Optimal Energy Transfer Pipe Arrangement for Acoustic Drill String Telemetry
Drill string acoustic telemetry is an effective transmission method to retrieve downhole data. Finite-difference simulations produce the comb-filter-like channel response (patterns of pass bands and stop bands) due to the presence of coupling joints in the metallic drill string. Practical pipes used for drilling deep wells have slight variation in length. The selection and arrangement of downhole pipes is important for improving the transmission efficiency of extensional waves transmitted through the drill string. Downhole drill string channel is studied using the transmission coefficients calculated from the transmission matrix method, and the resultant transfer function produces identical results similar to the finite-difference simulations. Reciprocity of the drill string structure is proved by comparing the pass band responses using the ascend-only (AO) and descend-only pipe arrangements. Transferred energies calculated up to 180 pipes of random length at the end of the drill strings using transmission coefficients for the three different pipe arrangements, namely, AO, descend-then-ascend, and ascend-then-descend (ATD), are compared to find the optimal pipe arrangement for single measurement. For the situations when pipes are distributed in sets, multiple measurements are required. In this paper, two sets of AO and two sets of ATD arrangements are analyzed for multiple measurements. ATD and nxATD arrangements are proposed as optimal pipe arrangements to produce the best possible telemetry performance in terms of optimal acoustic energy transfer via one- and two-way acoustic communication for single and multiple measurements, respectively.
A telegrapher equation for electric - telemetering in drill strings
A reliable method of transmitting downhole information to the surface while drilling is essential to improve drilling applications. We have designed a numerical algorithm for simulation of electric-signal transmission through the drill string, based on the telegrapher equation (a circuit model). The drill string is then represented by a transmission line with varying geometrical and electromagnetic properties versus depth, depending on the characteristics of the drill string/formation system. These properties are implicitly modeled by the series impedance and the shunt admittance of the transmission line. The telegrapher equation is solved in the high-frequency range by using a direct method.
Propagation of electromagnetic waves along a drillstring of finite conductivity
An evaluation of the electromagnetic communication capability of a drillstring used to transmit data from the bottom to the surface of a well, or measurement while drilling (MWD), must take into account the finite conductivity and permeability of the drillstring. The choice of frequency is a compromise between a relatively fast transmission rate and a great depth of operation. Theory and experiments show that, because of the finite conductivity of the drillpipe, decreasing the frequency below a few hertz does not improve the communication because the current distribution along the string remains nearly constant. The current distribution is resolved numerically by a matrix formulation that takes into account the series or internal impedance of the drill, and a comparison with experimental results is made.