Upstream Oil and Gas Drilling Mud System

In this lesson, we’ll discuss the drilling mud system.

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Drilling Mud System

At the rig floor, the rotating, hollow drill pipe string continuously circulates drilling fluids, called mud. Drilling mud is used to:

lubricate, cool, and clean the drill bit
control downhole pressure
stabilize the wall of the borehole
remove drill bit cuttings from the bottom of the hole

Drilling fluid is a highly engineered substance that is tailor-made for each specific well application. The mud chemical makeup is monitored closely as it changes throughout the drilling process. The mud system reflects any new geological and operational conditions that inevitably occur down-hole.

Two common classifications of drilling mud is whether it is oil or water-based mud, each of which are used depending on reservoir formation characteristics. Generally speaking, oil based muds are used in horizontal and shale wells and water based in vertical wells.

Air is also used in the initial vertical stages in many of the wells in some formations such as the Marcellus Shale in Pennsylvania.

The Drilling Mud System

The mud circulation system in any rig is as important to a drilling rig operation as the blood circulation system is to the human body. Any mechanical or technical breakdowns in the mud system can immediately stop the drilling.

Once reaching the drill bit through the hollow drill string, the mud then travels back to the surface around the outside of the drill pipe, called the annulus.

Because of its corrosive and toxic nature, drilling mud is carefully controlled on and off the drilling rig. Lined mud pits on land rigs, and tanks on offshore rigs, capture and store the mud that is not currently being used in the active drilling operation.

Mud motors are a hydraulically powered device positioned just above the drill bit. They use the mud pressure to spin the bit independently from the rest of the drill string. Mud motors are essential to effective directional drilling operations.

Mud pressure must be kept greater than the borehole formation pressure to prevent oil and gas escaping into well while being drilled. If the formation pressure exceeds the mud system pressure, a ‘kick’ is observed at the rig floor and corrective action is taken to add additional weight to the mud in the system.

Well Logging

Examining rock chips extracted from the mud is known as mud logging. The device that separates these cuttings is called a ‘shale shaker.’

Electronic well logging is frequently employed to evaluate the existence of possible oil and gas deposits in the borehole. Electronic logging can be ‘open hole’ within the wellbore or ‘cased hole’ when done after the casing string is installed.

Electronic logging can take place by lowering logging measurement tools into the newly-drilled hole when the drilling is stopped or while the well is being drilled, using Logging While Drilling (LWD) tools, which we’ll talk more about now.

MWD and LWD

Drilling progress is monitored and managed ‘on the fly’ using Measurement While Drilling (MWD) and Logging While Drilling (LWD) technology.

MWD instruments monitor variables in the borehole, such as depth, position, temperature, and pressure.

LWD measures characteristics of the geological formation such as water saturation, resistivity and porosity.

The ability to do mud logging and real-time cuttings analysis provides immediate data on the rocks being penetrated. This eliminates the need to stop drilling and pull the drill string out of the borehole in order to take these readings.

This, of course, saves time and money. Because the rig is the highest cost component in the drilling operation, and a day saved means a lot of dollars saved.