U.S.S.R. TURBODRILLING ROP EXCEEDS U.S. ROTARY RATE

Alexey Alexandrovich Pogarskiy, Anatoliy Michailovich Yasashin

All Union Scientific & Research Institute for Organization, Management, and Economics of Oil & Gas Industry
Moscow

It is common practice in the U.S.S.R. to evaluate drilling estimates by a mere comparison of the footage drilled on a per-rig, per-month basis because the figure integrates the techniques, technology, and management used.

According to this approach, the rate of penetration (ROP) in the U.S.S.R. is 2.3-2.8 times less than that in the U.S.

Based on the statistics published in the "Neftanaya Promishlennost" annual, in 1988 the average rotary drilling rate in the U.S. was 4,404 m (1 4,449 ft)/rig/month (this figure is the U.S. annual footage drilled by all rotary rigs divided by the annual average rig count and converted from yearly to monthly figures), and in the U.S.S.R. it was 1,595 m (5,233 ft)/rig/month.

This type of comparison is misleading because the drilling areas in the two countries differ greatly in geographic, geologic, and climatic characteristics. The same holds true for comparisons of average well depths.

The preceding characteristics influence drilling estimates differently. The most difficult task is to evaluate these factors as an average in each country. Our approach to the comparison takes into account only the average footage drilled for each country. In the U.S.S.R. more than 85% of the wells are drilled with turbodrills, and in the U.S. most wells (approximately 80%) are rotary drilled.

SOVIET ROP

According to the Ministry of Oil Industry data, in 1987 the average development well depth in the U.S.S.R. was 2,207 m (7,241 ft) with an average ROP of 17.51 m/hr (57.5 ft/hr). However, the average penetration time was only 5.57 hr/day or 23.2% of a 24-hr day.

This low figure of 23.2% is caused by poor organization, including an irregular supply of tubular goods and equipment. The situation is further complicated by a severe climate and the absence of a well-developed infrastructure--there are few good roads.

Thus, it takes 126 hr of actual penetration (2,207 m/17.51 m/hr = 126 hr) or 22.6 total days (126 hr/5.57 hr/day = 22.6 days) to drill a well that is 2,207 m (7,241 ft) deep. The average footage/rig/month is therefore 2,930 m (9,613 ft), and this is close to the actual field data (Table 1).

To compare the level of management and organization of drilling, technical means, and technology used through average depth/rig and average footage drilled/rig/month, we have used a chart that shows the daily footage of a medium-depth well at different depths (Fig. 1).

The final point of the curve is the intersection of the coordinate 22.6 days and the ordinate 2,207 m (7,241 ft). The remainder of the curve represents data from the Tyumen region. 2 For these data, ROP is about four times less at depths of 2,000-2,500 m (6,562-8,202 ft) than at surface intervals.

The average depth (2,207 m) of exploration wells in 1987 is divided into three intervals of 735 m (2,411 ft) each. Next, the average ROP is determined for each interval using the following conditions: in the first interval, ROP is four times higher than in the third interval, and the average ROP for the entire well from the surface to the total depth of 2,207 m (7,241 ft) is 17.5 m/hr (57.5 ft/hr).

We formulate the following equations:

735 m/V1 + 735 m/V2 + 735 m/V3 =

22.6 days x 5.57 hr/day =

126 hr, and V1 = 2V2 = 4V3

Solving the equations we find that:

V1 = 40.8 m/hr (133.9 ft/hr)

V2 = 20.4 m/hr (66.9 ft/hr), and

V3 = 10.2 m/hr (33.5 ft/hr)

From these calculations, the first interval of 735 m will be drilled in 3.3 days [735 m/(40.8 m/hr x 5.57 hr/day)]. The second interval will take 6.4 days, and the third will take 12.9 days. These values are then plotted to make the curve in Fig. 1.

In 1987, the average well depth in the U.S.S.R. was 2,207 m (7,241 ft), but in the U.S. it was 1,450 m (4,757 ft). If wells in the U.S.S.R. were drilled to 1,450 m like their U.S. counterparts, then by intersecting the curve in Fig. 1 at 1,450 m, we obtain a drilling time for this well of 9.2 days.

