Modeling Open Fracture Response in Non-Conductive Mud with LWD Dual Imager

Fracture classification into open vs. closed with borehole images in non-conductive mud has always been challenging, due to established workflows seeking leakage of current into fractures in wells drilled with conductive mud system. Often there are questions raised regarding open fracture classification with resistivity imagers in non-conductive mud. A dual-physics logging-while-drilling imager that utilizes resistivity and ultrasonic measurement principles collocated on a single collar provides complementary information to classify fractures.

The high-resolution electromagnetic waves are sent through the mud at optimal frequencies. The impedivity images are a function of the distance between the sensors and the borehole wall, thereby allowing a standoff image to help interpretation of open features. With mud acoustic slowness knowledge, high-resolution ultrasonic pulse-echo images deliver also standoffs information.

Modelling and laboratory experiments with both physics were performed for different resistivity and acoustic contrast against variable standoff. The results suggested that detection limit of open fractures is way finer than the image resolution. Based on detection limit and available contrast of feature against the host matrix; features manifested themselves on different frequencies distinctly. It can be concluded that the fine-aperture fractures present in the subsurface can be detected and characterized to a large extent.