过井眼洞穴型地层钻头电阻率测井响应特征分析

Response in at-bit resistivity logging for cavernous formations through the borehole

钻头/近钻头电阻率测量是实现复杂储层中实时高效测量和实时地质导向的重要技术手段。为分析钻头电阻率测井钻遇洞穴型储层时的响应特征,基于有限元法考察了钻头电阻率测井仪的径向探测深度、纵向分层能力和前视探边能力,通过数值模拟分析了洞穴尺寸、形态、填充物电阻率等因素对不同井底钻具长度的钻头电阻率测井的影响。研究结果表明:①随井底钻具长度增大,仪器的径向探测深度增加,纵向分层能力变差;固定井底钻具长度,目的层相对围岩低阻时,仪器的分层能力更强;地层厚度大于两倍井底钻具长度时,仪器响应受层厚/围岩影响相对较小;②井眼泥浆电阻率越大,仪器探测到层边界的灵敏度越高;层界面倾斜时,仪器更容易探测到地层边界,但穿过地层后恢复到下部地层真电阻率的速度变慢;③井底钻具长度越小,仪器响应对洞穴的敏感性越强,越容易判断洞穴的存在;④洞穴形态变化对钻头测量视电阻率曲线有不同影响,洞穴纵向延伸时,视电阻率明显降低,此曲线确定的地层视厚度增大;径向延伸时,视电阻率下降程度更大,仪器钻出洞穴时的视电阻率曲线极化角增大;洞穴填充物电阻率与基岩电阻率对比度小于1∶50,洞穴半径大于下部钻铤长度时,视电阻率会进一步下降,且测井曲线出现两个低谷;⑤洞穴边界位置可通过测井曲线变化率最大处、出现极化角的位置和井底钻具长度来判断。研究结果可为钻头电阻率测井钻遇洞穴时的定性判断提供一定参考。

Measuring at-bit or near-bit resistivity is an important method for real-time measurements and geosteering in complex reservoirs.Igneous and carbonate reservoirs consist of formations characterized by caverns and fractures.FEM was used to analyze the radial sounding depth,longitudinal stratification capability,and boundary detection capacity to study the characteristics of the at-bit resistivity measurement when drilling encounters caves.The results showed that①The detection depth of at-bit resistivity logging was positively correlated with the length of the bottom hole assembly(BHA),but the change in detection depth was smaller when the BHA length was less than 1 m.The layering ability of the instrument was stronger when the resistivity of the target layer was lower than that of the surrounding rock.When the stratum thickness was more than twice the BHA length,the instrument response was not significantly affected by the stratum thickness or the surrounding rock.②The higher the borehole mud resistivity,the greater the stratification capability of the tool.The instrument could detect the stratum boundary more quickly if the stratum interface were inclined.However,the instrument was more severely affected by the upper stratum when crossing the stratum interface.③The shorter the length of the BHA,the higher the instrument’s resolution,and the easier it was to determine the presence of caves.④The apparent resistivity curve measured by the at-bit depended on the cave shape.When the cave extended longitudinally,the apparent resistivity decreased significantly,and the apparent thickness provided by the apparent resistivity curve increased.The longer the radial extension of the cave,the greater the polarization angle of the instrument’s apparent resistivity curve and the apparent resistivity decreased more significantly.When the contrast between the cave filling resistivity and bedrock resistivity was less than 1∶50,and the cave radius was greater than the length of the lower drill collar,the apparent resistivity further declined,and the logging curve had two valleys.⑤The location of the cave boundary could be determined from the location of the maximum change rate of the logging curve and the polarization angle.The results of this study can help provide qualitative judgments when drilling resistivity logging to encounter caves.