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A mud logging unit (MLU is the short name of Mud logging unit ) is installed on the rig when geologic information must be retrieved on a timely basis. If the formations drilled are not well known or if a specific geologic horizon is targeted, mineralogical data from cuttings brought to the surface with the mud might be essential. For example, secondary hydrothermal mineralization is a key indicator to locate a viable horizon as a geothermal reservoir. Changes in the mineralogical composition of the formations can require adjustments in the mud program.

The main task of a mud logging unit is to monitor and collect all necessary information to ensure an uninterrupted drilling process. This includes the measurement of gases, mud flow rates (in and out), temperatures, drill rates, depth, and pressures.

Gases monitored include hydrogen sulfide (H2S) and carbon dioxide (CO2). H2S is poisonous, very corrosive when dissolved in water, and flammable as a gas. When dissolved in water (of the mud) it lowers the mud pH increasing corrosion tendencies. As a gas, it is transported to the surface when air is applied as the drilling fluid. Carbon dioxide is soluble in water and can form carbonic acid in the drilling fluid which reduces the mud pH and makes it more corrosive.

It is important to detect the gases in a timely manner using a mud-gas trap during mud drilling or an air-gas trap during air drilling, so appropriate abatement measures can be taken.

The depth and the drilling rate of the drill rig are also monitored by the mud logger. A sudden increase in the drill rate might help indicate fracture zones, which are of great interest in the geothermal drilling process

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