Petroleum Drilling Techniques, Issue 2, 2018

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The Influencing Factors on the Polarizing Angle of Array Laterolog Curves


The polarizing angle of a laterolog curve is helpful in the interpretation of geological structures and the assessment of formation interfaces,but the distortion of the resistivity curve and the features of "false thin layers" can also occur.Therefore,it is of great significance for evaluating oil and gas reservoirs to identify the influencing factors of the polarizing angle by using a forward calculation.Three-dimensional finite element numerical simulation is used to set up a layered media model,which facilitates evaluating the influence of a series of factors on polarizing angle characteristics,such as the surrounding rock resistivity,drilling fluid resistivity,invasion zone characteristics,instrument characteristics,layer thickness,well diameter,well angle,etc.The forward calculation results showed that when the ratio of the resistivity of the target rock to that of the surrounding rock is greater than 4,the polarizing angle will be sharper and the amplitude increased rapidly.They also showed that the resistivity of drilling fluid mainly affects the shape of the polarizing angle so that the greater the resistivity of drilling fluid,the sharper the polarizing angle.The influence of invasion zone on the polarizing angle is more complex.Well designers should carefully consider the invasion resistivity and depth.The detection depth of the instrument has little effect on the polarizing angle shape,but the higher the vertical resolution of instrument,the flatter the polarizing angle,and the smaller the amplitude.Further,the bigger the well diameter,the less obvious the polarizing angle is.Revealingly,when the formation thickness is less than the vertical resolution of the instrument,the polarizing angle of the array laterolog disappears.With the increase of formation thickness,the polarization angle becomes larger,and it tends to be flat.However,when the angle of inclination is more than 60°,the polarizing angle basically disappears.Likewise,the smaller the inclination angle,the more obvious the polarizing angle.The research results indicated that the effect of surrounding rock resistivity,instrument vertical resolution,and layer thickness had the most obvious effect on the polarizing angle.This could provide guidance for the correct application of the polarizing angle to interpret oil and gas reservoirs.