Range and Design of Untethered Downhole Vehicles

The paper explores the feasibility of an untethered, neutrally-buoyant, propeller-driven vehicle for data acquisition in oil wells. The limited well diameter requires long, thin cylindrical hull geometries, where the drag force comes mainly from displacing fluid through the annulus between the vehicle hull and the wellbore wall. Analytical, simulation, and experimental results for the drag are presented. The implications of this drag force for optimizing hull dimensions are considered. The vehicle range is shown to be independent of vehicle length, but proportional to the fraction of length available for energy storage. There is also an optimal vehicle diameter for maximizing range. This analysis suggests that it is feasible to construct a neutrally-buoyant autonomous downhole vehicle capable of traversing the laterals of wells under its own power.