Cotton Valley Sand Play
Production Characteristics
The Cotton Valley Sand along the east flank of the East Texas Basin is characterized by good statistical numbers, including an average Cum of over 875 MMCFGE (at P60) and average EUR of over 1 BCFGE. The median (P50) Cum is about 700 MMCFGE. A P10 Cum, or lowest 10% of performers, lies at over 100 MMCFGE, with the bottom quarter of all wells yielding less than 300 MMCFGE. The P75 Cum, or best quartile of wells, is over 1 BCFGE, and the best well should produce over 20 BCFGE. These numbers are displayed in Figure 1, a probability distribution plot for this area.
Figure 2 shows a log-log plot of practical Initial Potential (Best Monthly Cum) versus. The Power Law fit of data suggests that natural fractures, as well as induced hydraulic fracturing, can enhance production fairways. A deeper look at the probability distribution reveals some interesting trends.
Figure 3 shows the probability distribution rescaled with a log-log relationship. Looking at the data range in this way reveals subtle differences in the rock that plays a role in gas deliverability. The chart clearly shows three trends, implying three different reservoir-rock types.
Figure 4 shows the relationship between Gas Cum and the Gas to Oil Ration (GOR). On average, as the Gas Oil Ratio increases, the cumulative production increases.
Figure 5 shows the relationship of Gas Cum to the Perforation Interval Size, in feet. As the perforation size increases, the upper envelope on the Cum also increases; but, so does the overall scatter in the data. The reason for this is simple: the amount of pay increases, but operators are also exposed to greater risk of water influx, with shallower perforations. Figure 6 shows how water production relates to Perf Size. With more perforated interval, there is a greater risk of increased water influx. As an illustration, some statistics were calculated on wells with enough history to draw some conclusions. Only 200 feet of difference in the upper perforated depth can make a difference. When looking at wells where the upper perforation is no shallower than 9,600 feet, versus wells with perfs shallower than 9,400 feet, the wells with more aggressive (shallower) completion attempts have a median Cum of only 691 MMCFG, versus 734 MMCFG for wells with perfs below 9,600 feet. The median water Cum was also higher for the shallower completions – 20,718 barrels of water versus 20,041 barrels of water.
A look at the trend's geology will help to explain these observations; but first, it is helpful to gather more facts about the reservoir by looking at the decline curves types.