1. Field of the Invention
The present invention relates to an apparatus for detecting and indicating the difference in densities between two well fluids and, more particularly, to a detection apparatus within a drill collar for use within a well.
2. Setting of the Invention
In rotary drilling of oil and gas wells, the key to safe well control is an early detection of a downhole "kick" (fluid influx into the annular space between the drill collar and the wellbore) so that appropriate prevention procedures can be initiated to avoid a possible blowout. The most positive surface indication of a kick is a net gain in the mud volume returned from the well compared to the volume pumped downhole through the drill string; however, gas kicks are often difficult to recognize because gas is highly compressed at typical bottomhole pressures. Thus, the mud system gains little additional volume until the compressed gas present in the drilling fluid has been circulated uphole to relatively shallow depths where it undergoes rapid expansion; however, by this point, a large gas kick is often impossible to control if well-kill procedures have not already been initiated. In addition, oil-based muds pose a particular problem because natural gas is highly soluble in the diesel oil phase common to such muds. This means that at the onset of a kick, a considerable volume of gas can dissolve in the mud with little or no detectable volumetric gain at the surface for a considerable time to come, that is, until the gas-impregnated mud arrives at or near the surface.
In addition, obtaining various measurements at the bottom of a borehole during the course of drilling operations have become beneficial. Many commercial devices are now available which contain information at or adjacent the drill bit, such as the weight on the bit, the torque on the drill string, inclination or azimuthal direction of the borehole, borehole pressure, temperature, and other parameters, which are transmitted by telemetry to the surface for use by the drilling operators. These telemetry systems are now available for signaling data from the region adjacent the drill bit, for example, the drill collar, to the surface of the earth. For example, U.S. Pat. No. 3,713,089 of Claycomb discloses a pressure signaling system providing encoded representations of data measured at or near the drill bit which are sent up the well to the surface where the pressure pulses are detected and converted into meaningful indications or records by suitable surface apparatus, for example, those disclosed in U.S. Pat. Nos. 3,488,629 and 3,555,504. None of this series of patents reveals what is measured at the bottom of the well, although the first-mentioned patent does disclose measuring the weight of the drill bit, the drill string torque, the inclination and the azimuthal direction of the borehole, borehole pressures and temperatures, as well as, various characteristics of the formation being penetrated. There is no discussion within this series of patents relating to measuring the differential densities of the annular fluid and the drill pipe fluid for detecting gas or liquid kick.
There are numerous patents dealing with measurement of radioactivity in electrical well logging; however, since these patents are all discussing the ordinary forms of well logging, it is apparent that none are capable of measuring the differential density between the annular fluid and the drill pipe fluid.
It has been known before to measure the change in gamma radiation received at a detector when a source of radiation has been spaced at some appropriate distance from it, so that variations in the received gamma ray signal are indications of the density of the fluid. U.S. Pat. No. 3,123,709 of Caldwell and Sippel and U.S. Pat. No. 3,617,746 of Janssen, et al., are of interest as disclosing logging means for determining density of fluid in a well, but there is no mention therein for measuring differential densities of well fluids for detection of gas or liquid kicks.