2021 is coming to its end, and, despite the pandemic, internal and external problems the country has faced, it has turned out to be very productive for us – both from the point of view of the number of wells, and due to the implementation of the new technological solutions.
We have performed at 17 wells during this year, 9 of which were drilling, 4 workover wells and 4 offshoots. It was completed for the fields of DTEK Oil&Gas, Smart Energy and Ukrnefteinvest LLC.
This year’s history starts with the drilling of Well 79 of Semyrenky gas-condensate field, however it has been with us since last year. We do not usually speak about such carry-over wells as we tend to attribute them to the previous year. However, this well influenced so much the entire technology of drilling in Ukraine that we will have to make an exception.
Drilling started with the attempt to make changes in the well design and to stich together the sediments of Triassic colloidal clays and detrital mudstones of the Carboniferous in the interval where aqueous highly inhibited drilling mud was used. The idea was far from being successful. Yet another time it proved the incompatibility of such types of rock and the fact that different approaches are required to ensure their stability.
Today, thanks to the experience gained at Well 79 of Semyrenky gas-condensate field, it became obvious that colloidal clays of the Jurassic and Triassic, within a short period of time after opening, must be secured with a casing. Otherwise, with prolonged contact with a water-based mud, complications in clays are inevitable, due to their hydration and swelling.
At the same time, inhibition of the drilling fluid with salts does not improve the situation. More often, on the contrary, complications increase. Clays still lose stability, but their behavior changes from plastic flowing to brittle fracture. The crumbling of embrittled clays is very intense and, in the most complicated cases, resembles an avalanche. It is extremely difficult to stop the development of such processes using traditional methods.
The solution to this problem lies in separating clays and mudstones with a casing, or by switching to hydrocarbon drilling fluids, which exclude contact of clays with aqueous medium. Well 79 of Semyrenky gas-condensate field became the first well at DTEK’s fields, where our IEDM Witer II, oil-based system was used to open non-productive Carboniferous deposits. The results have surpassed all technological and geological expectations and became the starting point for a radical expansion of the boundaries for the application of oil-based drilling fluids. (Drilling Results: Well 79 Semyrenky Gas-Condensate Field).
Today, we start working on the IEDM Witer II system directly from under the surface casing, we open Jurassic and Triassic clays, Carboniferous mudstones and finish after reaching the design depth. (Utilization of the OBM Witer II for drilling colloidal mesozoic clays). In other words, the use of hydrocarbon drilling mud allowed us to combine incompatible intervals, abandon one technical casing string, increase the drilling speed and, in general, reduce the cost of wells.
THE WIDESPREAD USE OF OBM
Last year, at the fields operated by DTEK Oil&Gas, we worked on the implementation of a single technological complex for drilling, casing and well completion using oil-based systems, which allows us to achieve full compatibility of process fluids at all stages of well construction.
An important element of such a technological complex is IOECS-2 system – invert oil emulsion cement slurry developed by us. This bridging system ensures high quality of casing for wells drilled using hydrocarbon drilling fluids, and eliminates the contamination of productive formations due to water filtrate influence. This is confirmed by the gas inflow with a record condensate ratio upon perforation of the interval cemented using IOECS-2.
The widespread use of the IEDM Witer II has demonstrated that wellbore stability can be achieved at a significantly lower density than if water-based drilling fluids are used. For the conditions of Ukraine, where a significant part of the fields is at the final stage of operation, the transition to low-density hydrocarbon systems, without regard to the factor of rocks stability, is a real chance to ensure high productivity of wells. A striking example of this is DTEK Oil&Gas, where the use of the IEDM Witer II system makes it possible to put into operation reservoirs with abnormally low reservoir pressures and very low filtration characteristics, which would be impossible under alternative conditions of primary opening.(V Kiev Conference. Speech by Ph.D. I.P. Gafich, the Director of Exploration and Prospective Development of DTEK OIL&GAS.).
This conclusion is based on the results of the study on the effect that various drilling fluids systems produce on the reservoir properties of productive formations, obtained by us under real temperature and pressure conditions, using a unique machine for studying core samples. This unit was designed to meet our requirements by the American enterprise DCI Test Systems and is completely unique in terms of its characteristics (1000 atm and 200 °C).(Equipment for core material research). For the study, we are able to simulate real pressure drops that occur at opening the formations with abnormally low formation pressures at a depth of 5000 – 6000 m.
In the past year, several oil and gas producing companies started new fields. Our flushing systems were first used at Kovalevsko-Sulimovskaya (DTEK Oil&Gas), Svistunkovsko-Chervonolutskaya (Smart Energy) and Sergeevskaya area (Ukrnefteinvest LLC).
Sergeevskaya Square occupies a special place in this list. Geographically, it is part of the old Saratsk oil field, located in the Preddobrudzha trough, on the border with Romania, and discovered in the 70s of the previous century by Krymgeologiya association of the Mingeo of the Ukrainian SSR. The field has been plugged and abandoned for a long time. Over the years, attempts to restore it have been made several times, but they always ended up in failure. There were serious concerns that there was no industrial production at the field.
In our opinion, those failures were caused by the wrong choice of traditional polymer drilling fluids for penetrating productive deposits. Reservoirs, represented by fractured, sulfate-carbonate rock strata, sharply lost their permeability as a result of contact with the water phase of drilling fluids.
When we were approached by Ukrnefteinvest LLC – the new owners of the license – we immediately proposed to use the IEDM Witer II system, which we provided on a lease basis. The result was received immediately. Water contact elimination made it possible to preserve the natural state of the reservoirs and provide optimal conditions for a stable industrial inflow. Perhaps we are witnessing the beginning of a new oil and gas region development.
Ukraine has a huge number of deep abandoned wells completed with Ø139.7 mm casing. Recovery of such wells by sidetracking is extremely difficult due to high hydraulic resistances during flushing. The problems increase with weighted muds.
During the joint project held with the Ukrainian-Belarusian enterprise Service Oil, we had a task to ensure the technical feasibility of planned hydraulic flushing program, to maintain rock stability and high-quality opening for sidetrack drilling from Ø139.7 mm casing string (sidetracking depth – 5600 m, target depth – 6300 m). The density of the drilling mud was expected to increase gradually – from 1370 to 1520 kg/m3. For such conditions, we proposed a system of biopolymer drilling mud Biokar-TF, weighted with a complex of highly soluble salts and free of barite. (Drilling of a Sidetrack).
Biocar-TF made it possible to successfully drill to design depth. Rock resistance was high. Bore diameter was nominal. The supply pressure of 8 – 9 l/s was 250 – 280 atm. Drilling speed 1.5 – 2 m/h. Drilling was completed within two months. The experience of using Biocar-TF system has proven its technical feasibility of restoring deep wells from small-diameter casing strings.
The unique technical characteristics and environmental safety of the Biocar-TF system makes it a real alternative to weighted hydrocarbon-based systems. Today, DTEK Oil&Gas is considering the utilization of Biocar-TF in the coming year for productive horizons in the density range of 1300 – 1700 kg/m3.
‘Don’t let yourself the thought that God will show us disfavour. Your life is gravely short for a defeat.’
Lina Kostenko
Happy New Year! May this year bring you peace, good luck and prosperity!
