RU2335624C1 - Method of well operation - Google Patents

Abstract

FIELD: mining.

SUBSTANCE: according to invention there are determined intervals of border between poor and adequate cohesion of cement with casing pipe. Intervals of exterior corrosion of the casing pipe under influence of corrosive bed waters are detected. Then intervals of internal corrosion of the casing pipe are revealed. Also there are determined intervals of potential breaks of the casing pipe at excessive loads, for example, during repair works, pressure tests and during well operation. All such intervals are considered as potential sources of casing pipe breaks, particularly those where cement adequately cohered with the casing pipe is located lower, than cement poorly cohered with the casing pipe. At that all intervals with revealed interior and exterior corrosion are either subject to limited loads or isolated from excessive loads.

EFFECT: increased service life of casing pipe and well in whole.

1 ex, 3 dwg

Description

The invention relates to the oil industry and may find application in the operation of the well.

A known method for determining leakage of a casing string of a well equipped with a tubing string, comprising registering a flow rate of a fluid pumped into a well by a flow meter that is lowered below the funnel of the tubing string. The registration of the flow rate of the injected fluid is carried out simultaneously at the wellhead with a flowmeter of the same design as that launched into the well, on a pipe segment in the injection line located in the gap between the pump unit and the wellhead. By the mismatch of the injected water costs recorded by the flow meters, the leakage of the production string is judged. As flowmeters use flowmeters of electromagnetic action, and the diameter of the pipe segment is chosen equal to the diameter of the production casing of the well. In this case, electrically conductive fluid is selected as the injected fluid into the well (RF Patent No. 2211327, published. 2003.08.27).

The method is based on the determination of flow. The method is complex, requires equipment with special devices, is not always accurate enough.

A known method of research on the tightness of an injection well equipped with tubing, which involves the selection of fluid from the well and pumping it back through the annulus. Before taking the fluid, the background temperature of the well is first determined along its wellbore after a preliminary stop and restoration of the quasi-stationary mode. In this case, the selection of fluid is carried out in an amount up to one volume of the wellbore, and in parts equal to the volume. The selected fluid is pumped into the well in the same way as during the selection. After each selection and injection, temperature changes along the wellbore are recorded by moving the thermometer inside the tubing string. Then, the obtained thermograms are compared with the background value of the temperature of the well (RF Patent 2166628, published. 2001.05.10).

The method is based on determining the temperature. The method is not accurate enough due to the influence of a large number of factors on thermometry.

Closest to the invention in technical essence is a method for monitoring the tightness of an injection well, which includes measuring pressure at the mouth at the inlet to the tubing string and in the annulus. Pressure changes are recorded by comparing pressures before and after stopping the well at the rate of pressure drop at the wellhead and in the annulus after stopping a working well and comparing pressures before and after starting the well for injection by the rate of increase in pressure at the wellhead and in the annulus after starting the well for download. For the criterion for assessing the tightness of the annular space take the calculated value of the flow rate of fluid entering or leaving the annular space of the well (RF Patent No. 2246613, published. 2005.02.20 - prototype).

The known method does not take into account possible violations of the tightness of the casing due to destruction due to increased pressure during the tightness control.

The proposed invention solves the problem of increasing the durability of the casing string and the entire well.

The problem is solved in that in the method of operating the well, they determine the interval where the boundary between the cement that is poorly coupled and well coupled to the casing passes, identifies the external corrosion interval, determines the internal corrosion interval of the casing, determines the interval and causes of the casing rupture under increased loads, taking into account such intervals during the operation of the well as a potential source of disruption, during repair work, pressure testing and during the operation of the well, isolation o interval from increased loads or limiting loads on identified intervals.

SUMMARY OF THE INVENTION

Quite often, when pressure testing is carried out without taking into account the condition of the well casing, a gust occurs at intervals weakened by corrosion from external or internal sources. The same is observed when a well is operated under forced injection modes of a working agent or during repair work related to the injection of intensifying or insulating materials under increased pressure, during hydraulic fracturing, etc. In this case, instead of a repaired or previously operational well, a well with casing violations is obtained, which should be repaired again. At the same time, compliance with the well operating conditions, under which the casing is not destroyed by increased loads, allows to extend the overhaul period or even maintain the well’s working capacity for many years, i.e. increase well durability. The proposed invention solves the problem of increasing the durability of the well. The problem is solved as follows.

