Unusual methods treat Russian drilling mud losses

June 5, 2000
In the oil fields of the Volga-Urals and Near-Caspian basin regions, Russian drillers use determinative methods and plug-back systems to treat lost-circulation zones.

In the oil fields of the Volga-Urals and Near-Caspian basin regions, Russian drillers use determinative methods and plug-back systems to treat lost-circulation zones.

Much of the difficulty lies in the nature of the lithology where fluids exit into fissured and cavernous carbonate rocks.

Once circulation is lost, the typical practice is to change from clay-based to water-based drilling muds, then continue drilling blind (without returns) until the casing string is run. During this process, however, the cavings that drop out, settle to the bottom of the hole, in thicknesses up to 30-50 ft, hindering further deepening. Thus, potential problems associated with stuck drillstrings become distinctly possible.

To eliminate this problem, the uphole caverns are cemented in.1 Then, once the caverns are sealed off, the drill cuttings and water can move upward from the borehole bottom into the lost circulation zone, often resulting in the partial plugging of the minor fissures and channels.

Romashkinskoye

In the Zelenogorskaya area of the Romashkinskoe field, Tatarstan, wells that contain caverns 8-12 m thick were drilled and cemented in intervals of 700-750 m and 1,000-1,100 m. In this area, many wells with cemented caverns reaching 30 m thick were drilled to the depth of 1,600 m.

In well No. 6, in the Krasno-Kutskaya area of the Saratov oil field, drillers encountered a lost-circulation zone at a depth of 3,206 m. Additionally, below this zone, another cavern was encountered. As the driller began pumping water down the hole, a 30-m deposit of drilling cuttings began to form in the openhole section.

But by using the cavern-cementing method, the operator drilled the well to 3,590 m without losing circulation.2

Hydro-mechanical packer

In the oil fields of Tatarstan and Bashkortostan, once circulation is lost, a hydro-mechanical packer is used for hydraulic analysis, providing information as to whether the zone can be realistically treated.

First, the packer is set at the top of the lost circulation zone; then water is pumped into the zone under various flow rates under a fixed pressure.

According to these data, the absorption factor (C) recorded in cu m/hr is determined at a pressure of 1 kgf /sq cm. Typically, when C > 2-3 cu m/hr, the lost circulation is considered difficult to eliminate.

Another method used to determine the degree of lost circulation includes an analysis of the plugging ability within the zone, using a plug-back mixture. In many regions, a clay-latex mixture is used for this purpose.3

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In wells up to 2,000 m in depth, the mixture is pumped through a hydro-mechanical packer. The mixture consists of latex, which is brought into the plug-back mixture with fillers and CaCl2 additives through a special device (Fig. 1).

Coagulating the latex in the mixture makes possible the creation of a space lattice. In this way, the plugging ability of the mixture is raised. Yet it remains difficult to treat lost circulation when C > 2-3 cu m/hr at the depth of more than 3,000- 4,000 m.

Therefore, in the Saratov oil field, a different packer is used.4 This device is placed near a wellhead under the liquid level. The packer prevents dilution of the plug-back mixture between the drillstring and the lost circulation zone.

The depth (H) where the packer is set is calculated with the formula:

H = h1 + h2 (g1/g2 -1)

where:

h1 = Depth of a level in a well

h2 = Distance from the pipe end up to a lost circulation zone top

g1, g2 = Densities of the drilling mud and the plug-back mixture, accordingly.

This packer technology can be safely applied in depths of 3,000-4,000 m. For deeper wells, however, other technologies, such as expensive drilling mud, must be used.

To avoid mixing the expensive drilling mud with the cement slurry, a plug-back mixture with 10-20% filler is used to eliminate lost-circulation. In the lost-circulation zones, the efficiency of the plug-back mixtures depends on the filler concentration. The greater the filler concentration, the higher its plugging ability.

Separator

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The Russian Scientific Research Institute of Drilling Technologies (VNIIBT), Moscow, has developed a separator, which permits the withdrawal of some portion of a liquid while pumping the plug-back mixture into the well (Fig. 2). Therefore, this tool can raise the filler concentration in a plug-back mixture.

To treat lost circulation, a drillstring with an open-ended joint of pipe is lowered into the well. Then the separator (1 in Fig. 2) is connected between working pipe (2) and the drillstring (3).

The plug-back mixture with filler is then pumped into the well through the working pipe, passing through the separator in the drillstring into the lost-circulation zone.

During the pumping process, the auger (4) rotates the separator. A liquid portion from the plug-back mixture is then filtered off through the filter (5) and moves to the lateral branch (6).

Under field conditions, it has been noted that the filler concentration in the plug-back mixture of the separator increased from 20% up to 40%. The separator was applied in several oil regions as follows.

Field tests

On well No. 413 in the Orenburg oil field, lost circulation appeared at a depth of 1,653 m. A 70-cu m plug-back mixture contains drilling mud and 15% filler was unable to treat the lost-circulation zone. Installation of the separator, however, with a plug-back mixture, reduced losses twofold, by up to 2.5 cu m/hr.

In well No. 430, lost circulation of 15 cu m/hr was encountered at depth of 2,017 m. As drilling continued, the thickness of the zone increased, and mud circulation was lost completely.

Without a separator, the operator then carried out the first stage of the sealing-off operation, pumping 50 cu m of drilling mud with filler into the zone. The intensity of the losses, however, did not change.

Subsequently, the operator used the separator with 50 cu m of plug-back mixture and filler. At this point, lost circulation was reduced by 2 cu m/hr.

References

  1. Safonov, A.G., Kurochkin, B.M., and Habibullin, R.A., "Drilling of wells up to design depth with water washing in Romashkinskoe field," Journal of Drilling, No. 11, 1964.
  2. Kurochkin, B.M., Poliakob, L.P., and Bochkarev, V.F., "Cementing of zones of chip's accumulation at full lost circulation," Journal of Drilling, No. 11, 1964.
  3. Kurochkin, B.M., "Clay/latex mixture stops lost circulation in large carbonate fractures," Journal of Oil and Gas, Aug. 28, 1995.
  4. Kurochkin, B.M., Poliakob, L.P., Raev, L.M., Shishakin, B.A., and Turaev, V.V., "Technology of lost circulation elimination under equality of pressure in a system formation-well," Journal of HX, No. 12, 1980.

The Authors

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Boris M. Kurochkin is head of the laboratory on solving drilling problems as the Scientific Research Institute of Drilling Techniques (VNIIBT) in Moscow.

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Nina L. Prusova is an engineer at the Scientific Research Institute of Drilling Techniques. She holds a master of science degree.