Paradoxes of Anomalous Subsidence at Long-Developed Oil and Gas Fields

A review of anomalous subsidence (more than 1 m) of the earth’s surface at long-developed oil and gas fields measured in different regions around the world is presented. It is shown that the overwhelming majority of cases were obtained at oil fields. Anomalous surface subsidence (about 1 m) was found only at one gas field. This is an unexpected result. Usually, the development of oil fields is accompanied by fluid injection, which should reduce the amplitude of subsidence. At gas fields, fluid injection does not occur, and this should contribute to the accumulation of large amplitudes of subsidence. Two paradoxes are formulated: large subsidence in oil fields and small subsidence in gas fields. Geomechanical models for explaining these paradoxes are presented. It is shown that, to explain the paradox of large subsidence, it is necessary to consider the processes of destruction of reservoir rocks during the periodic injection of liquid into the formation. The paradox of small subsidence in gas fields is due to the need to take into account the initial stress–strain state of the field and the dynamics of mechanical properties of rocks (porosity and compressibility) over the entire period of development.