1963 Paper (P)

FORMATION EVALUATION IN WELLS DRILLED WITH INVERTED EMULSION MUD COMPARED WITH WELLS DRILLED WITH WATER-BASE MUD

Rush George Sinclair Oil & Gas Company Houston, Texas

ABSTRACT

Formation evaluation in wells drilled with inverted emulsion mud is essentially the same as for other muds; however, since no water is lost from this mud to the formation, the results should be more realistic for the following reasons:

1. Filtrate from water-base mud is water and may have a wide range of salinities or resistivities.
2. Filtrate water may be changed drastically by being forced through the mud cake.
3. Filtrate water is miscible with formation water and may be changed by it.
4. Filtrate water may induce radical changes in the formation.
5. Filtrate water may induce radical changes in the mud cake while being forced through it.
6. The volume of filtrate lost from inverted oil mud is substantially less than the loss from water-base mud.
7. Filtrate from inverted oil mud is all oil and is not miscible with formation water; therefore, the formation water is not changed by it.
8. Filtrate from inverted oil mud is all oil and is miscible with oil in the formation and may change its properties.

Log examples are presented from central Mississippi, south Louisiana, and the Texas Gulf Coast. These examples show some of the serious errors in formation evaluation that may occur if the well is drilled with water-base mud. The examples are as follows:

1. Self Potential values are too high, and formation water salinity calculated from this Self Potential is too high. Result: a calculated water saturation that is too low; but water only is produced.
2. Self Potential values are too low, and formation water salinity calculated from this Self Potential is too low. In extreme cases, the Self Potential may be completely