On the Natural Gas Reservoir of the Niigata Gas Field

Abstract

The gas reservoirs consist of coarse sands and gravels. It is supposed that the porosity is more than 20% and that the permeability is several m.d. in sand layers and 300 m.d. or more in gravel layers. The reservoir fluid is salt water, which has gas (CH4) in solution, and its gas water ratio (GWR) is 0.35 in the depth of 100m and 1.95 in the depth of 1, 000m.The production rate of gas per well amounts to 1, 000∼6, 000m3 per day. At the initial stage of exploitation, wells were flowing. As the fluid level became lower, however, most wells are gas-lifted at present.The factors concerning the decline of production of soluble gas in Niigata gas field are as follows.1. Lowering of the reservoir pressure.2. Decrease of the GWR of the producing water.3. Deposition of the sands at the bottom of the well which intruded into the casing pipe through anchor hole.4. Growth of the scale in the casing pipe.5. Decrease of the permeability of the gas producing layers around the anchor pipe owing to the deposition of fine materials.6. Interferences of the neighbouring wells.