ANALYTICAL PLOTS FOR ANALYSING CONING & CHANNELING IN DIFFERENT RESERVOIRS OF UPPER INDUS BASIN OF PAKISTAN
Muhammad Osama Jafri (NED University), Hafiz Haleem ud Din Farooqi (NED University), Waleed Anwar Saleem (NED University)

Abstract Water and gas drive mechanisms pose different problems for a petroleum engineer. The most common tribulations encountered are water and gas coning, high permeability layer breakthrough and near wellbore water channeling. In order to identify and differentiate between these tribulations different techniques can be applied. In this extended abstract, the characteristic trends of log-log plots of WOR (Water Oil Ratio) vs. Time will be analyzed for different mechanisms on the basis of proposed techniques by K.S. Chan (SPE Paper No. 30775). The derivatives of the plots show different trends for coning and channeling which makes it easier to interpret the situation. The identification of the problems will contribute to optimize water/gas coning treatments and other encountered problems. This technique has been applied on the wells of Upper Indus Basin (Potwar region) of Pakistan for the identification of coning or channeling in the wells. Introduction In a hydrocarbon reservoir, water is the most abundant fluid existing in the pores with the oil and as a separate phase in an aquifer. Broadly, the produced water can be classified as good water and bad water. Good water is the water which sweeps or displaces the oil and is produced within the economic limit. Bad water is the water which is beyond the economic limit, thus leading to potential revenue losses. Bad water may be produced due to many reasons, the major of which are water coning and channeling or a combination of both. When oil is produced at higher rates, water is drawn from an adjacent, connected zone towards the completion in the shape of a cone. When the cone becomes unstable, the water rises up and breaks through the perforations replacing all or part of the hydrocarbon production from that well. This is called water coning. High permeability layers can cause premature water breakthrough to the well by providing a channel or a path of least resistance to the water. This results in the water bypassing potential production zones, thus reducing the overall recovery. This is called water channeling. Field experience showed successful water control design would not be the same for different mechanisms. If the wrong mechanism is diagnosed, the treatment could even harm the production instead of helping to improve it. Therefore, characterization of the different mechanisms is very important to ensure proper treatments are applied. One of the simplest characterization methods of differentiating between coning and channeling is the Chan Water Diagnostic Plots, introduced by K.S. Chan in the year 1995. The basic idea of the plots is that if the production history ratios (WOR/WGR/GOR) and their time derivatives (WOR’/WGR’/GOR’) are plotted on the log-log graph against time, the graph shows specific trends which can be used to identify what type of

2

ANALYTICAL PLOTS FOR ANALYSING CONING & CHANNELING IN DIFFERENT RESERVOIRS OF UPPER INDUS BASIN OF PAKISTAN

mechanism is responsible for the water production. For an oil well, if the slope of WOR’ is positive, it means that channeling is taking place. If the slope is negative, it indicates coning. In a new approach we tried, we used the plots to discern whether a spike in the WOR was due to change in the channeling or coning mechanism, or due to some other factor like reservoir pressure decline, skin buildup, choke size adjustment or gas lift etc. Analysis for Oil Reservoir A well XYZ from an Upper Indus Basin oil reservoir was chosen as a candidate for the analysis of water production. To accomplish this, Chan advices that first the entire production history of the well should be plotted. From the plot, definite stages of production should be separated and analyzed in more depth. The production history for the XYZ well is shown in Figure 11. Keep in mind that this data is not from the beginning of production so water is already being produced. Looking at the plot, we can define four different periods where sudden change is taking place. The first period comprises the first 8 days of the production data. This data is shown in the Figure 1-1. As this figure quite clearly shows, the WOR’ has a positive slope. According to Chan, a positive slope shows that channeling is taking place. The second period starts after 8 days and goes on until day 65. It can be seen by almost a horizontal WOR line in Figure 1. The period is separately given in Figure 1-2. Here, the WOR’ curve is evidently going down, indicating a negative slope. Chan states that a negative slope dictates that coning is taking place. The third period goes on from day 65 to about day 1100. This plot is shown in Figure 1-3. Here, a problem arises. The WOR’ data is scattered to the extent that we cannot figure out whether the slope is positive or negative. The same can be said for the last period shown in the Figure 1-4. To overcome this problem, we applied a new strategy, not used by Chan in his paper. The data of the third and fourth period was divided into intervals of 30 days and then the slope of the WOR’ was measured for each month. The slope of each interval was plotted on a graph against time. This plot is shown in Figure 2. There is no abrupt slope change taking place in Figure 2 for about the first 600 days. The slope not suddenly changing shows that almost the same mechanism as before is taking place, so the WOR spike in Figure 1-3 should be due to something that changed the production of the well. Therefore, we conclude that the WOR spike in figure 1-3 at about the 100 days point is due to some factor, like well stimulation, possibly acidization, resulting in the increase of the water produced. After about 600 days, there is a spike again in the third period. This is clearly shown by the increasing slope in Figure 2, indicating that channeling is occurring. In the fourth period, there is again a spike in the WOR at about 1300 days. Again, we refer to Figure 2. It shows a very steep increasing slope, thus indicating that channeling is taking place.

1

Figures at the end of the paper.

3

ANALYTICAL PLOTS FOR ANALYSING CONING & CHANNELING IN DIFFERENT RESERVOIRS OF UPPER INDUS BASIN OF PAKISTAN

Figure 1: Log-log plot of entire production history of XYZ well

Figure 1-3: Third period of production history

Figure 1-1: First period of production history

Figure 1-4: Fourth period of production history

Figure 1-2: Second period of production history

Figure 2: Change of slope with time