基于四层钻孔模型的体应变潮汐因子差异性原因分析

在钻孔应变观测的实际标定分析中,一般是通过对钻孔应变观测资料采用固体潮调和分析的方法直接获得钻孔面应变耦合系数,该方法未能充分反映观测的钻孔应变与地层应变之间的内在联系。本文基于“地层-等效弹性层-水泥环-钢筒”四层耦合介质钻孔模型,利用TJ-2C型体应变仪器的测量钢筒内外径和弹性参数、钻孔半径以及水泥环弹性参数,给出不同地层中钻孔耦合系数与钻孔等效弹性层弹性模量之间的变化关系。对我国现有运行的40多套TJ-2C型钻孔体应变观测资料的分析表明,潮汐因子分布范围在0.01至1.86之间,基岩为花岗岩类的潮汐因子均值为0.83,基岩为灰岩或砂岩类的潮汐因子均值为0.57,即硬地层中的潮汐因子高于软地层。基于对该四层模型探讨发现,等效弹性层的等效弹性模量大小反映了钻孔的耦合状态,对于同类型基岩地层,探头与地层耦合不佳将导致其等效弹性层弹性模量下降,进而导致钻孔潮汐因子下降。本文以砂岩弹性模量0.5 GPa作为判定钻孔耦合状态的下限,即认为等效弹性模量低于0.5 GPa时钻孔处于脱耦状态,统计结果表明我国现有钻孔体应变观测中有约30%的钻孔处于脱耦状态,这一研究结果对预报人员应用这些钻孔资料开展地震分析预报工作有重要指导意义。

Reasons for the Difference among Volume Strain Tidal Factors Based on a Four-layer Drilling Model

In actual calibration analysis of drilling strain observation, the strain coupling coefficient of the drilling surface is directly obtained from drilling strain observation data through harmonic analysis of tide. However, the method is difficult to use to fully reflect the internal link between the observed drilling strain and stratigraphic strain. This study measures the inner and outer diameter and the elastic parameters of the steel tube as well as the drilling radius and the elastic parameters of the cement sheath paper by using the TJ-2C strain gauge and a four-layer coupling medium drilling model of "stratum-equivalent elastic layer-cement sheath-steel tube." The relations between the drilling coupling coefficient in different strata and the elastic modulus of the drilling equivalent elastic layer are obtained. The analysis results of the volumetric strain from 40 TJ-2C strain gauges show that the tidal factors are distributed from 0.01 to 1.86. The tidal factor mean value of the granitoid bedrock is 0.83, and that of limestone or sandstone bedrock is 0.57. Hence, the tidal factor in the hard formation is higher than that in the soft formation. Based on the four-layer model, the equivalent elastic modulus of the equivalent elastic layer reflects the coupling state of the borehole. For the same type of bedrock formation, the poor coupling between the probe and the formation will lead to the decrease in the elastic modulus of the equivalent elastic layer, leading to a decrease in the tidal factor. The borehole is assumed to be in the decoupling state when the equivalent elastic modulus is less than 0.5 GPa. The statistics show that about 30% of boreholes in China are in decoupling state. The study results are important in conducting seismic analysis and forecasting work by using drilling data.