白云西洼油藏的“近临界流体”特征及成因分析

Characteristics and genetic analysis of the“near critical fluids”of Oil 1 reservoir in western Baiyun sag

白云西洼X1井的PVT实验报告中将"油样"Oil1命名为"近临界流体",为了解决该流体到底属于何种流体相态类型这一问题,首先尝试用常规方法进行流体相态判别,Oil 1均判定为凝析气藏;然而,通过对X1井的PVT报告进行仔细研究和分析发现,Oil 1在储层温压下完全呈液态,只有在低于饱和压力时才逐渐挥发,故该流体应该定名为近临界挥发油。基于地层流体高压物性分析,发现该挥发油具有收缩率高、气油比高(1379.6~1846.9 m3/m3)、体积系数大(6.52)、泡点压力高(14.82 MPa)、储层原油密度低(0.4588 g/cm3)、黏度低(0.120 mPa·s)等特点。在调研大量文献的基础上,对Oil 1挥发油的多种成因进行了初步研究,认为成藏期成熟—高成熟的烃源岩对其形成起决定作用,Ⅱ2-Ⅲ型烃源岩是其形成的物质基础,埋藏深、盖层厚和后期不受构造运动影响也是其形成的重要条件。建议:用常规方法进行流体相态判别时,务必参考流体PVT实验结果;挥发油藏在开采过程中压力要时刻保持高于泡点压力,否则会有大量油挥发成气、原油体积收缩、原油黏度增大,不利于开采。

"Oil sample"Oil 1 of Well X1 was described as"near critical fluid"in the PVT report of Well X1 in western Baiyun sag.In order to determine types of fluid phase state,and to determine the fluid phase with conventional methods,Oil 1 is judged to be a condensate gas reservoir.However,through comprehensively analysis and research of the PVT report of Well X1,it is found that Oil 1 is completely liquid under the temperature and pressure of the reservoir,and only gradually volatilizes when the pressure is lower than the saturation pressure,so the fluid should be named as near-critical volatile oil.Based on the high-pressure physical property analysis of formationfluids,it is found that the volatile oil has the characteristics of high shrinkage ratio,high dissolved gas-oil ratio(1379.6-1846.9 m3/m3),large volume coefficient(6.52),high bubble point pressure(14.82 MPa),and low reservoir crude oil density(0.4588 g/cm3),low viscosity(0.120 mPa·s),etc.On the basis of investigating large literatures,the origins of Oil 1 volatile oil are preliminarily discussed and studied.It is considered that the mature to highly-mature source rocks in the reservoir forming period play a decisive role in its formation.TypeⅡ2-Ⅲsource rocks are the material basis.These conditions are important for formation,such as deep burial,thick caprock and no tectonic movement influence.It is recommended that when using conventional methods to determine the fluid phase behavior,it is necessary to refer to the results of the fluid PVT experiment.For volatile oil reservoirs,the exploitation pressure must always be higher than the bubble point pressure.Otherwise,a large amount of oil will evaporate into gas,the volume of crude oil will shrink,and the viscosity of crude oil will increase,which is not conducive in exploitation.