Resistivity Determination Methods in Original Formations
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摘要: 由于钻井液滤液侵入会引起低渗气层电阻率发生变化，因此测井测得的电阻率已不是低渗气层的原状电阻率。为求得低渗气层的原状电阻率，根据达西定律、质量守恒定律、状态方程建立径向数值模拟的数学模型，利用测井、测试等数据模拟钻井液滤液侵入对低渗气层电阻率的影响。模拟结果和钻井液滤液侵入动态模拟试验的对比表明，数值模型的参数设定合理，可以利用数值模型预测钻井液滤液侵入后低渗气层的电阻率。利用该方法反演了西湖凹陷XH-1井H4气层的钻井液侵入前的电阻率，并根据反演结果计算了气层初始含水饱和度，计算出的气层含水饱和度与相对渗透率试验获得的气层束缚水饱和度接近，并且气层测试结果证明计算出的初始含水饱和度比较合理，这说明采用反演方法得到的钻井液侵入前的电阻率与低渗气层原状电阻率非常接近。Abstract: Mud filtrate invasion of formations may lead to changes in resistivity of those formations, so normal resistivity determined from well logging are not true resistivity of the original formation. To determine resistivity of the original formation, mathematic model for radial numerical simulation was constructed in accordance with Darcy’s Law, Mass Conservation Law and State Equation. In addition, well logging and testing data were used to simulate the impact of invading mud filtrate on formation resistivity. By comparing simulation results and results of dynamic simulation of invading mud filtrate, the the newly developed numerical model demonstrated that, with proper setting of relevant parameters, it could be used to determine resistivity of the formation with invasion of mud filtrate. By using the numerical model along with relative permeability test data, well logging and testing data, formation resistivity before and at different times after invasion of mud filtrate can be determined through inversion. Resistivity of Formation H4 encountered by Well XH-1 in the Xihu Sag was determined by using the method. In addition, the initial gas saturation in gas-bearing layers was determined in accordance with the inversion results. The calculated water saturation coincided well with the bound water saturation determined through tests of relative permeability. These results showed that the resistivity before the invasion of mud infiltrate determined through inversion is very close to that in the original formation.
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