Analysis of Fracturing Effect of Recovered Wells in Class II Oil Reservoirs and Practices to Safeguard Fracturing Result

In recent years, as the polymer drive of the main class I oil reservoir has been transferred to the subsequent water drive development, the class II oil reservoir has gradually become the main target of the three times oil recovery development, compared with the class I oil reservoir, the class II oil reservoir has the characteristics of relatively poor oil reservoir development and strong non-homogeneity. With the effect of the second-class oil layer successively, the fracturing target is mainly the second-class oil layer, but the fracturing effect becomes worse year by year, and it is difficult to produce in the next step. Therefore, it is important to optimize well selection for fracturing. In this paper, we analyze the effect of fracturing wells in Class II reservoirs in recent years and summarize the factors affecting the effect of fracturing in Class II reservoirs and the practices to ensure the effect of fracturing in Class II reservoirs, so as to provide a reference basis for the selection of wells for Class II fracturing in the future, which is of certain significance.


Basic information on fracturing in previous years
Extraction well fracturing as one of the main production tools for oilfield development, the second-class oil layer has entered the peak of effectiveness one after another and has become the main fracturing target. The oil recovery rate is rising, but the fracturing effect is getting worse year by year, and it is difficult to go up to production next since 2005. The oil recovery rate has been rising, but the fracturing effect has become worse year by year, making the next step to production difficult. 2018, through optimizing the fracturing selection, the average single well fracturing of the extraction wells increased oil by more than 6t per day. In terms of fracturing scale, the proportion of fracturing in the late injection block reached more than 70%. Based on the analysis of the fracturing oil increase effect, the proportion of "high efficiency wells" can be effectively increased by optimizing the well selection conditions and implementing measures to ensure the fracturing effect.

Summary of experience with fracturing effects
Based on the fracturing results of Class II reservoirs in previous years, the experience is summarized in terms of "factors affecting the fracturing effect of recovery wells" and "practices to ensure the fracturing effect of recovery wells". The analysis concluded that the effect of fracturing on the oil increase in the recovery well is mainly related to factors such as "the remaining oil in the fractured well, the degree of control, the timing of fracturing, and the injection status of the well area". The approach to ensure the fracturing effect of the extraction well mainly includes "pre-fracturing training, timely post-fracturing adjustment and optimization of the fracturing process".

