when gas dissipates. Finally, the experimental data confirm that gas exsolution and expansion is a key mechanism for mud generation. Highlights ► Fracture geometry is controlled by the degree of gas saturation and preconsolidation pressure ► Gas exsolution provokes sediment damage and change in mechanical behavior ► Sediments do not recover their initial behavior completely after reloading ► Gas exsolution is able to generate mud from compacted sediments

Evolution Model for the Absheron Mud Volcano: From Stratified Sediments to Fluid Mud Generation

“…Blouin, Sultan, et al. (2019) observed the same behavior on sediments from the AMV, mud being generated for a degree of gas saturation of 38%.…”

“…First, dissolved methane distribution is disturbed and partly depleted around the faults compared to the simulation proposed in Figure 6 (see Figure 7b) due to the fact that, locally, a part of the initially dissolved methane exsolved to form free gas. Moreover, as the simulation calculates the preconsolidation pressure ( σ ' p ), based on Equation 1 (Blouin, Sultan, et al., 2019) linking S g with a preconsolidation ratio, we observe a local decrease in preconsolidation pressure in response to the presence of free methane (Figure 7d). This decrease in σ ' p is observed in the area where S g is nonzero.…”

“…In addition to available data from literature, we constrained the study using information from two exploration wells (pressure and stratigraphy) and subsurface geometry from the interpretation of a 3D seismic volume (Blouin, Imbert, et al., 2019) and using laboratory testing results showing the impact of gas exsolution on the mechanical properties of compacted sediments (Blouin, Sultan, et al., 2019). Two different numerical models were used to model mud generation at depth and to simulate local processes related to mud remobilization toward the surface.…”

“…The degree of gas saturation is consequently used to calculate the impact of gas exsolution on sediment damage through the equation obtained from laboratory testing on the AMV sediments (Blouin, Sultan, et al., 2019): …”

“…The degree of gas saturation was taken at 38 %, that is, the limit for mud generation based on the laboratory testing by Blouin, Sultan, et al. (2019). It allowed the calculation of the mud density using Equation 4.…”