茂原型水溶性ガス田の産出挙動における毛細管圧力と浮力の働きについて

Abstract

The Mobara gas field is one of the dissolved-in-water type natural gas fields. This field exists in Chiba Prefecture, Japan. The wells in this field have particular production performance curves called “Mobara type”. The gas water ratio curves are initially low, and then increase to be so high as to blow up a large amount of water.The Mobara gas deposit consists of alternate layers of sandstones and mudstones. We applied mud logging to exploration of the Mobara gas deposit. As the result, a small amount of free gas was detected in the mudstones.In addition to exploration, we carried out laboratory experiments using an X-ray CT scanner and mudstone cores. In the experiments, gas and water flow rates and CT values were measured at the same time. Gas water ratio increased with incremental drop of pore pressure, and gas saturation calculated from CT values also increased.From analyzing the exploration and the experiments, following conclusions were obtained.(1) In the Mobara gas deposit, gas bubbles of extremely small size are stored in pore bodies of the mudstones.(2) Upward buoyancy forces and downward forces given by capillary pressure difference act on gas bubbles at all times. These forces are initially in equilibrium.(3) With the start of production, hydraulic gradients occur and capillary pressure actions change in mudstones. The gas bubbles begin to move through pore throats toward sandstones consequently.The above-mentioned experiments suggest that buoyancy and capillary pressure have an important effect on the Mobara type production performance.