带油环底水凝析气藏开发方式优化及均衡开采对策——以东海盆地西湖坳陷CX北区块为例

Development mode optimization and balanced production strategies for the gas condensate reservoirs with bottom water and oil rim: A case study of CX North Block in the Xihu Depression, East China Sea Basin

东海盆地西湖坳陷CX北区块H3b、H3c油气藏为海上带油环底水凝析气藏,采用油气同采且初期优先采油的开发方式共设计了4口水平井,水平段位于油气界面以下的油环上部。为了尽快回收投资,该气藏在开发初期采用较高速度开采,油气井生产动态开始出现产量递减加快、气油比快速上升、含水率快速上升等现象,表明气顶和油环开采出现不均衡状态,而且位于油环下部的井已开始出现底水锥进。为此,基于该区块油气藏地层油气流体相态特征,同时结合油气井生产动态特征,分析反凝析、气顶气下窜、底水上升锥进等因素对该油气藏开发动态的影响,进而提出对策:(1)利用气顶气和油环油溶解气的弹性能量以及边底水天然水驱方式来开采油气藏,并合理控制采油采气速度,尽可能地实现油气藏的均衡开采;(2)由于过早打破了油气井的均衡开采状态,导致了气顶气过早下窜和边底水过早侵入,需要通过实时调整采油气速度来平衡开采状态,从而使得油气藏气油比和含水率得到有效控制。实施结果表明:上述对策的成功应用,确保了凝析气藏进入了稳定均衡的开发状态。

H3b and H3c reservoirs in CX North Block, Xihu Depression, East China Sea Basin, are offshore gas condensate reservoirs with bottom water and oil rim. According to the design, four horizontal wells are developed in the pattern of oil and gas co-production with oil production as the priority target in the early stage, and their horizontal sections are arranged in the upper part of oil rim below the oil-gas contact. For early investment recovery, these gas reservoirs were produced at a higher rate in the early development stage, so the production performance of oil and gas wells are characterized by fast production decline, gas-oil ratio increasing and water cut increasing,indicating unbalanced production of gas cap and oil rim. Besides, water coning is observed in the wells located in the lower part of oil rim. In this paper, the effects of retrograde condensation, downward channeling of gas cap gas and upward coning of bottom water on the development of these oil and gas reservoirs were analyzed based on fluid phase characteristics of reservoir oil and gas in this block, combined with production performance characteristics of oil and gas wells. And then several countermeasures were proposed. First, these oil and gas reservoirs should be produced by using the elastic energy of gas cap gas and the gas dissolved in oil rim and the natural energy of bottom water, and oil and gas producing rate should be controlled in a rational range so as to realize balanced production. Second, balanced production of oil and gas wells are damaged too early, so downward channeling of gas cap gas and breakthrough of bottom and edge water occur too early. To maintain balanced production, therefore, it is necessary to adjust oil and gas producing rates so as to control gas–oil ratio and water cut effectively. It is demonstrated that successful application of the above countermeasures ensures the stable and balanced development of gas condensate reservoirs.