![]()
Acoustic logging tools can assist in evaluating porosity because the compressional velocity of sound in fluid is less than the velocity in rock. If there is pore space in the rock, and it is fluid-filled, the acoustic energy will take longer to get from the transmitter to the receiver (i.e., low velocity indicates high porosity). The recorded velocity or travel time represents the sum of the velocity of: * Solid part or framework of the rock (i.e., the rock matrix) * Rock lining the pores * Fluid filling the pore space In turn, travel time in the rock matrix, ฮtma, is influenced by variations in [1]: * Lithology (i.e., the chemical composition) * Confining pore pressure (i.e., compaction) These factors are related through an empirical relationship known as the Wyllie time-average equation.[2] When the velocity (transit time, ฮt, or travel time, t) of the rock matrix and borehole fluids are known, porosity can be computed the following ways (Eq. 1 to Eq. 4). The velocity of most borehole and reservoir fluids (except gas) does not vary greatly: * A fluid velocity (ฮtf) of 189 ฮผsec/ft (5,300 ft/sec) is generally assumed for fresh drilling fluids * A slightly lower value, 185 ฮผsec/ft, is used for salt muds.
Site News