2. 中海油研究总院有限责任公司, 北京 朝阳 100028
2. CNOOC Research Institute Ltd., Chaoyang, Beijing 100028, China
油水相对渗透率曲线是油藏开发中的重要参数,准确给定油水相对渗透率曲线,有助于对油田开发规律的正确认识。相对渗透率曲线可通过室内实验方法获得,但由于天然岩芯数量有限,实验获得的相对渗透率曲线难以反映整个油藏的实际情况[1-2]。为此,有学者提出了基于油田生产数据来计算相对渗透率曲线的方法[3-9],但目前计算方法中的残余油饱和度端点值通常采用初始相渗实验得到的数值,未能考虑长期水驱引起的油藏残余油饱和度变化[10-13],同时目前方法主要通过多元回归拟合等数学方法,缺乏严谨理论推导,应用有一定的局限性。为此,本文基于新型的近似理论水驱曲线,推导了油相指数、水相指数、残余油饱和度下的水相相对渗透率等相对渗透率曲线关键参数的数学表征公式,同时结合残余油饱和度计算方法,构建了一种广泛适用的考虑储层参数时变效应的油水相对渗透率曲线动态计算方法。
1 理论推导 1.1 近似理论水驱曲线近似理论水驱曲线是基于油水相对渗透率指数表达式直接推导得到的一种新型水驱曲线,通过引入两个可变参数
$ {N_{\rm{p}}} = {N_{\rm{R}}} - a\dfrac{{N_{\rm{p}}^p}}{{W_{\rm{p}}^q}} $ | (1) |
近似理论水驱曲线4个特征参数
基于近似理论水驱曲线表达式(式(1)),可得水油比具有如下关系
$ \dfrac{{{f_{\rm{w}}}}}{{1 - {f_{\rm{w}}}}} = \dfrac{{{Q_{\rm{w}}}}}{{{Q_{\rm{o}}}}} = \dfrac{{\partial {W_{\rm{p}}}}}{{\partial {N_{\rm{p}}}}} = \\\quad\quad\quad\quad \dfrac{p}{q}{a^{\frac{1}{q}}}N_{\rm{R}}^{\frac{p}{q} - \frac{1}{q} - 1} \times \dfrac{{\left( {1 + \dfrac{{1 - p}}{p}R} \right)}}{{{{(1 - R)}^{\frac{1}{q} + 1}}}} \times {R^{\frac{p}{q} - 1}} $ | (2) |
其中,
当含水率小于98%时,可忽略分子中的
$ \dfrac{{{f_{\rm{w}}}}}{{1 - {f_{\rm{w}}}}} = \left( {\dfrac{p}{q}{a^{\frac{1}{q}}}N_{\rm{R}}^{\frac{p}{q} - \frac{1}{q} - 1}} \right) \times \dfrac{{{R^{\frac{p}{q} - 1}}}}{{{{(1 - R)}^{\frac{1}{q} + 1}}}} $ | (3) |
油水两相相对渗透率曲线可采用指数形式表达[1]
$ {K_{{\rm{rw}}}} = {K_{{\rm{rw}}}}({S_{{\rm{or}}}}){\left( {\dfrac{{{S_{\rm{w}}} - {S_{{\rm{wi}}}}}}{{1 - {S_{{\rm{wi}}}} - {S_{{\rm{or}}}}}}} \right)^{{n_{\rm{w}}}}} $ | (4) |
$ {K_{{\rm{ro}}}} = {K_{{\rm{ro}}}}({S_{{\rm{wi}}}}){\left( {\dfrac{{1 - {S_{\rm{w}}} - {S_{{\rm{or}}}}}}{{1 - {S_{{\rm{wi}}}} - {S_{{\rm{or}}}}}}} \right)^{{n_{\rm{o}}}}} $ | (5) |
忽略重力和毛管力作用的分流量方程可表示为
