Development and service of drilling fluid

Speech by the Deputy Director of Geosynthesis Engineering LLC Ph.D. Luban Yu.V. on the topic:




Slide 1
It is generally known that the depletion of old fields generates the need to increase drilling depths and develop hard-to-recover reserves associated with the reservoirs demonstrating a complex structure, occurring in conditions of high temperatures and abrupt fluctuations of formation pressure – from abnormally low to abnormally high. In such conditions, the role of technogenic factors in the future productivity of wells increases significantly, which requires the introduction of new “sparing” technological solutions at all stages of their construction.

Slide 2
Among other things, this is the use of oil-based drilling fluids, that ensure the maximum preservation of the natural state of the formation due to the absence of the negative influence caused by the technogenic aquatic environment. Thus, the experience of using Witer II, the invert emulsion mud, at the DDD deposits confirms its positive effect on rock stability and high efficiency during the opening of productive horizons even in the zones with incompatible mining and geological conditions, with repressions that can reach 40 – 50 MPa and higher.

Slide 3
However, the problem of high-quality isolation of productive intervals exposed by oil-based drilling fluids and preservation of their natural reservoir characteristics during casing remains unresolved. The existing technology of casing cementing, based on the utilization of traditional water-cement suspensions, does not allow to avoid the contact of productive formations with harmful water filtrate as well as the formation of superviscous non-pumpable emulsions in the pore space of reservoirs, as a result of which the effect of OBM use of can be significantly reduced or even completely lost.

Slide 4
Thus, maximum effect from the impact of the hydrocarbon environment on the formation is possible only if a single technological complex is created. It covers the processes of both drilling and casing of productive formations using drilling muds and oil-based cementing systems. These mud systems should be compatible with each other, similar in technological properties and have the same, hydrocarbon type of filtrate.

Slide 5
These requirements are met by the new IOECS-2 invert oil emulsion cement slurry system, developed by our company in cooperation with Texcor LLC, by the order of DTEK Neftegaz.
The development of IOECS-2 is based on the following theoretical provisions.

Slide 6
1. In order to ensure the necessary fluidity, traditional cement slurry contains 1.5 – 2 times more water than it is necessary for the complete hydration of the binding agent. Consequently, a cement stone can be obtained with water phase content of 20 – 25%, and the fluidity of the system is ensured by hydrocarbon liquid.
2. As a result of emulsification of aqueous suspension of a mineral binder in a hydrocarbon fluid, a stable inverted emulsion with rheological and filtration properties similar to the ones of oil-based drilling fluids can be obtained in the presence of a surfactant.
3. Stone formation occurs inside the emulsion without its destruction, phase reversal and water separation. The hydrocarbon fluid remains the external environment at all stages of cementing.
4. Low hydration rate of the binder is due to the blocking of its surface by adsorbed surfactant molecules. An increase in temperature decreases the strength of interfacial films and accelerates hydration process. Regulation of thickening and hardening processes is carried out by changing the emulsion stability.

Slide 7
The main purpose of IOECS-2 system is to stabilize and isolate deep-occurring productive intervals with high reservoir pressures and temperatures, which were penetrated with the help of OBMs. It is also possible to use during the installation of cement plugs and during cementing of sidetracks and unproductive intervals in wells filled with OBMs.

• Binding agent.
• Weighting agent.
• Inverted emulsion is the dispersion medium containing a complex of surfactant emulsifiers and water repellents of the solid phase.
Depending on the temperature and pressure conditions within the cementing interval, the aqueous phase of the emulsion may contain electrolyte agents (salts, acids or alkalis).
• In order to regulate filtration losses and the rate of binder hydration, it is possible to use substances that increase the viscosity of a dispersion hydrocarbon medium, in particular oxidized bitumen.

