﻿ 复杂孔隙结构低阻油层含水饱和度解释新方法
 西南石油大学学报(自然科学版)  2018, Vol. 40 Issue (1): 97-103

A New Method of Explaining the Water Saturation of Low Resistivity Reservoir with Complex Pore Structure
ZHANG Hengrong , HE Shenglin, ZHENG Xiangwei, HU Xiangyang, ZENG Shaojun
Zhanjiang Branch, CNOOC Limited, Zhanjiang, Guangdong 524057, China
Abstract: The gravelly sandstone reservoirs of the Liusha Formation in Wushi, Beibu Gulf are subject to complicated changes in lithology, sorting and distribution of pore throats, the resistivity of the oil layer is lower than that of the mudstone and is similar to that of the water layer, the porous tube microstructure model was used to simulate the response of pore structure and rock resistivity. It is proved that the complex pore structure of rock is an important reason for the formation of peculiar low resistivity reservoir of gravel sandstone. The Archie correction equation is proposed based on experimental "bent" rock electrical data caused by complex pore structures. The modified Archie formula is applicable to reservoirs with different pore structures. The Wushi A Oilfield gravel sandstone reservoir is evaluated by a new method, can effectively improve the oil saturation, and the results are more compatible with core capillary pressures and NMR-bound water.
Key words: gravel bearing sandstone     ball tube model     pore structure     bent rock resistivity     Archie formula

1 复杂孔隙结构油层岩芯实验分析

 图1 含砾砂岩岩芯铸体薄片 Fig. 1 Core cast thin section of sandstone cores

 图2 含砾砂岩岩芯核磁共振 $T_{2}$ 谱图 Fig. 2 The nuclear magnetic resonance $T_{2}$ spectra of sandstone cores
2 复杂孔隙结构储层导电数值模拟

2.1 岩石饱和盐水的导电数值模拟( $m$ )

 $F = \dfrac{R_0}{R_{\rm{w}}} = \dfrac{1 - 2 r_{\rm{s}} \sqrt{1 - C_{\rm{d}}^2}}{{{{ π} }} C_{\rm{d}}^2 r_{\rm{s}}^2} + \dfrac{1}{{{{ π} }}{r_{\rm{s}}}} \ln \left( \dfrac{1 - \sqrt{1 - C_{\rm{d}}^2}}{1 + \sqrt{1 - C_{\rm{d}}^2}} \right)$ (1)

$F$ —地层因素，无因次；

$R_0$ —岩石饱含盐水的电阻率，Ω·m；

$R_{\rm{w}}$ —饱含盐水的电阻率，Ω·m；

$r_{\rm{s}}$ —球管模型的孔半径，无因次，且 $0 <{r_{\rm{s}}} \leqslant \dfrac{1}{2 \sqrt{1 - C_{\rm{d}}^2}} \leqslant \sqrt{\dfrac{1}{{{{ π} }}}}$

$r_{\rm{c}}$ —球管模型的喉半径，无因次；

$C_{\rm{d}}$ —孔隙喉道比值，无因次，且 $C_{\rm{d}} = r_{\rm{c}}/{r_{\rm{s}}}$

 图3 不同孔隙结构的阿尔奇参数值模拟结果对比 Fig. 3 Comparison of simulation results with different pore structure parameters of Archi
2.2 岩石逐步饱和油气的导电数值模拟( $n$ )

 $\left\{\begin{array}{l} I={R_{\rm{t}}}/{R_{\rm{o}}}\\ {R_{\rm{t}}}=\frac{2 r_{\rm{so}}\sqrt{1 - C_{\rm{do}}^2}}{{{{ π} }}\left(r_{\rm{s}}^2 - r_{\rm{so}}^2\right)} + \frac{1 - 2 r_{\rm{s}}\sqrt{1 - C_{\rm{d}}^2}}{{{{ π} }}\left(C_{\rm{d}}^2r_{\rm{s}}^2 - C_{\rm{do}}^2r_{\rm{so}}^2\right)} + \frac{1}{{{{ π} }}\sqrt{r_{\rm{s}}^2 - C_{\rm{do}}^2r_{\rm{so}}^2}}\\\left(\ln\frac{\sqrt{r_{\rm{s}}^2 - C_{\rm{do}}^2r_{\rm{so}}^2} - r_{\rm{s}}\sqrt{1 - C_{\rm{do}}^2}}{\sqrt{r_{\rm{s}}^2 - C_{\rm{do}}^2r_{\rm{so}}^2} + r_{\rm{s}}\sqrt{1 - C_{\rm{do}}^2}} - \ln\frac{\sqrt{r_{\rm{s}}^2 - C_{\rm{do}}^2r_{\rm{so}}^2} - r_{\rm{s}}\sqrt{1 - C_{\rm{d}}^2}}{\sqrt{r_{\rm{s}}^2 - C_{\rm{do}}^2r_{\rm{so}}^2} + r_{\rm{s}}\sqrt{1 - C_{\rm{d}}^2}}\right)\\ {R_{\rm{o}}}=\frac{1 - 2 r_{\rm{s}}\sqrt{1 - C_{\rm{d}}^2}}{{{{ π} }} C_{\rm{d}}^2r_{\rm{s}}^2} + \frac{1}{{{{ π} }}{r_{\rm{s}}}}\ln\left(\frac{1 + \sqrt{1 - C_{\rm{d}}^2}}{1 - \sqrt{1 - C_{\rm{d}}^2}}\right) \end{array}\right.$ (2)

$r_{\rm{co}}$ —管喉中油的半径，无因次；

$r_{\rm{so}}$ —球孔中油的半径，无因次；

$R_{\rm{t}}$ —含油气纯岩石的电阻率，Ω·m；

$C_{\rm{do}}$ —油珠孔隙喉道比值，无因次，且 $C_{\rm{do}} = r_{\rm{co}}/{r_{\rm{so}}}$

3 基于复杂孔隙结构的修正阿尔奇公式提出

 图4 含砾砂岩储层测井曲线特征对比图 Fig. 4 Comparison of logging curves of sandstone reservoirs with gravel

 图5 含砾砂岩储层岩芯地层因素与电阻率增大系数 Fig. 5 Formation factor and resistivity increase coefficient of core formation in gravel bearing sandstone reservoirs

 $\ln F = \ln a' - m'\phi$

$\phi$ —孔隙度，无因次。

 $\ln I = \ln b' - n'{S_{\rm{w}}}$

$S_{\rm{w}}$ —含水饱和度，无因次。

 图6 WS17-A井利用修正后阿尔奇公式计算的含水饱和度 Fig. 6 Well WS17-A water saturation calculated by the modified Archie formula
4 结论

(1) 复杂孔隙结构的储层也能产生低阻油层。

(2) 同是含砾砂岩油层，电阻率测井曲线差异大的原因是孔隙结构差异大，孔隙结构越复杂，喉道越小，束缚水越多，电阻率越低。

(3) 阿尔奇公式在低渗复杂孔隙结构储层不适用，修正后地层因素、电阻率系数分别随着孔隙度、饱和度变化，更符合实际地质情况。

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