We would then find that typical wells at that average depth can be drilled with a total footage/rig/month of 4,728 m (15,512 ft). This figure is obtained by dividing 1,450 m by 9.2 days and then multiplying by 30 days/month.

This is 1.38 times higher (4,728 m divided by 3,426 m = 1.38) than that for U.S. rotary drilling.

The dashed curve in Fig. 1 represents the data from 1988, which is analyzed in a similar manner.

TURBODRILLING

In 1987, the average footage/rig/month from turbodrilling in the U.S.S.R. was 1.1 times that of the U.S. figures (3,783 m divided by 3,426 m = 1.1). In 1988, the U.S.S.R. figures were only 0.925 times those in the U.S. (4,076 m divided by 4,404 m = 0.925) (Table 1).

However, the difference between the average well depths must be taken into account. Thus, we find that wells in 1987 with an average depth of 1,450 m could be turbodrilled in 8 days, for an average footage/rig/month of 5,520 m (1,450 m/8 days x 30 days/month).

This is 1.6 times (5,520 m divided by 3,426 m = 1.6) faster than in the U.S.

In 1988, the corresponding figures result in an average footage/rig/month of 5,728 m. Thus, 5,728 m divided by 4,404 m 1.3, which is also faster than in the U.S.

We should also stress here that drilling operations are better organized in the U.S. than in the U.S.S.R. (i.e., wells drilled by the Ministry of Oil Industry). This is demonstrated by comparing the actual penetration time to the entire time for drilling operations.

For instance, in 1987 in the U.S.S.R. the average time spent drilling was only 23.2% of the total time necessary to complete a well, whereas in the U.S. the drilling time is over 50%. In spite of this very substantial difference, turbodrilling figures in the U.S.S.R. are significantly better than those of rotary drilling in the U.S.

If the oilmen in the U.S.S.R. managed to implement the opportunities they have and organize and manage drilling operations as effectively as those in the U.S., the U.S.S.R. turbodrilling estimates (1,450 m divided by 9.2 days x 30 days/month = 4,728 m/rig/month) in the interval of 1,500-2,000 m would be more than 1.5-2.0 times higher than those in the U.S.

The higher effectiveness of U.S.S.R. turbodrilling over U.S. rotary drilling is a result of better applications of bit rotational speed, axial load, and hydraulics in a given interval.

This statement can be substantiated by a mathematical model of drilling which was field-tested for roller cone bits (Fig. 2).

In Fig. 2, the horizontal lines represent formations of different hardness. The lower lines are for softer formations, and the upper lines are for harder formations. The inclined lines represent optimum drilling practices at different depths. Drilling practices vary with the time necessary to trip the bit: T1 T2 T3 T4 T5.

The cross-hatched area in Fig. 2 is the range of optimum drilling practices in wells 1,500-2,000 m (4,921-6,562 ft) deep.

The range is characterized by relatively low formation hardness and high-speed bit rotation. This entire range is good for turbodrilling, but it is not effective for rotary drilling.

In conclusion, we can state that attempts to convert from turbodrilling to rotary drilling in 1970-80 have not produced good field results. Such attempts cannot be justified at present either, even more so because Stratapax drill bits have specifications and applications that suit turbodrills better.

For deeper intervals, these drill bits are even more effective for turbodrilling than for rotary drilling.

REFERENCES

Neftanaya Promishlennost U.S.S.R., Al] Union Scientific and Research Institute for Organization Management and Economics of Oil and Gas Industry, Moscow, 1987-88.
Safiullim, M.N., et al., Ctroitelstvo Neftyanich Squadgin v Zapadnoy Sibiri, All Union Scientific and Research Institute for Organization Management and Economics of Oil and Gas Industry, 1987.
Pogarskiy, A.A., Avtomatizatsia Processov Burenia Glubokich Squadgin, Nedra, Moscow, 1972.