Slide 8
Since the setting of IOECS-2 does not occur for a long time at ambient temperature, its preparation is carried out on the surface, before the start of the cementing operation. It is possible to store ready-to-use cement slurry during several days.
Preparation technology:
• Mixing fluid – which is an inverted emulsion with a certain electrical stability index – is delivered to the rig site.
• The required amount of binder and weighting agents is introduced into the emulsion.
• If necessary, surfactants or electrolytes can be additionally introduced into the system.
• Dispersion of the components is carried out until a uniform and homogeneous liquid with desired technological properties is formed.

Slide 9 
The composition and properties of IOECS-2 significantly depend on the required density, permissible setting time and temperature at the bottom of the well. At the same time, even a slight change of the content of solid components of the system, their composition or ratio, leads to the need to adjust binder activity, change the type and amount of surfactants, and the concentration of electrolytes. As a result, the importance of laboratory verification of the formulation prior to each cementing operation is extremely important.
LABORATORY TESTING of IOECS-2 have certain methodological features and are carried out using the simulation of real thermobaric conditions.
• Thus, IOECS-2 is characterized by a higher initial consistency value than it is inherent in water-based cement slurries. However, should binder hydration be retarded, such values do not threaten the technological process, since their value remains unchanged during the entire period of cementing.
• A characteristic feature of IOECS-2 system is preservation and even a slight increase in the value of the electrical stability indicator, which is determined after conducting studies on a thermobaric consistometer.
• There should be no separation of water in the sample after the consistometer. This fact confirms the absence of emulsion destruction signs or reversal of its phases under temperature.

Slide 10
Filtration of IOECS-2 is characterized by extremely low filtration losses, comparable to the filtration of OBMs.
• filtration of IOECS-2 at the temperature of 20 ± 5 °C and a pressure differential of 0.7 MPa: not more than 6 cm3/30 min;
• HPHT filtration of IOECS-2 at reservoir temperature and pressure differential of 3.5 MPa: not more than 50 cm3/30 min;
• IOECS-2 filtrate is a hydrocarbon liquid. The presence of water in the filtrate is unacceptable. The appearance of water indicates the destruction of the emulsion and the reversal of its phases.

Slide 11
Formation of the cement stone out of IOECS-2 should be carried out in a hydrocarbon environment, under the temperature and pressure conditions corresponding to the one in an actual well. All other conditions being equal, any increase in temperature and pressure leads to the acceleration of the cement system setting and the increase in the stone strength. In any case, the temperature and pressure should not be less than 110° C and 2 MPa respectively.
At the beginning of the process of stone formation out of IOECS-2, its strength is inferior to that of the stone formed out of water-based cements. However, within a few days, these indicators level out, and over time, the strength of a stone formed out of IOECS-2 may even exceed the strength of a stone formed out of water-based cements.

Slide 12
The process of strength generation under the influence of temperature requires considerable amount of time. Observations revealed no cessation of strength growth during the period of 14 days, although in the first 3 days the growth rate is the highest.
It should also be noted that the set strength of cement formed from IOECS-2 is greatly influenced by the conditions of its formation precisely at the initial stage of the strength gain. In case of the destruction of primary crystallization structure of the stone for any reason, its strength significantly decreases. Pilot determination of the stone strength must be carried out no earlier than after 3 days of its autoclave molding, which corresponds to the minimum duration of WOC period in the wells.

Slide 13
When a cement stone formed from IOECS-2 comes into contact with water or even moisture in the air, the processes of binder hydration become accelerated, which leads to an increase of the sample strength. To avoid this phenomenon, stone samples formed from IOECS-2 should be stored in a hydrocarbon medium at normal temperature. With this method of storage, the strength of the samples does not change over time.
Gas permeability of a stone formed from IOECS-2 equals 0.03 – 0.06 mD and decreases with the increase of pressure at which this stone was formed. The strength of contact, its adhesion to steel, including in the case of its wetting with a hydrocarbon liquid, exceeds the strength of the stone itself. Adhesion strength increases over time and with the increase of temperature.
Resistance to corrosion is very high, even after 5 days of exposure to hydrochloric acid with a concentration of 15%, the stone formed from IOECS -2 retains almost half of its original strength. For comparison, a stone of similar composition, formed from a water-based cement slurry, cracks in similar conditions and completely loses its strength indicators.