During well operation, a determination is made of the interval where the boundary between the poorly coupled and well-coupled cement with the casing passes. The interval of external corrosion from aggressive formation waters of various horizons is revealed, which can lead to corrosion of the outer side of the casing and its weakening. The interval of internal corrosion of the casing string, which is usually located in the interval between the packer and the perforation interval of the reservoir, is determined. Above the packer, in the annulus between the casing and tubing, there is usually an anticorrosive fluid or a non-corrosive fluid such as dehydrated and desalted oil. These intervals are analyzed for corrosion activity and the degree of danger of corrosion damage to the casing in these intervals is determined. Identify the most dangerous intervals, which allows you to determine the intervals and the reasons for the rupture of the casing string at high loads. They record such intervals during well operation as a potential source of disturbance. During repair work, pressure tests and during well operation, isolate such intervals from increased loads or limit the load on the identified intervals. To protect against internal corrosion, reduce the interval between the perforation interval and the packer on the tubing string to the minimum possible value.

Concrete example

There are 766 wells on the balance of the oil and gas production department. Of these, 486 wells are registered in the production fund, 280 wells in the non-operational fund. The problem of casing gusts was always relevant during the operation of injection wells. This is affected by several factors, the main ones being the influence of an aggressive environment and high pressure. At the moment, there are 121 wells on the balance that have ever had casing disturbances. Of these, 94 wells are located in the Bavlinskoye field, 27 wells in the Sabanchinskoye field. Out of 121 wells: 77 wells are in the production fund, 44 in the non-operational fund. 38 non-production wells at the Bavlinskoye field, 6 non-production wells at the Sabanchinskoye field.

Of the wells of the production fund that have ever had a violation of production cores, 4 wells are inactive - all in the Bavlinskoye field. Of the wells of the non-production fund that have ever had a violation of production casing, 11 wells are in conservation at the Bavlinskoye field, 6 are piezometric wells, and 21 have been liquidated. At the Sabanchinskoye field, 4 wells were mothballed, 2 wells were abandoned.

Out of 56 production wells at the Bavlinskoye field, fresh water is pumped at 8 wells, and sewage is pumped at the remaining 48 wells. Of the 21 production wells of the Sabanchinskoye field, fresh water is pumped in 2, Devonian produced water is injected in 1, and sewage coal is pumped in the remaining 18 wells.

Thus, in the Bavlinskoye field, the ratio of production wells with production line disturbances at which fresh water is pumped to wells at which wastewater is pumped in the percentage ratio is: 14.2% -85.8%. At the same time, for the entire fund of the Bavlinskoye field, this ratio is: 14.1% -85.9%.

They conclude that the influence of the injected aggressive environment, by which we mean the injection of waste water, is minimal. The injection of anticorrosive fluid, the installation of packers cut off the influence of external factors by almost 100%.

Of the 121 wells that have ever had production casing violations, 93 wells had 1 violation, 22 wells had 2 violations, 4 wells had 3 violations, 1 well had 4 violations and 1 more well 5 violations. Thus, there were a total of 158 violations.

Determine the horizons, in front of which there were the most violations.

Kazan layer - 8.2% of violations, Ufa suite - 6.8%, Kungur layer - 5.2%, Artinsky horizon - 8.3%. But, as expected, most of the gusts were opposite the horizons, the waters of which are more corrosive. Tula horizon - 8.3%, Tournaisian layer - 10.5%. But the Serpukhov-Oka substage is the absolute record holder here - 32.3% of violations. Less disturbances were opposite the coal-bearing horizon - 3%.

The thickness of the layers is different, therefore, the number of gusts is recalculated per 1 m. Randomly selected geological sections of the wells were taken and average thicknesses of the selected horizons were obtained, on which there were most gusts:

1. Kazan Tier - 124 m.

2. Ufa Suite - 107 m.

3. Kungursky layer - 63 m.

4. Artinsky horizon - 175 m.

5. Serpukhov-Oka sub-tier - 294 m.

6. Tula horizon - 32 m.

7. Carboniferous horizon - 17 m.

8. Tournaisian tier - 95 m.

Recounting the number of gusts at these horizons for the thickness of the layers, we obtain the following values:

1. Kazan Tier - 0.09 gusts per 1 meter of reservoir power.

2. Ufa Suite - 0.08 gusts per 1 meter of reservoir power.

3. Kungursky layer - 0.11 gusts per 1 meter of reservoir power.

4. Artinsky horizon - 0.06 gusts per 1 meter of reservoir power.

5. Serpukhov-Oka sub-tier - 0.15 gusts per 1 meter of reservoir power.

6. Tula horizon - 0.34 gusts per 1 meter of reservoir power.

7. Carboniferous horizon - 0.24 gusts per 1 meter of reservoir power.

8. Tournaisian layer - 0.15 gusts per 1 meter of reservoir power.

To confirm the possibility of external corrosion, we will look at the quality of cement adhesion to the column in the Serpukhov-Oka sub-tier opposite to the failure of production casing. The analysis showed that good adhesion of cement in the studied intervals opposite to disturbances is observed only in 45.5% of the wells. The remaining 54.5% was distributed as follows: partial adhesion - 33.5%, poor adhesion - 15%, and cement was absent in 6% of the wells.

