Changes

Near wellbore flushing occurs when the pressure or weight of the mud column exceeds the fluid entry pressure of the formation (for information on calculating mud weight, see chapter on [[Wellsite math]] in Part 3). This flushing by mud filtrate occurs above and ahead of the bit and is a function of time. If the mud system is overbalanced, gas shows can be reduced or totally suppressed. Even in a carefully balanced mud system where the fluid loss is minimized and the radius of wellbore flushing is small, problems can still occur in evaluating gas show quality if the rock's petrophysical properties are not considered. In zones of low effective porosity, even relatively small volumes of filtrate loss may result in deep invasion profiles. This causes a zone with a good gas show when drilled to recover only mud filtrate or to appear water saturated when later tested or electric logged. In zones of high effective porosity and permeability, the rocks will initially be flushed, then return to their native state soon after drilling, with little or no gas liberated. This causes a zone with minimal gas show when drilled to appear productive on electric logs or when later tested. Low permeability overpressured zones will not flush and will give high gas readings.

Near wellbore flushing occurs when the pressure or weight of the mud column exceeds the fluid entry pressure of the formation (for information on calculating mud weight, see chapter on [[Wellsite math]] in Part 3). This flushing by mud filtrate occurs above and ahead of the bit and is a function of time. If the mud system is overbalanced, gas shows can be reduced or totally suppressed. Even in a carefully balanced mud system where the fluid loss is minimized and the radius of wellbore flushing is small, problems can still occur in evaluating gas show quality if the rock's petrophysical properties are not considered. In zones of low effective porosity, even relatively small volumes of filtrate loss may result in deep invasion profiles. This causes a zone with a good gas show when drilled to recover only mud filtrate or to appear water saturated when later tested or electric logged. In zones of high effective porosity and permeability, the rocks will initially be flushed, then return to their native state soon after drilling, with little or no gas liberated. This causes a zone with minimal gas show when drilled to appear productive on electric logs or when later tested. Low permeability overpressured zones will not flush and will give high gas readings.

Information that can aid in the interpretation of flushed anomalies includes the following<ref name=pt03r17 />:

Information that can aid in the interpretation of flushed anomalies includes the following:<ref name=pt03r17 />

* Pump pressure

* Pump pressure