南苏丹高凝油油藏冷伤害机理研究

Study on Reservoir Cold Damage Mechanism of High Pour Point Oil Reservoir in Southern Sultan

通过室内实验对南苏丹高凝油油藏因析蜡导致地层冷伤害的可能性进行研究。实验结果表明,温度低于析蜡点后,高凝油中蜡晶体析出,呈现非牛顿流体特征,具有剪切稀释性和触变性,随着温度降低,析出的蜡颗粒半径逐渐变大,很容易在孔隙喉道处被捕集,造成地层冷伤害;渗流曲线表现出明显的非线性渗流特征,渗流体系存在启动压力梯度,油水相渗两相区范围变窄,等渗点右移,油水两相渗流能力降低;当温度低于析蜡点时,油水流度比增大,导致见水早、无水采收率和最终采收率都明显低于温度高于析蜡点的水驱效果。建议南苏丹高凝油油藏在实施注水开发时应保证注入水到达井底的温度高于地层原油的析蜡点,防止地层冷伤害的发生。

The possibility of the reservoir cold damage caused by wax precipitation in the high pour point oil reservoir in southern Sultan was studied by laboratory experiments. The experiment resuhs show that ,when the temperature is lower than the wax precipitation point, the wax crystal in high pour point oil will precipitate, the high pour point oil has the characteristics of non Newtonian fluid, that is, the characteristics of shear thinning and thixotropy;with the temperature reducing, the size of the precipitated wax particle becomes larger, the larger wax crystal particle is easy to gather at the throat of pore to cause the cold damage of reservoir; the oil-water seepage curves show obvious nonlinear seepage characteristics, the seepage system has start-up pressure gradient, the two-phase region becomes narrow, the isotonic point moves to right, the seepage capacity of oil and water all decreases. When the temperature is higher than the wax precipitation point,water-oil mobility ratio increases,which will lead to water breakthrough earlier, water-free recovery factor and ultimate recovery factor of water flooding lower than that when the temperature is lower than the wax precipitation point. Therefore it is suggested that in the water injection development of high pour point oil reservoir in southern Sudan, the bottom-hole temperature of injected water should be higher than the wax precipitation point so as to prevent the cold damage to the reservoir.