阵列感应测井响应的井洞影响分析

Analysis of the Cave Effect on the Array Induction Logging Response

井洞影响是井径变化致使阵列感应测井响应曲线出现异常的主要原因。在建立井洞影响计算模型的基础上,数值计算分析了井洞径向深度、纵向宽度、井眼泥浆电导率和地层电导率变化对阵列感应测井各子阵列的影响特征。结果表明,井洞影响在子阵列1和2的曲线上显示为假层存在;其他子阵列会出现凸或凹尖峰异常,在高电阻率地层出现负值;数据合成处理异常,深探测深度曲线分辨率高于浅探测深度;低泥浆电导率与地层电导率差别大,井洞影响严重;当地层电导率较低时,井洞的负影响会使响应出现负值异常,致使后续处理出错;随着井眼半径的增大,短子阵列表现为假层,较长子阵列的异常明显。通过几何因子解释了井洞影响导致曲线异常的原因。实际测井曲线的井洞影响实例验证了井洞影响数值模拟结果的有效性。

The cave effect is the main factor which results in the abnormity of array induction logging responses when the borehole irregularity exits. On the basis of building the cave effect model, the response characteristics of all subarrays are calculated and analyzed when the cave radial radius, the cave vertical width, the mud conductivity in borehole and formation conductivity are changed. The results show that the cave effect generates the false formation in the curves of the subarray 1 and 2, the convex and concave abnormal phenomena in other subarray curves, or the negative value in high resistivity formation. It also makes the synthesizing result abnormal. The resolution of deep investigation curves is greater than shallow curves. With increase of the formation conductivity, negative abnormal value appears easily due to the mud conductivity is lower than the formation conductivity. When the formation conductivity is lower, the negative cave effect will make the log response negative abnormal, so we often think the log curves were poorer. With increase of the borehole radius, the short subarray indicates the false formation, so the abnormal response is more obvious than the longer subarray. The abnormal reason is explained by the 2D geometric factor of the subarray. Finally, an actual logging curve validates the numerical calculation results of the cave effect.