Abstract：

 With the development of petroleum exploration, logging-while-drilling (LWD) technology has become an important means to obtain timely and accurate drilling and geological data. Resistivity imaging logging-while-drilling plays a key role in real-time well site data acquisition, interpretation, field decision-making and guidance of geological steering. However, there are few studies into the factors influencing resistivity imaging logging while drilling. The factors influencing the logging response of resistivity imaging while drilling in fractured formations are not well understood at present. This paper uses the finite element simulation platformCOMSOL  MULTIPHYSICS based on a 3D finite element method (3D FEM) to simulate the response of resistivity imaging while drilling in single crack and reticular fractured formations. The influence of borehole size, mud resistivity, resistivity contrast, fracture opening,  fracture dip angle and extension distance on logging response are analyzed. In the simulation, local mesh refinement technology is adopted to improve the solution accuracy and the solution speed. The numerical simulation results show that the resistivity of borehole mud will affect the fracture resolution, and due to the existence of mud, when the borehole size is small and the button electrode of the instrument is close to the borehole wall, the resolution of the instrument to the fracture is higher. When the borehole size is kept within 7 inches, the effect of the apparent resistivity curve is credible. The influence of the borehole environment on button electrodes with different sizes is different. When the fracture opening is less than the resolution of the button electrode, the apparent resistivity at the fracture gradually approaches the true value with an increase of crack opening. When the fracture opening is greater than the resolution, the resistivity imaging logging while drilling is greatly affected by the opening degree. The influence of fracture extension distance on imaging results is limited, and the influence can be ignored when the extension distance exceeds 0.3 m. The Schlumberger interpretation model is used to investigate the relationship between the abnormal current area A  and the fracture opening degree, which shows a good positive linear correlation and a negative correlation with resistivity contrast. With an increase in the dip angle, the apparent opening of a fracture on the image increases, and the influence of the dip angle on the apparent opening of a low angle fracture is less than that of a high angle fracture. The research results have certain guiding significance for logging interpretation and model establishment of resistivity imaging with drilling tools in complex fracture formations.

Key words：LWD resistivity imaging; logging responses; 3D FEM; numerical simulation; complex fractures

Received： 2019-09-16

Corresponding Authors： wksz@vip.sina.com

Cite this article：CAI Yalin, KE Shizhen, KANG Zhengming, LI Xin, LI Mingyu, MA Xuerui. Logging response simulation of a LWD resistivity imaging tool in fractured formations. Petroleum Science Bulletin, 2020, 03: 327-336.

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