Underground gas storage involves the storage of natural gas within the pore structure of a large volume of suitably porous rock, termed the "reservoir rock," located at some depth below the surface. Strata of relatively low porosity and permeability, termed the cap rock, normally directly overlie the reservoir rock itself. In practice, natural gas is injected under pressure from the surface down a cased borehole into the reservoir rock. Due to the permeable nature of this rock, the gas eventually fills the available pore volume, but is prevented from leaving due to the impermeable nature of the...
Underground gas storage involves the storage of natural gas within the pore structure of a large volume of suitably porous rock, termed the "reservoir rock," located at some depth below the surface. Strata of relatively low porosity and permeability, termed the cap rock, normally directly overlie the reservoir rock itself. In practice, natural gas is injected under pressure from the surface down a cased borehole into the reservoir rock. Due to the permeable nature of this rock, the gas eventually fills the available pore volume, but is prevented from leaving due to the impermeable nature of the cap rock. As a result, stresses develop within the reservoir rock, and these are transmitted to the cap rock and hence to the surrounding strata. From a rock mechanics point of view, it is these stresses and their effect on the reservoir structure as a whole, which determine the structural stability of the reservoir and the maximum level to which it may be pressurized. In order to more realistically evaluate the optimum pressure for a new or existing reservoir facility, an objective means for field evaluation of reservoir stability is required. Since field techniques were not available for the measurement of storage reservoir response directly in terms of such basic mechanics parameters as stress, strain or deformation, an indirect method involving the measurement of microseismic activity was utilized in the New Haven studies. Such microseismic techniques are based on the physical fact that many materials, including rocks, generate transient vibrations (microseismic activity) in the audible and sub-audible range when stressed and/or deformed. This research represents Phase III of a study involving the rock mechanics aspects of optimization of gas storage pressures in reservoirs and involved the detailed microseismic study of an underground gas storage reservoir. The main objectives of the overall program have been the development of techniques for evaluating optimum pressures for underground gas storage reservoirs and the evaluation of the mechanical stability of such reservoirs under operating conditions. Due to the detailed nature of the study, the associated monograph has been prepared as a two-volume set. The primary purpose of Volume 1 was to provide a detailed discussion of the microseismic technique and its application during studies at the New Haven site. The present volume (Volume II) deals almost exclusively with the acquisition and detailed processing of microseismic data from the New Haven site, and the analysis of this data in terms of various reservoir parameters. The two volumes, although complementary, were prepared to be utilized independently if desired.