Thus, the possibility of external corrosion is obvious.

As the analysis showed, very often the violation occurred at the place where the boundary between the poorly cemented cement and the well-cemented cement passed, and the well-cemented cement was lower. It is analytically possible to imagine the possibility of formation corrosive-active water flowing through poorly coupled cement into the production string area and keeping it in this interval with well-coupled cement, which led to external corrosion of the casing metal.

Corrosion activity of formation waters of the Serpukhov-Oka, Tula, coal-bearing, Tournaisian horizons is due to the presence of hydrogen sulfide, oxygen, sulfate-reducing bacteria in these waters. According to the chemical composition of water, calcium chloride type.

But the analysis shows that the peak of the number of violations falls on the Tula horizon.

One of the reasons for the increase in the number of violations in the Tula horizon may be increased loads in this interval during pressure testing of the column. Increased loads in this interval during crimping are possible if:

1. operational horizon - Bobrikovsky;

2. Packer for crimping during repairs was installed below the violation interval;

3. Violations were identified during pressure testing of the casing string.

10 wells of the Bavlinskoye field were analyzed, on which violations were revealed opposite the Tula horizon. Of these, 3 are non-operational.

All wells have an operational horizon - Bobrikovsky. As it was found out, increased loads could arise not only during pressure testing, but also during other operations - treatment of the bottom-hole zone, hydraulic fracturing.

In addition, a mechanical load on the casing during hydraulic scraper treatment is possible.

Well analysis is as follows:

1. well 104 - during the overhaul of the well in October 1988, during the treatment of the bottom-hole zone, the initial pressure was 14 MPa, during the treatment it decreased to 11 MPa, after processing the geophysical surveys during the same repair revealed a violation. The maximum load fell precisely on the violation interval;

2. well 105 - during the overhaul of the well in November 1995 in the interval 1140-1160 m, the casing was worked out with a hydraulic scraper. After working at a depth of 1158 m, geophysical studies revealed a violation;

3. well 106 - during well overhaul in February 1970, hydraulic fracturing was carried out. The packer was installed at a depth of 1272 m. The pressure was 27-30 MPa. Immediately after hydraulic fracturing, geophysical studies identified a violation at a depth of 127.4 MPa;

4. well 107 - during well overhaul in June 1983, the treatment of the bottom-hole zone and then the test for injectivity were carried out under a pressure of 15 MPa. At the conclusion of the column is broken during the work to increase the throttle response;

5. well 108 - during well overhaul in September 2001, a hydroscraper was developed in the range of 1050-1235 m. After the development, hydrodynamic studies revealed a violation in the range of 1175-1176 m;

6. well 109 - during well overhaul in June 1991, after lifting the tubing string, geophysical surveys were conducted. They showed the tightness of the column. Then, pressure testing was carried out with a pressure of 15 MPa. The column was leaking. Then again geophysical studies were carried out. They showed a gust of casing at a depth of 1280 m. The packer was installed at a depth of 1288 m. It is clear that the gust arose during pressure testing.

So, in 6 out of 10 cases, the cause of gusts were increased loads on the casing string, which arose:

a) in two cases - due to the development of a hydraulic scraper,

b) in one case - due to hydraulic fracturing,

c) in two cases - due to increased pressure during the treatment of the bottom-hole zone.

d) in one case - during crimping.

External corrosion played a role. But in the case of violations opposite the Tula horizon, there is another factor - internal corrosion. As the analysis showed, this applies both to the wells into which sewage is pumped, and to the wells into which fresh water is pumped. The fact is that if the operational horizon is Bobrikovsky, and the shoe of the tubing string is 20-30 m above the perforation interval, i.e. just opposite the Tula horizon, this section of the casing string remains unprotected with anticorrosive fluid pumped into the annulus. In fresh wells, when injection is stopped, formation water gets into this section, which may contain hydrogen sulfide, oxygen, and sulfate-reducing bacteria.

The total effect of external, internal corrosion and increased loads on the electric power was the reason for the increased number of casing disturbances opposite the Tula horizon.

