Numerical Modeling with a Grid-Characteristic Method of Elastic Wave Propagation in Geological Media with Gas Cavities
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rical Modeling with a Grid-Characteristic Method of Elastic Wave Propagation in Geological Media with Gas Cavities P. V. Stognii1* , N. I. Khokhlov2** , and I. B. Petrov1, 2*** 1
2
Moscow Institute of Physics and Technology, Institutskii per. 9, Dolgoprudnyi, Moscow Region, 141700 Russia
Scientific Research Institute for System Analysis, Russian Academy of Sciences, Nakhimovskii pr. 36/1, Moscow, 117218 Russia Received April 11, 2019; in final form, June 12, 2019; accepted April 16, 2020
Abstract—Shallow gas in ground water geological layers is of great danger for drilling rigs in the case of accidental opening of gas deposits. The gas may rise towards the water surface, and its emission into the atmosphere may threaten the environment. It is very important to forecast the gas emissions in order to prevent catastrophic destruction of drilling rigs and loss of human lives. This paper presents the results of numerical simulations of seismic wave propagation towards the water surface in models with gas deposits in a layered soil for the 3D case. The modeling is carried out for a 4-year period for layers located at a depth of 1000 m from the sea bottom. The calculation results (wave patterns and seismograms) show that the gas may approach the water surface in the 4th year of the calculations. The results for a 3D problem are in agreement with those for a 2D problem obtained by the authors earlier. This is very important, since the calculations can be greatly simplified. DOI: 10.1134/S1995423920030076 Keywords: gas pockets, numerical 3D modelling, grid-characteristic method, Arctic shelf.
1. INTRODUCTION Gas pockets or near-surface gas are gas reservoirs with anomalously high reservoir pressure (AHRP) [1–4]. Such gas reservoirs can be located at small (30–100 m) and considerable (1000 m) depths from the sea bottom. To date these gas pockets have been considered not dangerous, since spontaneous emissions have been very rare. However, arbitrary gas emissions into the atmosphere become more frequent due to global warming. This is because many gas pockets hidden in frozen rocks are no longer gas-tight and begin to spread to the water surface. There are also gas emissions into the atmosphere due to earthquakes [5]. Accidental opening of gas reservoirs by drilling rigs often takes place. Due to high pressure in the reservoirs, the gas rises to the surface and enters the atmosphere. This may cause loss of human lives, damage to the rigs, and increase the gas content in the water. A severe accident took place in the Gulf of Mexico in 2010 largely as a result of a gas emission with AHRP. it killed 11 people, destroyed a modern drilling rig, and caused the penetration of a large amount of oil into the gulf (about 0.7 million tons). To avoid serious consequences from such emissions and undertake timely measures, systematic monitoring of the territories with gas deposits is required. Gas pockets are widespread in the North Seas. For instance, the number of only known gas deposits in the Sea of Okhotsk is about 200 [6], and on the N
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