# Matching of Water Breakthroughs in a Low-Resistivity Oil Reservoir Using Permeability Anisotropy
## 2 Geologic and Production Overview
### History of LHS Reservoir Production and Modelling  
At the start of production in 1980, the first eight wells produced from the Lower Haushi Sand (LHS) reservoir comingled with the overlying reservoirs of the Middle Gharif, which comprise the following from top to bottom: Upper Haushi Sandstone (UHS), Dolomite Drain (DD), Sandstone Drain (SD), and Basal Gharif Sand (BS). The dates of comingled production from the reservoirs are given in Table 1.  
The wells are characterized by high initial oil rates with varying degrees of water encroachment. In 1996, Well D was converted to an injector and was the only injector for the next 11 years. Water breakthroughs, however, were recorded before the water injection started. The sources and paths of water inflow remain uncertain.  
The comingled production significantly complicated the interpretation and simulation of the production results. The biggest challenge in the simulation was matching the Lower Haushi water breakthroughs, specifically, the timing of the breakthroughs and the magnitude of the influx. Different explanations for the water breakthroughs and several static and dynamic reservoir properties have been considered to improve the matching in the past [16].  
\begin{table}
\begin{tabular}{c c c c} \hline \hline
**Well Name** & **Production Start Date** & **Commingled Production** & **Reservoir** \\ \hline \multirow{3}{*}{Well G} & \multirow{3}{*}{11/1/1980} & Lower Haushi & LHS \\  & & Dolomite Drain & UHS-DD \\  & & Gharif Basal & UHS-GB \\ \hline Well D & 11/1/1980 & Lower Haushi & LHS \\ \hline Well C & 9/1/1982 & Lower Haushi & LHS \\  & & Gharif Basal & UHS-GB \\ \hline Well B & 10/1/1982 & Lower Haushi & LHS \\  & & Dolomite Drain & UHS-DD \\ \hline Well E & 3/1/1987 & Lower Haushi & LHS \\  & & Dolomite Drain & UHS-DD \\ \hline \multirow{3}{*}{Well A} & \multirow{3}{*}{3/1/1988} & Lower Haushi & LHS \\  & & Upper Haushi & UHS \\ \cline{1-1}  & & Dolomite Drain & UHS-DD \\ \hline \hline \end{tabular}
\end{table}
Table 1: Comingled production from the Upper Haushi and Lower Haushi reservoirs.  
In the early wells drilled, the Lower Haushi appeared to have an oil-down-to (ODT). This suggests the presence of the oil-water contact (OWC) in the underlying Al-Khlata Formation. However, the OWC remained uncertain due to the complexity of the mineralogy, uncertainty around the formation water resistivity at that time, potential changes in the contact, and saturation changes due to production.  
The additional water observed was believed to be a result of failure in the plug and wellbore integrity issues, which led to the influx from either the overlaying Khuff Formation or the underlying Al-Khlata Formation. The step change in the water-oil ratio (WOR) was indicative of sudden failure more than likely associated with wellbore integrity as opposed to water coning or water channeling through the wellbore [16; 17].  
Thus, the high water influx is attributed to a combination of aquifer influx and water encroachment through defective wellbores, most likely due to poor well cementing [16]. If a significant amount of water is attributed to wellbore issues and not the reservoir, then there is potential for additional oil recovery from the Lower Haushi.