Outside of the horizons, let us consider another important section of the casing string - the section between the shoe of the tubing string and the perforation interval, and the thickness of this section is conventionally assumed to be 20 m. We find that 10% of violations occur in this interval.

At the same time, counting the number of gusts in this interval by 20 m, we get: 0.65 gust per 1 m, i.e. almost 2 times more than opposite the Tula horizon.

The main reason for this, of course, was internal corrosion. Apparently, the increased number of violations opposite the Tula horizon is explained by the fact that it is located just in the interval of the shoe of the tubing string — the roof of the perforated formation. However, the influence of the formation itself with its aggressive formation water cannot be cast off in the last role - there were practically no violations in the wells where the Devonian formations are operated between the shoe of the tubing string string and the perforation interval. Due to the absence of violations in the interval of the shoe of the tubing string — the roof of the perforated formation in the wells where the Devonian formations are operated, it is nevertheless necessary to isolate into a separate problematic section of the casing string, which had the most violations: the interval between the shoe of the tubing string compressor pipes and the roof of the perforated formation in wells where the Bobrikovsky horizon is operational. An interval of 20 m-40 m from the roof of the perforated area is also quite problematic - 0.2 violations per m. due to the combination of external and internal corrosion, the casing section opposite the Tula horizon in wells operating the coal-bearing horizon is completely a risk zone for disturbances.

Based on the results of the analysis, the following conclusions can be drawn.

1. The main causes of violations are:

a) external corrosion due to the action of formation water,

b) increased load on the casing string, by which it is necessary to understand not only increased injection pressure, but also increased pressure during technological operations of overhaul wells, as well as mechanical damage to the casing string during well repair,

c) internal corrosion occurring in unprotected sections of the production string exposed to anticorrosive fluid.

2. An increased number of gusts is noted in the casing section, which almost always remains unprotected by both corrosion protection and the packer, that is, in the interval of the shoe of the tubing string — the roof of the perforated formation in the wells where the coal-bearing horizon is operational.

3. During technological operations of well overhaul, it is necessary to take into account the risk of a casing string disturbance opposite the horizon adjacent to the production one. Therefore, in some cases, carry out operations with the installation of the packer 5-10 m above the perforation interval (usually they are carried out with the installation of the packer 20 m above the perforation interval). When crimping the casing, on the contrary, the packer can be installed 30-50 m above the perforation interval, leaving the Tula horizon below the packer. Very often, casing pressure testing is carried out in conjunction with geophysical surveys. Geophysical surveys cannot detect casing disturbance if it is above the water level, therefore, often casing pressure testing is additionally carried out. If geophysical studies have shown that there are no violations opposite the Tula horizon, then you can install a packer above this horizon.

Under these conditions, the following models are offered.

The first model (figure 1): installation of the packer 1 when crimping the casing 2 through the string of tubing 3 under the sole of the Tula horizon 4 when performing bottom hole treatment, hydraulic fracturing in wells where the coal-bearing horizon 5 above the Tournaisian layer is operational 6, and where in this case a reserve of 5-10 m remains until the perforation interval. In this case, the entire load during the treatment of the bottom-hole zone, hydraulic fracturing will fall on the operational horizon.

The second model (figure 2): installation of the packer 1 when crimping the casing 2 through the string of tubing 3 above the roof of the Tula horizon 4 at the wells where the coal-bearing horizon 5 above the Tournaisian layer 6 is operational. to the casing 2 opposite the Tula horizon 4, and the tightness of this section is determined by geophysical studies of the well.

The third model (Fig. 3): installation of a packer 1 during pressure testing of the casing string 2 during the descent of the tubing string 3 to the upper boundary of the exploited horizon at the wells, where the exploitation horizon 5 is located below the Tula horizon 4 and above the Tournaisian layer 6. In this case, the injected corrosive wastewater does not affect the sections of the casing 2 located above the carboniferous horizon 5 and protected by an anti-corrosion fluid.

Application of the proposed method will increase the durability of the casing string and the entire well.

Claims (1)

A method of operating a well, including determining the interval where the boundary between the cement poorly adhered and well adhered to the casing, identifying the external corrosion interval of the casing from aggressive formation water, identifying the internal corrosion interval of the casing, determining the interval of possible rupture of the casing with increased loads - during repair work, pressure tests and during well operation and taking into account such intervals as a potential source of casing failure especially those intervals where cement well adhered to the casing is lower than cement poorly adhered to the casing, while limiting the loads on the identified intervals of external and internal corrosion or isolating these intervals from increased loads.

Images