University of Sulaimani Well logging practical College of Science 4th Year Geology Dept. of Geology Lab No.-2-

Subject 2: (Borehole Environment……….Invasion and Resistivity Profiles) . Lecture: Devan O. Hussein Objective: Drilling mud operation while drilling and understanding the Resistivity profiles. The Scientific contents: Where a hole is drilled into a formation, the rock plus the fluids in it (the rock-fluid system) are altered in the vicinity of the borehole. The borehole rock and the rock surrounding it are contaminated by the drilling mud, which affects logging measurements. Figure 1.1 is a schematic illustration of a porous and permeable formation that is penetrated by a borehole filled with drilling mud. Invaded zone: The zone in which much of the original fluid is replaced by mud filtrate. It consists of a flushed zone (of a Resistivity Rxo ) and a transition( Ri). Uninvaded zone: The uninvaded zone is located beyond the invaded zone, pores in the uninvaded zone are uncontaminated by mud filtrate, instead, they are saturated with formation water Resistivity (Rw), oil, and/or gas. The Resistivity values for drilling mud, mud cake, and mud filtrate are recorded on a log’s header (Figure 1.2) and are used in log interpretation. Ex (1): Draw the invasion profiles for three idealized versions of fluid distributions in the vicinity of the bore hole.

1- Step profile, the abrupt contact between invaded zone (Rxo) and uninvaded zone (Ro) in this sample is wet (100% water and no hydrocarbon).

2- The transition profile, the invasion of mud filtrate (Rxo) diminishes

gradually through a transition zone (Ri) toward the outer un-invaded zone (Ro).

3- Annulus profile, this reflects a temporary fluid distribution and denotes

the presence of hydrocarbon, the abrupt drop in resistivity (Ran) beyond the invaded zone (Rxo, Ri) due to high concentration of the formation water in the annulus zone. The un-invaded zone with true resistivity (Rt) will be high.

Ex (2): Draw two horizontal sections through permeable formations, once to the oil bearing and the second water bearing. Assume that the drilling muds are either fresh water or salt water based.

1- The concentric circles represent the bore hole, invasion zones (flushed and transition zone) plus the annulus of the second one (oil bearing case).

2- Two curves connected with the concentric circles, one to the fresh muds

and second to salt muds.

3- Distribute the four cases on the coordinate curves (vertical axis to resistivity values and the horizontal axis to the distance from the well to un-invaded formation.