(74c) Sensor Ball: An Autonomous Logging Robot for Oil Wells

Oil and gas production from subsurface reservoirs can be optimized by frequent measurement of downhole pressures, temperatures, flow rates and fluid properties. Unfortunately, obtaining this data by conventional logging methods is complicated and expensive. As a result, wells are rarely logged more than a few times per year. Opportunities may be missed for timely adjustment to field-wide production strategies and early intervention to correct production problems.

Most of the cost of well logging derives from the long wireline cable that connects the logging tool to the logging truck. This cable provides power to the tool and returns data to the surface. However, the cable is up to 4 miles long and weighs more than one ton, requiring a substantial logging truck, draw works, and crane to handle it at the surface. The cable must also slide freely in and out of a well which may be at a pressure of 1000 psi or more at the surface. Controlling pressure in the well during the logging job requires a large “lubricator” system and “blow out preventer” as well as a crew to deploy and continually monitor the safe operation of this equipment. All of this is necessary to deploy a sensor that weighs only a few grams into the well.

We developed an untethered, hand-held logging robot called “Sensor Ball”, which can be deployed and recovered by one person with no heavy equipment and autonomously logs the well with no need for human supervision. The robot is deployed through the “Christmas tree” valves at the top of the well. The valves are opened and closed in succession so that two pressure barriers are always maintained for safety as the robot is released into the well. The valves are left in normal operating or shut-off condition while the robot autonomously collects data. The field engineer returns after a typically 8 hour logging job to simply open and close the valves in reverse order to retrieve the robot from the well.

At deployment, the robot carries a small dissolvable weight which causes it to descend into the well. While descending, it logs data from several sensors—temperature, pressure, and magnetic field. The robot determines its depth by analyzing variations in the naturally occurring magnetic fields emitted by the steel casing or tubing joints that line the well. Upon reaching the desired depth, it releases the weight and floats back to the surface. After the robot is retrieved from the well, the data is downloaded over a wireless communication link.

The robot has been qualified for operation at pressures up to 5000 psi and temperatures to 125°C. It can continually log data for 200 hours before battery replacement. We have logged wells up to 3600’ deep and up to 29 degrees deviation from vertical. We are currently incorporating additional sensors for analyzing downhole fluid properties.

The sensor ball is an autonomous logging robot that provides data on demand from oil wells, without the cost and logistical delays of conventional logging operations. This easy and inexpensive access to downhole data enables timely detection and remediation of production problems, and moves us closer to real-time field-wide optimization of production.