The gathering of downhole information has been done by the oil well industry for many years. Modern petroleum drilling and production operations demand a great quantity of information relating to the parameters and conditions downhole. Such information typically includes the location and orientation of the wellbore and drilling assembly, earth formation properties, and drilling environment parameters downhole. The collection of information relating to formation properties and conditions downhole is commonly referred to as “logging”, and can be performed during the drilling process itself.
Various measurement tools exist for use in wireline logging and logging while drilling (“LWD”). One such tool is the electromagnetic logging tool, which includes one or more antennas for transmitting an electromagnetic signal into the formation and one or more antennas for receiving a formation response. When operated at low frequencies, the electromagnetic logging tool may be called an “induction tool”, and at higher frequencies it may be called an “electromagnetic wave propagation tool”. Though the physical phenomena that dominate the measurement may vary with frequency, the operating principles for the tool are consistent. The amplitude and/or the phase of the receive signals may be compared to the amplitude and/or phase of the transmit signals to measure electrical properties of the formation (e.g., conductivity, permittivity). Alternatively, the amplitude and/or phase of multiple receive signals are compared to each other to measure the formation properties. Where the tools are configured to provide measurements at multiple signal frequencies, multiple antenna spacings, and/or multiple orientations, additional information can be derived including, for example, the dependence of the formation properties on azimuth and radial distance from the borehole. Such information is particularly valued for guidance when steering the drilling assembly.
In certain situations, such as when drilling through formations in which the formation boundaries extend vertically, or when drilling from an off-shore platform, it is desirable to drill wells at a shallow angle with respect to bed boundaries in the strata. This is often termed “horizontal” drilling. When drilling horizontally, it is desirable to maintain the well bore in the pay zone (the formation which contains hydrocarbons) as much as possible so as to maximize the recovery. However, due to the drilling assembly's limited turning radius, such pay zone following can be challenging, particularly when the formations dip or divert.
While attempting to drill and maintain the well bore within a particular formation, the drill bit may approach a bed boundary. Such approaches are preferably detected as early as possible to provide the drill crew with sufficient time to react. As the detection distance can be increased with lower frequencies and larger antenna separations, certain tool designs employ multiple electromagnetic logging tools in the drilling assembly, optionally separating them by one or more intermediate tools or tubulars. Electromagnetic signals transmitted from one of the various electromagnetic logging tools are received by the others, so that they collectively operate as modules of one combined electromagnetic logging tool. These tool designs permit customizable antenna positioning and spacings, within certain limits set by the given lengths of existing tools and tubulars selected for use in the drilling assembly. These limits are believed to be inconvenient and overly restrictive.
It should be understood, however, that the specific embodiments given in the drawings and detailed description thereto do not limit the disclosure. On the contrary, they provide the foundation for one of ordinary skill to discern the alternative forms, equivalents, and modifications that are encompassed together with one or more of the given embodiments in the scope of the appended claims.