During the initial pressure buildup of a leak off test (LOT), the increase in wellbore pressure causes porous flow of drilling fluid into the formation as well as deformation of the formation due to the pressure gradient. When the borehole pressure reaches the fracture initiation pressure, the formation yields and fractures initiate and propagate, allowing drilling fluid to leak off more rapidly into the formation. The movement of drilling fluid into the formation during pressure buildup and leakoff results in a measurable change in resistivity, which may be detected by logging while drilling (LWD) resistivity at bit (RAB) tools. These tools may either be placed in the open hole section during the LOT or logged over the open hole section before and after the LOT to determine the extent of invasion and detect fractures. Fracture dip and azimuth may be determined from resistivity images taken around the borehole.
In this project we will simulate the response of a RAB tool to the changes that occur in the formation during a LOT. The results will then be incorporated into an inversion algorithm to determine formation properties from RAB data recorded before, during, and after a LOT.
(right) Resistivity image showing open, conductive fractures as dark sinusoidal features. Fracture dip and azimuth may be determined from the image.
Project PI: Hugh Daigle
Co-PI: K. E. Gray
PhD Candidate: Bishwas Ghimire