TECHNOLOGY Clay/latex mixture stops lost circulation in large carbonate fractures

Boris Kurochkin
Scientific Research Institute of Drilling Techniques
Moscow

A mixture of latex and drilling mud clay controls lost circulation in large carbonate fractures by coagulating when it mixes with the formation fluids.

The mixture forms large aggregates capable of plugging large formation fractures and is not prone to being washed out by formation water.

In the Ural-Povolzhie oil regions in Russia, drilling mud lost circulation occurs in several horizons located at different depths. The lost circulation zones at depths to 300-400 m are usually cased. The next interval is typically drilled with a 215.9-mm bit, and 146-mm production casing is then run.

Below the surface casing, the lost circulation zones are met at depths of 900 m, 1,500 m, 2,200 m, and more than 3,000-4,000 m.

These intervals, which contain stable carbonate rock, are mostly drilled with water circulation. Drilling then continues with mud circulation.

As a typical well is drilled deeper with water circulation, several lost circulation zones are encountered in different horizons with different formation pressures. The formation fluids from these zones are distinguishable by density and mineral composition.

Drilling through different lost circulation horizons without isolating them results in formation fluid cross flow between beds along the borehole. The formation fluid cross flow adversely impairs the efficiency of isolation operations because the plugging back mixtures become diluted and do not plug the fractures in the carbonate rocks. Lost circulation in large fractures, or karst cavities, in carbonate rocks is the most complicated to isolate.

Clay/latex mixture

The Scientific Research Institute of Drilling Techniques in Moscow (Vniibt) has developed a special lost circulation mixture combining a latex base and clay drilling mud for use in plugging these large fractures. Mud powders of bentonite and calcium clays can be used to prepare the clay-latex mixture (GPC).

The latex is mixed with drilling mud made from bentonite clay powder and is pumped to the lost circulation zone where it coagulates by reacting with the calcium ions in the formation water.

In mixing the latex with drilling mud made from calcium clays, the components are pumped through a T-joint on the wellhead. Because of the calcium ions present in the drilling mud, the mixture coagulates at once and is pumped downhole in that state to the lost circulation zone.

The latex used to prepare GPC must be compatible with the mud being used. After coagulation, GPC changes to a rubber mixture.

In the Tataria oil region, one of the drilling enterprises used GPC in 23 wells, with positive results in 20 of the wells.1

The bentonite powder base GPC mixture remains easily pumpable until the end of the operation. The mixture's plastic strength increases over time as it is pumped into the lost circulation zone. This type of mixture is typically used to control lost circulation at depths greater than 1,000-1,500 m.

GPC based on drilling mud made from calcium clay is used at shallower depths. For GPC pumped into a borehole, the latex/clay drilling mud ratio is typically between 1:1 and 1:2.

Hydrodynamic tests

GPC has several advantages compared to conventional cement mixtures used for lost circulation control. The GPC mixture forms rather large aggregates capable of plugging large fractures in the carbonate formations. The mixture remains in the fractures and is not easily washed out by formation fluids.

Fig. 1 (30610 bytes) shows some example data on short-time hydrodynamic investigations in wells in the Romashkinskoe field in Tatarstan, before and after isolation operations to stop lost circulation.1 The "taking-up capability" of lost circulation zones is one of the basic criteria for estimating the problem of isolating the zone.

The short-time hydrodynamic investigations were carried out by pumping fluid into the lost circulation zone at various pump capacities. The pumping occurs before constant pressure is established. Thus, a positive pressure is registered at the wellhead. The short-time hydrodynamic investigation data were plotted on a graph of wellhead positive pressure (P) against pump capacity (Q).

Based on data taken from numerous Tatarstan oil wells with lost circulation problems, three types of lost circulation zones were identified and are used to chose the isolation method:

The area in which the hydrodynamic curve falls determines the isolation method for the lost circulation zone isolation and the volume and type of plug-back mixture to be used. As shown in the example wells (No. 16547 and No. 14091) in Fig. 1 (30610 bytes), after the isolation operations with the GPC mixture, the taking up capability of the lost circulation zones sharply decreased. These zones then did not present further difficulties for full isolation by means of pumping several batches of drilling mud with filling agents.

Procedure

The isolation operation for wells with lost circulation problems typically uses the following plan: An hydraulic-mechanical packer is run on drill pipe into the well. The packer is then set about 50 m above the top of the lost circulation zone. The plug-back mixture is then pumped into the borehole through the drill pipe and packer and forced into the lost circulation zone. The packer is then released, and the drillstring is pulled out of the hole.

For isolation operations with GPC, a combination of plug-back materials is pumped in series. First, a batch of about 20-30 cu m of clay drilling mud with filling agents is pumped. Then, about 10-16 cu m of GPC and about 6-8 cu m of cement slurry are pumped.

The drilling mud with fillers plugs small and medium size fractures. The GPC plugs the large fractures. The cement slurry forces the GPC deep into the lost circulation formation and plugs the near-borehole part of the formation.

In some of the most complicated cases of lost circulation, gel cement is pumped following the drilling mud with fillers, and then GPC is pumped.

Because the standard latex is unstable with temperatures below zero, a special frost-resistant latex composition was developed. This material has been used successfully in winter conditions.2

GPC has been used successfully to stop lost circulation problems in many oil regions of the former Soviet Union.

References

1.Kurochkin, B.M., Gorbunova, I.V., Alekseev, M.V., Bikchurin, T.N., and Chirskov, G.V., "The experience of GPC application for the drilling fluid lost circulation control," PNTC Burenie, first edition, Vniioeng, Moscow, 1982, pp. 10-13.

2.Kurochkin, B.M., Faktulin, R.KH., Bikchurin, T.N., and Gabidullin, R.S., "Lost circulation control with frost-resistant latex composition," Neftyanoe Khozyaistvo, 1991, No. 1, pp. 14-15.

The Author