New wellbore temperature control design for preventing failure and poor performance of logging tools in high pressure - high temperature wells.

Failure and poor performance of logging tools in high pressure-high temperature wells; the associated technical consequences and the resultant high cost of logging operations are recurrent problems in the oil & gas industry worldwide. The aforesaid prompted this work to invent the "New Wellbore Temperature Control Design" for protection of logging tools against high bottom-hole temperatures. It hinges on the novel concept of thermal drainage radius and the temperature-drop effect of chilled oil-based mud to reduce bottom-hole temperature prior to logging. The methods applied include laboratory testing, mathematical modeling and design. The novel concept of "thermal drainage radius" quantified the radial extent of temperature-drop effect in the formation, away from the wellbore. Novel mathematical models were also invented. Laboratory results showed that the formation temperature-drop from bottom-hole temperature to thermal equilibrium temperature increased with bottom-hole temperature per chilled oil-based mud. The temperature-drop duration and temperature-rise duration increased with bottom-hole temperature per chilled oil-based mud. The temperature-rise duration increased with decrease in the temperature of the chilled oil-based mud per bottom-hole temperature. Job design results showed that when placed in the zone of interest in the wellbore the heat energy absorbed increased with quantity of the chilled oil based mud but the temperature rise duration decreased with increasing heat transfer rate from the formation and vice versa. Furthermore, results revealed that the higher the thermal conductivity of the formation, the longer the thermal drainage radius and vice versa. The logging operation should commence after placement of the chilled oil-based mud in the zone of interest (no circulation of the chilled oil-based mud) and be completed before the end of the temperature-rise duration. Oil-based mud is cheap and available in every country unlike the expensive vacuum flasks and thermal insulating jackets currently used for protecting logging tools against high temperature. The new wellbore temperature control design has global applicability.