Causes of Gas Reservoir With High Water Saturation and Low Gas Saturation in the High-Temperature and High-Pressure Zone of the Yinggehai Basin, South China Sea

Abstract

Natural gas reservoirs in the high-temperature and high-pressure zone of the Yinggehai Basin, South China Sea are characterized by high water saturation and low gas saturation, the cause of which is not fully understood. Some researchers support high condensate water content as the probable cause. In this study, the water content in the coexisting gas phase was measured under high temperature and high pressure, using an ultra-high pressure fluid PVT system, with the experimental temperature reaching up to 453K and pressure up to 130MPa. The maximum gas saturation was measured for samples with different physical properties using the micropore membrane technique. By combining experimental results with studied of geological data from the Yinggehai Basin, the causes of high-water saturation, low-gas saturation gas reservoirs in the high-temperature and high-pressure zone are proposed, as follows: The water content in the coexisting gas phase performs positive correlation with temperature, and negative correlation with pressure. The water content increases with the rise of temperature, and the increasing rate is slower at lower temperature and faster at higher temperature. However, the water content decreases with the rise of pressure, and the decreasing rate is faster at lower pressure and slower at higher pressure. In this experiment, the water content is not very high at the highest temperature of 453K and the highest pressure of 130MPa, because its mole fraction only accounts for 1.51% of the whole gas phase system. This indicates that the condensate water content in the gas reservoir is not very high at high temperature and high pressure. Hence, the output water of gas reservoirs is primarily due to pore water in the layers, and condensate water is not the main source. The gas saturation of gas reservoirs are mainly controlled by the reservoir physical property and the impermeable interlayers. When the reservoir physical property becomes poor, its maximum gas saturation will rapidly reduce. Besides, impermeable or poor permeable interlayers will also lead to the reduction of gas saturation in the gas reservoir. Therefore, low permeability reservoirs (mainly (0.1~100)×10–3μm²) and the occurrence of interlayers should be the main causes of high water saturation and low gas saturation gas reservoirs in the high-temperature and high-pressure zone of the Yinggehai Basin, rather than the high condensate water content.

AAPG Datapages/Search and Discovery Article #90216 ©2015 AAPG Annual Convention and Exhibition, Denver, CO., May 31 - June 3, 2015