$ {f_{\rm{w}}} = \dfrac{1}{{1 + \dfrac{{{K_{{\rm{ro}}}}}}{{{K_{{\rm{rw}}}}}}\dfrac{{{\mu _{\rm{w}}}}}{{{\mu _{\rm{o}}}}}\dfrac{{{B_{\rm{w}}}}}{{{B_{\rm{o}}}}}}} $ | (6) |
将式(4)、式(5)代入式(6),并转化为可采储量采出程度表达式
$ \dfrac{{{f_{\rm{w}}}}}{{1 - {f_{\rm{w}}}}} = \dfrac{{{K_{{\rm{rw}}}}}}{{{K_{{\rm{ro}}}}}}\dfrac{{{\mu _{\rm{o}}}{B_{\rm{o}}}}}{{{\mu _{\rm{w}}}{B_{\rm{w}}}}} = M\dfrac{{{R^{{n_{\rm{w}}}}}}}{{{{(1 - R)}^{{n_{\rm{o}}}}}}} $ | (7) |
式(7)中,
$ M = \dfrac{{{K_{{\rm{rw}}}}({S_{{\rm{or}}}}){\mu _{\rm{o}}}{B_{\rm{o}}}}}{{{K_{{\rm{ro}}}}({S_{{\rm{wi}}}}){\mu _{\rm{w}}}{B_{\rm{w}}}}} $ | (8) |
式(3)和式(7)联立,可得油水相指数与近似理论水驱曲线特征参数
$ {n_{\rm{w}}} = \dfrac{p}{q} - 1 $ | (9) |
$ {n_{\rm{o}}} = \dfrac{1}{q} + 1 $ | (10) |
$ M = \dfrac{p}{q}{a^{\frac{1}{q}}}N_{\rm{R}}^{\frac{p}{q} - \frac{1}{q} - 1} $ | (11) |
由文献[18]可知,极限驱油效率为可动油储量与动用地质储量比值,即
$ {E_{\rm{D}}} = \dfrac{{{N_{\rm{R}}}}}{N} $ | (12) |
根据极限驱油效率定义公式[19]
$ {E_{\rm{D}}} = \dfrac{{1 - {S_{{\rm{wi}}}} - {S_{{\rm{or}}}}}}{{1 - {S_{{\rm{wi}}}}}} $ | (13) |
由式(12)、式(13)可得,残余油饱和度为
$ {S_{{\rm{or}}}} = \left( {1 - \dfrac{{{N_{\rm{R}}}}}{N}} \right)(1 - {S_{{\rm{wi}}}}) $ | (14) |
综上,相对渗透率曲线动态计算方法包括如下步骤:(1)根据生产数据,计算近似理论水驱曲线四个特征参数
西江油田为边底水能量充足的海相砂岩油田,储层孔隙连通性好,孔隙度20.1%
根据油田累产油、累产水等历史生产数据,调节
对西江油田评价井15块岩芯做驱替实验,得到初始状态下的残余油饱和度(图 2)。由图 2可知,地质特征参数与残余油饱和度呈现出较好的对数关系,初始状态下油藏的残余油饱和度主要分布在0.25
根据近似理论水驱曲线四个特征参数,由式(9)、式(10)和式(11)计算可得
从图 4可以看出,根据理论相对渗透率曲线所得到的含水率随采出程度的变化关系与实际生产数据吻合,预测油田含水率达到98%时,采出程度可达到66%,西江油田仍有107
(1) 建立了残余油饱和度动态计算方法,可根据生产动态数据计算油藏长期水驱后的残余油饱和度,克服了现有相对渗透率曲线计算方法只能依赖实验测量残余油饱和度端点值的局限性,实现了油藏残余油饱和度的时变表征。
(2) 基于新型的理论水驱曲线建立了油相指数、水相指数、残余油饱和度下的水相相对渗透率等相对渗透率曲线关键参数的数学表征公式,并结合残余油饱和度计算方法提出了一种考虑储层参数时变的水驱油藏相对渗透率曲线计算新方法。
(3) 典型油田应用实例表明,该方法有助于解决长期水驱后油藏相对渗透率曲线有效表征的问题,也可广泛应用于疏松砂岩等取芯困难油田基础参数的获取,从而为油田可采储量标定、剩余油分布规律研究、特高含水期油藏数值模拟历史拟合等提供参考。
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