Method and device for obtaining measurements of downhole properties in a subterranean well

A method for operating an untethered tool in a well, the method comprising:

• releasing the untethered tool, wherein the untethered tool comprises:
• a housing having an interior portion, a signal generator disposed in the housing, and a changeable ballast attached to the housing and configured to control buoyancy of the untethered tool, and wherein releasing the untethered tool comprises:
• closing a master valve of a Christmas tree assembly associated with the well;
• opening a swab valve of the Christmas tree assembly;
• introducing the untethered tool into the well through the swab valve;
• closing the swab valve;
• opening the master valve;
• wherein the Christmas tree assembly comprises a screening element configured to allow fluids from the well to pass but to preclude passage of the untethered tool;
• and
• deploying a sensor at the well, wherein the sensor is configured to:
• sense an output signal within the Christmas tree assembly, wherein the output signal is generated by the signal generator of the untethered tool;
• and
• provide a sensor output;
• and
• recovering the untethered tool after a period during which the untethered tool has descended into the well, modified the changeable ballast, and ascended up the well;
• wherein recovering the untethered tool comprises determining, based at least in part on sensing the output signal, that the untethered tool is within the Christmas tree assembly,
• wherein controlling the buoyancy using the changeable ballast of the untethered tool comprises using a switching device of the untethered tool to release a weight attached to the untethered tool,
• wherein the switching device is a magnetic flux switching device comprising a first permanent magnet and a second permanent magnet, the first permanent magnet and the second permanent magnet connected in parallel, the first permanent magnet having a greater coercivity than that of the second permanent magnet, wherein a coil of wire is wrapped around the second permanent magnet so to polarize the second permanent magnet upon an electrical current being reversibly applied to the coil of wire such that the weight is selectively secured and released from the untethered tool.

An untethered tool for use in investigating a well, the untethered tool comprising:

• a chemical sensor configured to sense a chemical characteristic within the well;
• a physical sensor configured to sense a physical characteristic within the well;
• and
• a drag element configured to modify a position of the untethered tool within the well by using drag as the untethered tool moves within a material in the well;
• and
• a buoyancy element configured to modify a position of the untethered tool within the well by using buoyancy of the buoyancy element in the material in the well,
• wherein the buoyancy element comprises a changeable ballast including a switching device of the untethered tool configured to release a weight attached to the untethered tool,
• wherein the switching device is a magnetic flux switching device comprising a first permanent magnet and a second permanent magnet, the first permanent magnet and the second permanent magnet connected in parallel, the first permanent magnet having a greater coercivity than that of the second permanent magnet, wherein a coil of wire is wrapped around the second permanent magnet so to polarize the second permanent magnet upon an electrical current being reversibly applied to the coil of wire such that the weight is selectively secured and released from the untethered tool.