2.1
Analysis of factors affecting the effectiveness of fracturing of recovery wells 2.1.1 Good fracturing effect in the remaining oil-rich well area.
According to the oil increase effect of different fractured wells, when the water content before injection is less than 96%, the oil increase effect of fracturing is better, and the lower the water content before fracturing, the higher the initial oil increase after the measure. From the oil increase of wells with different water saturation before fracturing, the oil increase effect of fracturing is better when the water saturation is less than 56%, and the lower the water saturation, the higher the initial oil increase after the measure. In addition, the oil increase classification of fracturing wells with different water flooding thickness ratios shows that when the high-water flooding thickness ratio exceeds 40%, the oil increase is relatively low due to the low remaining oil, high water content saturation and relatively high-water content before injection. Therefore, when selecting wells for oil fracturing, try to select wells with a high-water flood thickness of less than 40% and wells containing less than 96% water before injection.   According to the results of the above analysis, the extraction wells are located near the original water-driven injection well rows or wells in the mainstream line of water-driven oil and water wells with poor fracturing effect. According to the statistics of fracturing effect of Class II oil layer in recent years, there are 10 wells affected by the interference of water-driven wells, the water content saturation of these wells is higher than other well areas, the water content before fracturing is greater than 96%, the effect of fracturing to increase oil is not satisfactory, so when conducting fracturing selection avoid water-driven interference wells as far as possible. According to the analysis results of the oil increase effect of fractured wells in different periods in recent years, the oil increase effect of fracturing wells in the declining and stable water-bearing periods is significantly better than that of wells in the non-effective, poorly effective and rebounding water-bearing periods. Based on this experience, 38 wells were fractured in 2018, accounting for 60.32% of the wells fractured, 5.17% more compared to last year. A better oil enhancement effect was achieved.  Through further analysis, the fracturing effect of the recovery wells with a high degree of effectiveness was good. From the oil increase effect of wells with different water content reduction rate before fracturing, when the water content reduction rate before fracturing exceeds 5%, the oil increase effect is better, and the higher the water content reduction rate, the greater the oil increase by fracturing the wells. Therefore, when selecting wells for fracturing, we should try to select wells with a water content decline and water content stabilization period, while the water content decline rate is greater than 5 percentage points for fracturing. According to the oil increase effect of fractured wells with different degrees of control, the oil increase effect of fracturing wells with a degree of control greater than 60% is higher than that of wells with a degree of control less than 60% at different stages, and the proportion of wells with oil increase greater than 6t is also higher, so fracturing wells with a degree of control greater than 60% should be selected for fracturing as far as possible.   Based on the analysis of fractured well results, among the fractured wells with similar recovery well see-through conditions, connection conditions and well relationships, the well group with good well zone injection conditions had good results. Take two fractured wells in Block D as an example.   Both wells are free from water-driven well interference. From the fracturing timing, D1 is fractured during the water-bearing decline period and D2 is fractured during the water-bearing stability period. In terms of effectiveness, both wells have shown good results. Based on the experience of the previous analysis, both wells are suitable for fracturing. However, poor injection in the well zone before D1 pressure, decreasing injection volume leads to a continuous decrease in fluid production and sinkage before pressure in the recovery well, and insufficient fluid supply in the well zone, resulting in a small increase in fluid after fracturing the recovery well. And the injection volume of the D2 well group was reduced at the beginning of the injection period due to the rapid rise in injection pressure, after which only one well in the well area had difficulty in injection after testing and recovered after well washing, and the pre-fracturing sinkage had an improving trend, proving that the injection volume was guaranteed and a better fluid and oil increase was achieved after fracturing. It has been proven that for wells with fracture selection conditions, but poor well zone injection conditions, prefracture cultivation can be implemented to ensure that the fractured well increases fluid and oil. Take well A-1 as an example. The A-1 well is an oil well located in the central area of Block A. The well is dominated by a thin differential layer development and the numerical model oil saturation results indicate that the well is relatively rich in remaining oil. Figure 12 Censored map of the A-1 well group Figure 13 Numerical simulation of oil-bearing saturation in Block A As can be seen from the production curve, the well has low initial water content and relatively enriched remaining oil. The significant drop in injection volume in the well area is mainly due to the implementation of dissection and controlled injection in three wells in the well area, injection difficulties in one well and operational reallocation, which resulted in a large drop in fluid volume in the recovered well. Figure 14 Pre-frac extraction curve for well A-1 In order to guarantee the fracturing effect, the injection wells in the well area were planned to implement programmed fluid withdrawal and fracturing to increase injection in the first five months of the measure, and after ensuring the fluid supply capacity, the fracturing of the extraction wells was implemented, and the daily oil increase reached 13.7t after the measure, and the water content dropped significantly. After the fracturing program is implemented, the production pressure differential should be amplified in a timely manner according to the dynamic characteristics of the well area to safeguard the fracturing effect and extend the fracturing period as far as possible. Take well D-3 as an example. The D-3 well is located at the edge of the fault adjacent to Block D and Block C. The well area is connected to a total of three injection wells, two of which belong to Block C and one to Block D. The well is mainly developed with medium and low water flooding, with a water content saturation of 48.5%, which is 7.33% lower than that of the whole area, and relatively rich in remaining oil. Influenced by the earlier injection and gathering time in Block C, the well started to see results two months after it was put into production, with a large drop in water content and relatively rich residual oil. After the implementation of fracturing in December 2017, the initial daily fluid increase of 24t and daily oil increase of 14.7t were achieved with good oil increase effect, Two main problems existed at the stage from the production curve, one is that the sinkage has been at the wellhead after fracturing and the production capacity has not been effectively released, and the other is that the fracturing The injection capacity of the well area before fracturing decreased, and the fracturing validity period was difficult to guarantee. In order to ensure the fracturing effect, firstly, in response to the actual high sinkage, timely implementation of the adjustment of the large participation to enlarge the production pressure difference, the initial period after the adjustment of the participation, the daily fluid increase of 22t, the daily oil increase of 12.6t, to achieve a better fluid and oil increase effect. Second, the timely implementation of fracturing transformation for injection wells in block D of the well area where injection has deteriorated, the injection volume was guaranteed after the measure, and the effect of fracturing the recovery well to increase oil lasted for 7 months, with a cumulative increase of 3,133t.

Figure 18
Extraction curve for the D-3 well group

Optimizing the fracturing process to guarantee fracturing results
According to the relationship curve between the fracture penetration ratio and the degree of recovery for Class II reservoirs, when the fracture penetration ratio reaches a certain value, the degree of recovery reaches its highest point, and when the penetration ratio is greater than that value, the degree of recovery begins to decline. This is due to the fact that when the penetration ratio is too high, it tends to cause the injected media to burst in, resulting in a rapid rise in water content. The penetration ratio is determined by the fracture radius. According to the formula for the relationship between sand addition and fracture radius, the amount of sand addition needs to be optimized to keep the fracture penetration ratio around 0.12-0.18 as much as possible to ensure the maximum fracturing effect, so when there is a high water absorption ratio layer in the injection well in the well area, the amount of sand addition should be controlled to prevent the penetration ratio from being too high and causing the injection media to break through. The effect of fracturing a well from a Class II formation is highly uncertain and is usually not determined by one factor, but rather by the co-existence of several factors that affect the fracturing effect. 3.2 Based on past experience with fracturing to increase oil, select wells with characteristics such as "rich residual oil, water content in the declining and rebounding phases with a high degree of effectiveness, and strong injection capacity in the well area" for fracturing as far as possible to increase the probability of producing fractured highefficiency wells. 3.3 According to the well area injection and extraction conditions, adopt pre-pressing cultivation, timely and effective post-pressing tracking and adjustment and optimization of the fracturing process, which can guarantee the fracturing effect of the recovery well.