Dissolvable Materials for Buoyancy Driven Downhole Robots

Abstract

Dissolvable materials made a positive impact in many downhole applications by reducing down-time, cost, and operation complexity, especially as frac plugs. For a new application for these materials, here we investigate their deployment as ballasts that can be attached to buoyancy driven untethered downhole robots, such as the Sensor Ball [1]-[2]. The Sensor Ball is an untethered downhole tool that takes a ballast to sink in the well fluids and releases it to become buoyant, hence it can return to the surface. Although the ballast is released on-demand via a magnetic actuator, using a dissolvable ballast is useful for two reasons: 1) The clutter left behind is minimized; 2) Provides a fail-safe mechanism such that the buoyancy is regained even when the weight releasing system fails. The Sensor Ball needs a ballast material that is as dense as possible to easily offset its effective density. Among the readily available materials, aluminum alloys are suitable, but pose problems such as highly exothermic reaction. These materials can cause extreme heating that can raise the local temperatures over 100 °C at downhole pressures. A secondary issue is the byproduct formation and aggregation. The aggregated solids inside water cause a mud-like viscosity that can entrap the Sensor Ball. Polymer spray coatings and separation of functions for clutter prevention and fail-safe mechanisms were effective in reducing the exothermic heating and mud formation. We also investigated 3D printed dissolvable polymer containers with metal powders. Although this method was useful in addressing the heat generation problem, the containers showed mechanical integrity problems.