ABSTRACT:

After nearly 30 years of measurement-while-drilling (MWD) operations, the market is still demanding an MWD telemetry system which can reliably operate in real time regardless wellbore hydraulics. The electromagnetic (EM) transmission system transmits downhole data by injecting current into the formation rather than modulating mud pulses. This method reduces survey time by transmitting data during pumps off and is inherently more reliable than standard mud-pulse telemetry, as there are no moving parts in the system. Unaffected by drilling-fluid loss, the EM system is also capable of transmitting logging-whiledrilling (LWD) formation evaluation data for geosteering in underbalanced environments, including wells drilled with foam, mist, air or aerated mud. This paper discusses the planning, operations and results of the first EM MWD application while drilling a well on the high enthalpy geothermal Larderello - Travale (Siena, Italy) field. The main objective while drilling the well was searching for the superheated steam within the metamorphic Basement in order to enhance the production in the geothermal area characterized by a drilling environment with the following main issues:

  • Total loss of circulation with the consequent absence of annulus return and hydrostatic head within drill pipe which in turn leads to the impossibility of using conventional mud pulse telemetry system.

  • Temperature increasing quickly above 150°C when circulation is off (connection and survey time), therefore causing the damage of conventional MWD equipment.

A feasibility study for the use of EM MWD was performed before drilling the well, in order to predict the level of signal attenuation while drilling through the expected layers of anhydrites. By the use of EM MWD it was possible to overcome the limit of mud pulse transmission system in total circulation losses and at the same time provide the operator with continuous (real time) directional control and temperature monitoring while drilling on a cluster well

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