Slide 14
Interaction of a cement slurry with the porous medium of reservoirs was simulated by means of filtering them through ceramic discs with a pore size of 50 microns at a temperature of 130°C and a pressure differential of 3.5 MPa. We compared filtration properties of IOECS-2 and weighted water-based cement slurry used for cementing wells in abnormally high pressure zones (density – 2100 kg/m3, w/c – 0.38, water separation – 0, spreadability – 25 cm, heat resistance – 150°C). Ceramic discs were saturated with kerosene prior to measurements. Technogenic pollution of the pore medium was simulated by means of prior filtration of oil-based drilling mud – OBM Witer II through a ceramic disk.

Slide 15
The results of the study established that filtration values of IOECS-2 and Witer II drilling mud are close in values. IOECS-2 filtrate is a homogeneous hydrocarbon liquid, which, after mixing with the drilling mud filtrate, becomes dark-colored. However, mixing the filtrate does not lead to the formation of clots or sludge, which can negatively affect the reservoir.
A different situation takes place during filtration of an water-based cement slurry. Even with the use of filtration-limiting agents, its value is more than three times higher than IOECS-2 filtration. The filtrate is represented by the aqueous phase of the cement slurry. As expected, mixing of the hydrocarbon and aqueous filtrates results in a stable and highly viscous emulsion that adheres well to the glass of the laboratory cylinder. This indirectly witnesses of the high strength of contact of such emulsion with the formation, the creation of conditions for blocking the pore space and reducing reservoir permeability.

Slide 16
The tests that were carried out made it possible to formulate basic technical requirements for oil-based cement slurries and to the conditions of their laboratory testing, which makes it possible to ensure reliable isolation and preserve productive characteristics of reservoirs, namely:
• electrical stability of cement slurry after preparation – not less than 150 V;
• cement slurry filtration at the temperature of 20 ± 5 °С and pressure differential of 0,7 МPа: not more than 6 сm3/30 min;
• HPHT filtration of cement slurry at formation temperature and pressure differential of 3.5 MPa: not more than 50 cm3/30 min;
• type of filtrate: hydrocarbon liquid, no water detected;
• electrical stability of the cement slurry filtrate: not less than 1000 V;
• electrical stability of the cement slurry after consistometry under thermobaric conditions: not less than 150 V;
• bending strength of a 3-day-old cement stone: not less than 1.5 MPa (hardening at a static bottomhole temperature and pressure, but not less than 110 °C and 2 MPa);
• set strength of a 3-day cement stone: not less than 5 MPa (hardening at a static bottomhole temperature and pressure, but not less than 110 °C and 2 MPa).
• cement slurry density, beginning and end of setting: determined by the cementing program, kg/m3;
Industrial use of hydrocarbon-based cement slurry is possible only under the circumstances that its characteristics comply with the abovementioned technical requirements.

One of the wells operated by DTEK Naftogaz, being drilled using Witer II invert emulsion drilling mud, in the 5238 – 5472 m interval, at the temperature of 147 °C, a cement bridge was successfully performed to plug the water-saturated formation.

Slide 18
According to the currently available data, as of today IOECS-2 is an unparalleled cementing system capable of creating a cement stone without phase inversion of the emulsion and, at the same time, corresponding to the thermobaric conditions of Ukraine.
Further implementation of the IOECS-2 system for cementing casing strings, which has already been planned at several DTEK Neftegaz wells, will ensure reliable bridging and preservation of the productive characteristics of reservoirs exposed by hydrocarbon-based drilling fluids, which, in turn, creates a solid foundation for increasing drilling depths when searching for oil and gas deposits.