Maury, V. M. R. and Grasso, Jean-Robert and Wittlinger, G. (1992) Monitoring of subsidence and induced seismicity in the Lacq Gas Field (France): the consequences on gas production and field operation. Engineering Geology, 32 (3). pp. 123-135. DOI: https://doi.org/10.1016/0013-7952(92)90041-V
Full text not available from this repository.Abstract
Between 1957, when the Lacq gas field was first put into production, and 1967, 2.5 cm of subsidence occurred. This corresponded to a period in which the pressure had been depleted by 30 MPa. Further depletion from 1967 until 1989, of 25 MPa induced an additional subsidence of 3.0 cm, which coincided with more than 1000 earthquakes; 44 with magnitudes greater than 3, and 4 with magnitudes greater than 4. The strain energy release rate by seismic events increased for the period 1969–1979 and now decreases yearly. The method used for processing and interpreting the seismological data takes into account the lithologies and the mechanical properties of the rocks and the source parameters. The regional mechanism of the gas field deformation appears to be the lowering of a rigid block located at the top of the structure by a succession of movements along pre-existing fractures and faults in the reverse direction to the original. From spectrum analysis of seismic recordings, the amplitude of the fault slips has been calulated to be of the order of millimetres. This may be the reason why there has been no damage to the wells. The radii of the sources may give an indication about the size of the blocks to be considered in the reservoir engineering models. In spite of the obvious non-elastic behaviour of the seismic events, the regional response of the rock mass is an elastic one. The rock mass can be modeled as a block assembly, with localized movements between some blocks being responsible for the induced seismicity. The size of the induced events is, therefore, limited by the pre-existing fractures. Close monitoring of subsidence and seismic events is a new and useful aid to understanding the reservoir response to fluid withdrawal, the practical consequences with regard to safety and gas production, and a contribution to our knowledge of reservoir behaviour over the course of time.