Formation Fluid Influx (Kick) One of the most common causes of oil on shale shakers is a formation fluid influx, also known as a kick. This occurs when the pressure in the wellbore is lower than the reservoir pressure, allowing oil to flow into the wellbore and be carried to the surface.
Underbalanced Drilling In underbalanced drilling, the hydrostatic pressure of the mud column is intentionally kept lower than the reservoir pressure to allow hydrocarbons to flow into the wellbore. This can lead to oil being carried to the surface through the drilling mud, appearing on the shale shakers.
Highly Permeable or Fractured Reservoirs Some reservoirs have high permeability or natural fractures that allow hydrocarbons to flow more easily into the wellbore during drilling. When drilling through such formations, oil from the reservoir can mix with the drilling mud and reach the shale shakers.
Oil-Based Mud (OBM) Contamination If oil-based mud (OBM) is being used, distinguishing between formation oil and the base oil of the drilling fluid can be challenging. Improperly maintained mud properties or contamination with formation oil can cause excess oil to appear on the shakers.
Differential Pressure Effects When there is a significant pressure difference between the wellbore and formation, oil can be forced into the wellbore, even if the mud weight is designed to provide adequate control. This effect is more pronounced in depleted reservoirs where pressure variations exist.
Wellbore Instability and Loss of Drilling Mud Wellbore instability can lead to fractures or cavings, allowing formation fluids, including oil, to enter the wellbore. If drilling mud is lost into the formation due to fractures or vugs, the reduced wellbore pressure can lead to an influx of oil.
Improper Mud Conditioning and Circulation Issues If the mud circulation is not properly maintained, cuttings and formation fluids may not be effectively removed from the wellbore, leading to an accumulation of oil in the system. Poorly managed gas-cut mud can also lead to increased oil content in the drilling fluid, appearing on the shale shakers.
Gas Cut Mud Leading to Oil Separation When drilling through a gas-bearing formation, the gas can expand as it rises to the surface, causing separation of oil components from the mud. This can result in visible oil on shale shakers, even if the well is otherwise stable.
