Just doing and assignment and got the following question
How can you determine from well logs the relative degree of mobility of oil or gas? Describe. Make a sketch to show what you mean.
Anwser
Hydrocarbon mobility can be seen by how much the formation as been invaded by the drilling fluid. The drilling fluids can be seen on the logs and this area is known as the invaded zone. There are 3 technique used in the industry
1. The transition zone is the area of the reservoir where the water saturation declines from 100% in the water zone to an irreducible water saturation which is above the free water level. The quantity of recoverable oil in a transition zone depends on the distribution of initial oil saturation (Soi) as a function of depth and the dependency of the oil's mobility, i.e., the residual oil saturation (Sor) and relative permeability, on initial oil saturation.
2. Step Profile, It is industry practise to simplify the geometry of the invasion profile and reduce it to just the flushed zone and uncontaminated zone therefore eliminating the transition zone, thus creating a step-profile See below picture. Any other type of profile will be too complex for the computation of resistivities due to the continuous changes in fluid mix with the depth of invasion. This invasion profile considers a cylindrical interface which moves laterally in the porous and permeable medium with filtrate invasion. This model is used for all petrophysical computations and evaluations.
Annulus Profile 3. Is the phenomenon of short duration which marks the presence of highly movable hydrocarbons (the mobility of many hydrocarbons is far greater than that of water due to relative permeability differences). See the below picture. It is usually indicative of low water saturation and high permeability. It is, however, not a criterion for the potential productivity of a formation and it does not occur in all oil-bearing formations. The annulus profile shows a low-resistivity zone (the\ annulus or ring) between the flushed and uncontaminated zones made up of formation water that has been pushed ahead of the invading fluid by a process of miscible drive. It dissipates in a very short time, minutes rather than hours. It is the oretical model that has little practical applicability.
That seems pretty discriptive to me, and there is picture in my awnser. Would you be happy if one of you jounior Geo,s gave you that anwser if you asked them??
