The need to create cement slurry with hydrocarbon-type filtrate arose as a part of the concept of ultimate preservation of the natural state of rock by means of preventing its contact with anthropogenic aquatic environment. In practice, this concept was partially implemented during the introduction of oil-based drilling fluids. However, under such conditions, the use of conventional cement slurry demonstrating tremendous filtration and aqueous filtrate, seemed illogical and too harmful. The problem of well drilling and cementing of wells using oil-based fluids required a comprehensive solution.
Numerous attempts to develop oil-based cement slurry, which does not wet the rock and creates a hydrophobic stone during hardening, were unsuccessful either due to the use of oil-cement emulsions of oil-in-water, or due to the circulation of inverted emulsions in the process of hardening. Since water in such case is the external phase of the emulsion, it was impossible to avoid its penetration into the reservoir.
The task was to develop the formulation of a cement slurry, which is inverted emulsion whose hardening occurs without phase invertion. That is, the hydrocarbon fluid shall remain the external medium at all stages of cementing. For the first time, such formulation of an inverted oil-emulsion cement slurry (IOECS) was developed by V. Luban. and E. Ogolikhin under the supervision of Professor L. Mukhin at Moscow Institute of Petrochemical and Gas Industry named after Ivan Gubkin in the 1970s. The implementation of the IOECS system was carried out in the fields of the North Caucasus and Ukraine, in particular in the Sagaidak, Brateshkovska and Radchenkovska areas.
Today, due to the unavailability of the key components required for IOECS preparation, it is impossible to recover its formula. Even in Russia, where IOECS was once developed, there exists no analogue of it. Existing oil-based cementing compositions are characterized by thermal stability not exceeding 100 - 120 °C and they form a stone either due to the destruction of the invert emulsion, or as a result of the contact with formation water. Accordingly, such systems do not satisfy the geological conditions of Ukraine, and the technology of their use, based on phase inversion, leads to the change of the filtrate type and the inevitable contact of water with the hydrocarbon medium both in a well and in the reservoir.
More than two years ago, we started our movement in the direction of developing a new oil-emulsion cement slurry system. The task of creating such a formulation was extremely difficult. Sometimes it was thought to be impossible at all. The only thing that supported us during such hard times and did not allow us to abandon this idea was the understanding that IOECS system really existed. Therefore, when the recipe was finally developed, it was called IOECS-2, as a tribute to those who were the first to do it.
Thus, after more than 40 years in Ukraine, the technology of using oil-based cement slurry has been restored. Performance tests were successfully carried out while installing a cement plug at Well 55 of Machukhy gas condensate field in the interval between 5238 – 5472 m, at a temperature of 147 °C. 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 oil-based drilling fluids.
The creation of a single drilling and cementing complex, which would use oiln-based drilling and cement slurry systems similar in nature and properties, establishes a solid foundation for extended capabilities regarding the search for oil and gas deposits at great depths.
TECHNICAL REQUIREMENTS TOWARDS THE CEMENT SLURRY AND STONE
In order to ensure reliable isolation and preserve productive characteristics of reservoirs, oil-based cement slurry and the stone obtained from it must meet the following technical requirements:
TECHNICAL REQUIREMENTS TOWARDS THE CEMENT SLURRY AND STONE
In order to ensure reliable isolation and preserve productive characteristics of reservoirs, oil-based cement slurry and the stone obtained from it must meet the following technical requirements:
- cement slurry density: determined by the cementing program, kg/m3;
- electrical stability of cement slurry after preparation: not less than 150 V;
- filtration of cement slurry at the temperature of 20 ± 5 °C and a pressure drop of 0.7 MPa: not more than 6 cm3/30 min;
- HPHT filtration of cement slurry at reservoir temperature and pressure drop 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 1500 V;
- initial setting: determined by the cementing program, hour-min;
- final setting: determined by the cementing program, hour-min;
- 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).
Industrial use of oil-based cement slurry is possible only under the circumstances that its characteristics comply with the abovementioned technical requirements. DEVIATIONS FROM THE SPECIFIED TECHNICAL REQUIREMENTS ARE UNACCEPTABLE.
