CONING EFFECTS EXAMINED FOP OIL-RIM HORIZONTAL WELLS

Sumbat Zakirov
Oil & Gas Research Institute, Russian Academy of Sciences
Moscow

Horizontal wells operating under critical gas-free rates permit the development of gas and oil reserves more efficiently.

These wells drain a larger area with fewer wells. Each well will have a greater cumulative oil production than a vertical well, but ultimate oil recovery from the reservoir can be less.

For a given formation zone with fixed reserves and number of horizontal wells, the location of these wells does not significantly influence cumulative oil production and ultimate oil recovery factor.

It should be remembered that total liquid rate, not just the oil rate, influences gas breakthrough

CONING

Gas and water coning hinder effective oil production. Since Muskat's 1 time, critical gas and water-free oil rates have been calculated and used for determining the effective oil production rates from oil-rim reservoirs.

A number of papers discuss the stationary coning theory of gas and/or water-free production of vertical wells." The coning theory was developed further for horizontal wells."'

Usually, oil and water properties are similar. There- fore, the critical water-free oil rates are low and unprofitable. As a result, oil recovery from oil rims involves producing water.

But gas-free oil withdrawal has proved to be real and effective for certain formation properties.12 11

TROLL GAS AND OIL

The North Sea's Troll gas and oil field is being devel- oped. In Troll, the oil rims with sizable reserves are in two parts of field: TWGP and TWOP. 12 13 15 These oil reserves are characterized as thin oil rims because their thickness is about 12-14 in (39-46 ft) and 22-26 in (72-85 ft). Plans include development of the oil rims in TWGP and TWOP with horizontal wells under critical gas-free oil rates."

Long-term horizontal well tests confirmed the practical- ity of this scheme. But some operational problems with critical gas-free oil rates remain unclear.

TWGP and TWOP oil rims serve as models for our studies. Data for our studies are similar to TWGP as given in References 12, 13, and 15. In the case of TWOP, only the oil rim thickness is changed to 22 in for our calculations.

NUMBER OF WELLS

In offshore fields, the number of the operating wells greatly influences net income. It is important to

know how much oil each well can recover. In other words, what should be the horizontal well pattern with critical gas- free oil production rates?

To determine this, we first consider the influence of the distance between horizontal well rows on the cumulative oil production and ultimate oil recovery.

For simplicity, we neglect the "end" effects concerned with the finite length of the horizontal wells. This allows us to use a profile formation model with one well and a two- dimensional, three-phase simulator.

In all cases, the well is placed 3 m above the water/oil contact (WOO and the horizontal hole length is 500 m (1,640 ft). The wells are operated under critical gas-free oil rates and the Distance L between rows of wells is equal to 600 rn, 1,200 m, 2,400 m, and 4,800 m.

Calculations are completed on one of the following conditions: water cut of 70%, and the economic cut-off oil rate of 50 cu m/day (315 bo/d) or a production life of 30 years.

Table 1 (24951 bytes) shows that in the case of the oil rim with 12 m oil-column thickness (similar to TWGP), the gas-cone breakthrough into a well does not depend on Distance L and is about 92-93 days. But in the case of the oil rim with 22 m thickness (similar to TWOP), the gas breakthrough time increases as L increases. The time increases from 543 days if L = 600 m to 1,051 days if L = 2,400 m. Further increases in L do not influence breakthrough time.

Initial rate in the TWGP case was equal to 500 cu m/day (3,145 b/d) and in the TWOP case was 4,000 cu m/day (25,162 b/d).

The distance between rows of wells significantly influences the production life of the oil rim in TWOP This is natural because an increasing L increases the oil reserves drained by each well. But L has much less influence on production life in the TWGP case.

Figs. 1 and 2 illustrate oil production under different distances between rows.

As the results show (Fig. 2c) there is a tendency to increase water cut with an increased L. But here one needs to consider that this is also connected with the increase in field life.

Data about the cumulative oil production and the ultimate oil recovery factors are important. First, these values are significantly dependent on the Distance L between rows of wells. Secondly, although cumulative oil production per well increases with L, the ultimate oil recovery factor from the reservoir is reduced.

The values also are affected by the oil rim thickness. In Table 1 (24951 bytes), cumulative oil production is compared for TWGP and TWOP We can see that the increase of the Distance L in the TWOP case gives more incremental oil production than in the TWGP case.

INITIAL OIL PATE

Calculation results show that the critical gas-free oil rates decrease sharply with time.

Therefore, the natural question is: Should we always start oil production at the critical gas free rate?

In the TWOP example, the initial oil rate of 500 cu m/day and 4,000 cu m/day are compared in Table 2 (22129 bytes).

It follows from Table 2 (22129 bytes) that the initial oil rate influ- ences the gas-cone breakthrough time. This time is not affected by L. But L does affect the cumulative oil recovery. Close distances between rows decrease cumulative production from each well.

With a fixed Distance L (namely L = 1,200 m), Table 3 (16873 bytes) and Fig. la and b (73644 bytes) shows the affect of several initial oil rates.

DISTANCE BETWEEN WELLS

The next case examines the effect of the distance between wells, D, for parallel rows of horizontal wells with a well length of 500 ft. D was examined for 500 m, 1,000 m, 2,000 m and 4,000 m with a fixed L of 1,200 m for the TWOP case. The previous case considered a Qi = 500 cu m/day, L = 2,400 rn corresponds to D = 0.

Table 4 shows the results from a three-dimensional, three-phase model.

The initial oil rates were 500 cu m/day. When gas breaks through, the well rates were set to the critical gas-free oil rates. When D = 4,000 m, initial oil rate equals 4,000 cu m/day.

Table 4 (17651 bytes) shows the influence of distance between wells on development considerations. Especially cumulative oil production and ultimate oil recovery depend on D.

As previously noted, these values have an opposite tendency as seen in Table 4 (17651 bytes) and Fig. 2f (192431 bytes).

Thus, both the Distance L between horizontal rows of wells and the Distance D between wells in rows have considerable influence on the cumulative oil production and ultimate oil recovery factor.

In other words, horizontal wells from offshore platforms with limited well capacity will increase cumulative production from each well but can decrease ultimate recovery from the oil rim.

WELL PATTERNS

In onshore fields, all zones in a field usually can be developed, but the influence of well patterns is still impor- tant.

As an example, four wells placed along two mutually perpendicular axis lines from one location are considered.

The reservoir conditions are assumed to be the same as TWOP The square area to be developed has a side of 3,162 m, horizontal hole length of 500 m, and initial oil rate of 4,000 cu m/day. The distance from the square's center to the nearest end of the horizontal well is equal to the following: d - 250 m, 300 m, and 1,01 8 m.

Table 5 (12638 bytes) shows the results for a well operating at the I critical rate. In this case, the well pattern has little influence on the cumulative oil production and ultimate oil recovery.

CRITICAL GAS-FREE RATES

Gas-free oil rates sharply decrease with time but these rates are not the only ones that prevent gas-cone break- through.

Produced bottom water also needs to be considered to obtain a gas-free liquid rate. Namely, liquid (oil and water) rate predetermines the intensity of the gas cone movement.

Figs. 1c (73644 bytes) and 2e (192431 bytes) demonstrate the dynamics of critical gas- free liquid rates for conditions similar to TWGP and TWOP. These indicate less rapid liquid rate decrease as compared with the oil rates.

ACKNOWLEDGMENTS

The author is grateful to den norske stats oljeselskap AS (Statoil) for permission to publish this article and to A. Henriques and O. Scontorp for their help and support in this study. Also thanks to Y. Gordon and Y. Levochkin for helping with the calculations.

AUTHOR