# Matching of Water Breakthroughs in a Low-Resistivity Oil Reservoir Using Permeability Anisotropy
## 4 Results and Discussion
### Well Logging Profile for LHS Reservoir  
Gamma ray logs, porosity, resistivity, water saturation, and perforation intervals in the LHS reservoir are shown for Wells G, D, C, B, H and E in Figure 7. Well A was not included because well logging was not performed in this well. Instead, Well H was shown for correlation purposes, but it did not produce from LHS.  
Core porosity closely followed the calculated log-derived porosity in Well G. In Well C, the core porosity also closely followed the log-derived porosity in the tight section of LHS-UD, but log-derived porosity was higher in LHS-LD.  
In the lower part of the LHS-UD and over the entire LHS-LD, the porosity is up to 23% and the resistivity is below 5 Ohmm. The resistivity in the porous layers can be less than 1 Ohmm. Porosity does not vary significantly across the LHS-LD section in Wells G and C. However, the porosity noticeably decreased from top to bottom in Well D accompanied by a resistivity increase, which can indicate a decrease in grain size with depth in LHS-UD.  
The resistivity decrease is often associated with an increase in grain size, since resistivity is inverse to average grain diameter when other parameters are kept constant. For example, quartz sandstones composed of big grains formed in the continental crust in the rift zones of Western Siberia and the Dnieper-Donetsk depression have a resistivity of 1-5 Ohmm at Rw = 0.01 Ohmm [30].  
The uppermost tight layer on top of LHS-UD has a Sw value close to unity in all wells in the oil zone, which can be due to the low contrast between oil and water caused by very low porosity. Except for this layer, the water saturation is consistently about 40-45%,  
Figure 7: Well logging profile of 6 wells in the following order from left to right (G, D, C, B, H, E): gamma ray (GR), resistivity logs, porosity, water saturation and perforation intervals. Well E was drilled 7 years after Well G, which explains the appearance of water at the base of LHS-LD as a result of OWC rise. Well H is shown for correlation purposes only, as it did not produce from LHS.  
indicating that the oil zone covers the entire LHS reservoir. Consequently, the OWC was set in the underlying Al-Khlata Formation in the previous model and was maintained in this study.  
Although it was assumed that the oil zone included both the LHS and UHS reservoirs, low resistivity or low contrast may mask the difference between oil and water, leading to misinterpretation of OWC. For example, 59 oil pools were recently discovered in Sudan in low-resistivity reservoirs, which were bypassed in the past because they did not exhibit any contrasts with the water zones [2].  
Among all of the wells, only Well D, which produced solely from LHS, did not experience early water breakthroughs. Other wells, produced in comingled mode with the reservoirs of Upper Haushi, as shown in Table 1, experienced early water breakthroughs. In Wells C, B, and E, perforations also covered the overlying shale and reservoirs of the Upper Haushi Formation. Perforations in Well C extended down to the Al-Khlata Formation. Hence, water can influx from the upper or the lower formations if the contrast between water and oil is not captured by the logging methods. For example, well testing performed in the uppermost tight oil zone of UHS showed only water.