异常高压对砂砾岩弹性性质影响的实验机理研究与压力预测新模型——以准噶尔盆地玛湖凹陷北斜坡三叠系为例

Experimental study on the effect of abnormal high pressure on the elastic properties of glutenite and a novel pressure prediction model:a case study from Triassic Formation in north slope of Mahu Sag,Junggar Basin

准噶尔盆地西北缘玛湖地区二三叠系的砂砾岩中已经发现了多个高效油气藏,勘探实践已经证实该区砂砾岩储层中广泛发育异常高压,尤其是在近生烃中心的斜坡区.正是由于异常高压的存在,玛湖凹陷斜坡区砂砾岩储层的弹性性质呈现出一些特殊的现象与规律.本项研究依据玛湖地区63块样品的岩石物理测试结果、10余口井的实测地层压力资料、常规测井和录井资料,对砂砾岩弹性参数随异常高压的变化规律进行了深入分析.结果表明,玛湖凹陷斜坡区砂砾岩受母岩区矿物成分和异常高压的影响,其弹性特征与Casatgna砂岩线和李庆忠砂岩线表征的常规砂岩储层的弹性特征具有较大差异.与以上两种砂岩相比,具有相同密度的研究区砂砾岩样品纵横波速度较低,异常高压的发育阻碍了岩石的胶结作用是造成速度较低的主要原因.在异常高压发育区岩石样品的胶结程度普遍较差,速度也普遍降低,并且这种现象对纵横波速度的影响规律还存在区别,对横波速度的影响更大.为了定量地研究异常高压对砂砾岩储层纵横波速度变化的影响规律,本文设计了系统的岩石物理实验,分别测量获得固定地层压力以及固定差应力等条件下样品的纵横波速度,从实验上解释了异常高压对玛湖凹陷斜坡区砂砾岩储层弹性性质特殊变化规律的影响机理.为了提高在研究区利用地震资料预测地层压力的精度,本论文总结了不同弹性参数随地层压力和差应力之间的变化规律,改进有效应力系数的计算方法,建立了适合玛湖地区砂砾岩储层的地层压力预测新模型.最后,将新模型成功应用于玛湖凹陷北斜坡M131井区.从预测结果与实测结果的对比来看,在钻井位置,预测的压力系数与实测的压力系数吻合较好.另外,预测的压力系数分布规律也符合玛湖凹陷斜坡区异常高压的形成机理.

Several high-efficiency oil and gas reservoirs have been found in the Permian and Triassic glutenite reservoir in Mahu Sag,Junggar Basin.The exploration practice shows that abnormal high pressure is widely developed in the glutenite reservoir,especially in the slope area near the hydrocarbon generation center.Due to the existence of abnormal high pressure,the elastic parameters of glutenite reservoir in slope area of Mahu Sag exhibit some special properties.In this paper,we conduct a comprehensive study of the variation law of glutenite elastic parameters with abnormal high pressure based on the petrophysical tests of 63 rock samples,the measured formation pressure from more than 10 wells,and conventional logging and mud logging data in Mahu area.The results show that the glutenite in the slope area of Mahu depression is affected by the mineral composition and the abnormal high pressure,and its elastic characteristics are quite different from those of conventional sandstone reservoirs which are usually represented by the Casatgna sandstone line and the Li QingZhong sandstone line.Compared with the above two kinds of sandstones,the samples having the same densities in the study area have lower velocity.The main reason for the low velocity is that the development of abnormal high pressure hinders the cementation of rocks.In the abnormal high-pressure area,the cementation of samples is generally poor,and thus the velocity of samples is low.Moreover,the influence of this phenomenon on P-wave and S-wave velocity is different,and it has a greater influence on S-wave velocity.In order to quantitatively study the influence of abnormal high pressure on the P-wave and S-wave velocity in glutenite reservoir,a systematic petrophysical experiment is designed to measure the velocity of samples under the conditions of fixed formation pressures and fixed differential stresses.The influence mechanism of abnormal high pressure on the elastic properties of glutenite reservoir in slope area of Mahu Sag is then explained experimentally.In order to improve the accuracy of the formation pressure prediction in the study area by using seismic data,we summarize the variation laws of different elastic parameters with formation pressure and differential stress,and developed an improve method for calculating the effective stress coefficient.Then,a novel model is established to predict formation pressure for glutenite reservoir in Mahu area.Finally,the novel model is applied in the well M131 zone at the slope of Mahu Sag.From the comparison of the predicted and measured pressure coefficients,we found that the predicted results have a good agreement with the measured data at the drilling position.In addition,the predicted pressure coefficient distribution also conforms to the formation mechanism of abnormal high pressure in the slope area of Mahu depression.