Hydrate Hazards in the South Caspian Basin

Abstract

Abstract The presence of c1athrates (gas hydrates) is well established observationally in the offshore region of the South Caspian Basin. This paper examines the potential hazards that can occur from hydrates for exploration rigs, production platforms and pipelines. Problems examined include hydrate formation zones, and explosive dissociation as a result of mud flows and/or uplift and rotation of submarine mud volcanoes. In addition, flame initiation by dissociation is also examined as a potential hazard, as are hazards due to drill penetration, warm circulation mud, and drill-bit heating of hydrates. Chemical composition of hydrates is shown to play a critical role in hazard evaluation, and seismic identification procedures for hydrate detection as bottom simulating reflectors (BSRs) are encouraged. Seismically-identified hydrates can be used to estimate thermal gradients in sediments without invasive procedures being required, and so to provide limits on the maximum thickness of hydrates in sediments. In addition, seismic procedures also provide a way of estimating hydrate composition. These non-invasive methods allow identification of worst-case hazard possibilities from hydrates. Introduction The offshore region of the South Caspian Basin is noted for a variety of hazards associated with gas-charged mud volcanoes. These hazards include:breccia ejecta;gas eruptions;mud flows;flame ignition;earthquakes associated with eruptions;kilometer-long fissure production to widths of a few meters and escarpments to a few meters or tens of meters;slumping, pull-away and rotation of volcanic crests;gas-charging of overlying waters;gas-charging of highly unconsolidated surficial sedimentary layers, to the point that the seismic impedance of the sediments is occasionally lower than that of overlying waters;clathrate production and dissociation;vertical and lateral motion of mud diapir crests;current washing away of surficial and near-surficial diapirs, volcanoes, and mud islands;massive overpressure caused by gas and fluid in the mud material Under particular conditions anyone or all of these hazard factors may be in effect at given times and for given locations. Based on Azeri historical data from mud volcanoes onshore, statistical probabilities were evaluated for breccia, gas eruptions, flaming, mud flows, earthquakes, and fissure production (Lerche et al., 1996), and were also used to provide preliminary assessments for hazards to exploration rigs, production platforms and pipelines in the offshore region of the South Caspian Basin. The remaining hazard factors in the above list do not have good statistical data bases of historical events from which to assess hazard probabilities in the offshore region, although isolated events are recorded. For instance, Dadashev et al. (1995) have compiled a temporal record of the appearances and disappearances of the offshore Chigil-Deniz Bank over the last 130 years, and of the numerous flame eruptions of Bakhar Bank and Zenbil Island. But, by-and-large, data are extremely sparse on mud diapirs and volcanoes in greater than 100 m of water. It then becomes difficult to provide hazard probabilities for the majority of the remaining hazards in the above list. An exception occurs for hydrates, because of direct gravity-core sampling of hydrates at or near the crests of the Buzdag, Elm and Abikh mud volcanoes (Ginsburg et al, 1992; Dadashev et al., 1995) in water depths of 480 m, 660 m and 600 m, respectively.