Development and service of drilling fluid

The well that has been spud today is unique for two reasons.
Firstly, this is the jubilee 100th well in our chronological list.
Secondly, at this well, we are planning to revive the technology of penetrating Jurassic and Triassic colloidal clays with OBM.
The fact that these two events coincided at the same well appears to be deeply symbolic to us.
For the first time in Ukraine, Mesozoic clays were opened using OBM by our friend and teacher A.G. Rozengaft in the early 70s of the previous century while drilling Well#35 in Mashevskaya area, Poltavaneftegazrazvedka Trust (thesis of  A.G. Rozengaft). The results of OBM testing, achieved in those distant times, may cause a condescending smile today, but in fact, it was a real revolutionary breakthrough. 

А.Г. Розенгафт на головну
A.G. Rozengaft

Here is the story.
The OBM was used in the clay-sandy rock interval of the Jurassic and Triassic periods: 794-1750 m. There was no special emulsion preparation unit. Mud preparation was performed through the well. The wellbore was filled with oil, where then the calculated amount of oxidized petrolatum was dissolved. During circulation, water and lime were introduced into the hydrocarbon liquid. Thus, about 170 m3 of OBM were prepared with a 50% water phase content (in the Soviet times, options for its reduction were not even considered).
OBM parameters: density – 0.94 – 0.96 g/cm3; funnel viscosity – 100 – 110 s; gel strength – 0/4 dPа; filtration loss – 1 cm3/30 min; electrical stability – 250 V.
As long as mud cleaning system did not actually exist in those times, in the process of drilling, OBM was enriched with 22% of finely dispersed clay cuttings. However, the emulsion retained its aggregate stability. The bit and BHA were free of oil seals, which was incredible for this section, whereas bit operating conditions and drilling performance improved significantly. As a result, the bit ammount decreased from 36 to 16; footage per bit increased from 24 to 59.7 m; and the mechanical speed increased from 3.3 to 6.9 m/h. 
Here is the story. The economic benefit from the use of OBM was 5 Soviet rubles per 1 drilled meter. And all this for the entire interval of 956 m!

Today, these intervals are drilled in one go, with daily penetration from 300 to 500 m. It may seem that the technology of fast opening of Mesozoic colloidal clays using muds with low solids content has been fully mastered, yet complications still arise from time to time.
However, the main issue is that, after drilling, these intervals must be secured with casing. Otherwise, in case of a prolonged contact with a water-based mud, complications in clays are inevitable due to their swelling.
Inhibition of drilling mud with salts does not improve the situation. More often, on the contrary, complications increase. Clays keep losing stability, however their behavior changes from plastic flow to brittle fracture.

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In his work, A.G. Rosengaft showed that the inertness of the OBM hydrocarbon phase with respect to clays helps to avoid the loss of their stability due to hydration. Based on this experience, as well as taking into account  successful application of the OBM Witer II when drilling nonproductive carboniferous deposits at Well#79 of the Semirenky gas condensate field, we are proposing a fundamentally new concept that suggests the use of OB fluids for drilling the entire wellbore, with the exception of fresh-water muds intervals of the Lower Cretaceous, overlapped by an elongated surfase casing.
As a result of such use of OBMs, it becomes possible to jointly penetrate previously incompatible intervals of Mesozoic colloidal clays and brittle Carboniferous mudstones.  This approach, at a minimum, allows to reduce the drilling diameter and save on one string. Now then, as a maximum, it opens up prospects for drilling wells with the depth of 7 - 8 thousand meters.
This idea was embodied in a project for drilling another well for DTEK Oil and Gas.

Surface casing Ø339,7 mm – 0 – 630 m; 
Production casing Ø244,5 mm – 0 – 3200 m; 
Liner Ø177,8 mm – 2900 – 5130 m;
Open hole – 5130 – 5300 m.

The elongated surface casing is run into the Jurassic clay roof and seals the Lower Cretaceous aquifers. Jurassic, Triassic, Permian and nonproductive Carboniferous intervals are combined and secured by a single casing Ø245 mm.

Surface casing Ø339,7 mm – fresh-clay-drilling fluid; 
Production casing Ø244,5 mm – OBM «Witer-II»; 
Liner Ø177,8 mm – OBM «Witer-II»;
Open hole (5130-5300 м) – OBM «Witer-II».

The surface casing is drilled using an environmentally friendly starch-based mud. After running the surface casing, the mud is switched into the OBM Witer II, which is used until the end of drilling. Mud density changes as the well deepens, with the adding of weighting agents.
We count on maintaining the stability of rocks, high drilling rates and effective penetration of productive zones.
The results of drilling will be announced later.
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