深盆气成藏数值模拟

深盆气藏是一种有别于常规气藏的特殊气体聚集类型,在靠近盆地中心的下倾部位致密砂岩内聚气、而在上倾方向渗透性较好的砂岩中含水,并且具有异常压力特征。数值模拟结果表明,深盆气的聚集是一种动态的气、水驱替过程,深盆气藏的异常压力特征也是这一过程中不同阶段气、水流动关系的一种表现。当气体供应速率大于散失速率时为气排水的过程,气、水同向流动,此时气藏表现为超压异常;当气体供应速率小于散失速率时为水排气过程,气、水逆向流动,此时气藏表现为低压异常。供气速率的自然变化导致深盆气的演化过程表现为静压→超压→低压→静压的压力变化过程,伴随着这一压力变化过程的含水饱和度表现为高→低→高的变化过程。大面积致密砂岩的存在、砂岩的低渗透性以及充足的天然气供应都是形成深盆气藏的有利条件。

Numerical simulation on accumulation mechanism of deep basin gas

Deep basin gas traps are the special gas accumulation styles different from those of conventional gas traps. Gas is commonly accumulated in tight sand rocks in the center of basin, and water is saturated down deep in the porous sand rocks. The deep basin gas traps are characterized by abnormal pressure. Numerical simulation indicated that the deep basin gas accumulation was a dynamic gas-water displacement process. The abnormal pressure is an indication of flow relationship between gas and water at different stage of the displacement process. When the gas supply rate is more than the gas loss rate, water is drained away by gas. At the same time, gas and water have the concurrent flow, and the gas trap is over pressurized. When the gas supply rate is less than the gas loss rate, water is imbibed with gas. Gas and water have the reverse flow, and the gas trap is under pressurized. The natural variation of gas supply rate can result in the deep basin gas traps changing from static pressure to overpressure to under-pressure and to static pressure. It also results in the water saturation changing from high to low and again to high. The existence of vast area of tight sand rock, low permeability of sand rock and plenteous gas supply are all favorable conditions for the formation of deep basin gas traps.