Challenge
An increase in exploratory drilling depths and the need to develop the potential of hard-to-recover reserves of existing fields require the introduction of new technological approaches to the problem of opening, securing and operating reservoirs with low filtration-capacity properties occurring in conditions of high temperatures and sharp fluctuations in reservoir pressures - from abnormally low to abnormally high. Among other things, this is the use of oil-based drilling fluids to ensure the maximum preservation of the natural state of rocks due to the lack of the negative impact of the aquatic medium. In particular, the experience of long-term use of the Witer II reversed emulsion solution (IES) system at the DDB deep drilling sites confirms its positive effect on the stability of rocks and high efficiency when opening productive layers even in zones with incompatible mining and geological conditions, under repressions that can reach 40 - 50 MPa and above.
However, the problem of high-quality isolation of productive intervals opened with oil-based drilling fluids and preservation of their natural reservoir characteristics at the stage of casing requires more advanced and effective solutions. The existing casing cementing technology based on the use of traditional water-cement suspensions makes it impossible to avoid contact of the hydrocarbon filtrate of the drilling fluid with the harmful aqueous filtrate in the wellbore zone of the reservoir, resulting in reduction or losing the effect of the use of oil-based drilling fluids. This negative effect of the aqueous filtrate of the cement mortar mostly appears when casing tightened, secondary and porous fractured reservoirs.
Thus, a high-quality opening of productive horizons is possible only if a technological complex is created that covers the processes of both drilling and casing productive reservoirs, using a single hydrocarbon base for drilling and cement systems.
Statement of the problem
The idea of developing a system of cement mortar, which is an emulsion of the second grade, hardening of which occurs without phase reversal, was first formed at the MINH and I.M. Gubkin SE in the 70s of the last century. Its practical embodiment was the formulation of the reversed oil-based emulsion cement mortar (ONECR) obtained by Luban V.Z. and Ogolikhin E.O. under the guidance of Prof. Mukhin L.K [1].
ONECR has been successfully tested in complex geological conditions of Ukraine and the North Caucasus, including in abnormally high pressure zones [2]. However, from the late 1980s to the early 2000s, when the use of oil-based drilling fluids almost completely ceased, these technologies were lost.
The task was to develop a cement mortar formulation, the external medium of which at all stages of cementing was a hydrocarbon liquid.
The ONECR technology shall ensure the aggregate stability of the system based on emulsifying aqueous suspension of cementing material in a hydrocarbon medium in the presence of surfactant emulsifiers and obtaining cement mortar with rheological and filtration properties similar to oil-based drilling fluids.
It was also necessary to seek high adhesion and cohesion to metal and rock, including when wetting their surface with a hydrocarbon liquid.
Current state of the issue
Today, due to the renewed interest in oil-based drilling fluids, the problem of ensuring reliable casing of productive intervals with maximum possible preservation of the natural properties of the reservoir is again urgent. Two opposite approaches to its solution have become widespread in Ukraine.
The first, which is offered by most specialized service companies, is the use of water-based cement mortar together with several special buffer fluids that are sequentially injected into the well. The purpose of such buffer fluids is to replace oil-based drilling fluid and wash the annular space behind the casing.
Another approach is to completely abandon the cementing of productive intervals opened with oil-based fluids and their overlap with filters (or liners with subsequent perforation) equipped with a set of water- and oil-swellable packers to isolate the annulus, separate aquifers or production facilities.
The first approach has significant restrictions, first of all, under casing of low-permeability reservoirs and productive horizons with abnormally high pressures. Among other things, this is the possibility of solid phase precipitation of drilling and cement mortars when they come into contact with buffer fluids and the formation of plugs; poor quality of displacement of hydrocarbons out of the annulus of the well, especially in the presence of caverns; possible decrease in hydrostatic pressure when pumping a large volume of buffer fluids, etc.
But the main problem lies in the water-based grouting mortar. Its aqueous filtrate, when mixed with hydrocarbon drilling mud filtrate, will form extremely viscous emulsions in the compacted pore space of reservoirs, which can significantly reduce their permeability (Fig. 1).
Figure 1 – Formation of extremely viscous emulsions when layer water enters a well filled with oil-based solution
The use of swellable packer systems to overlap and isolate productive intervals opened by oil-based drilling fluids completely avoids the problems associated with the negative impact of water-based cement mortars. The introduction of this technology at DTEK Oil&Gas’s fields allowed one to increase the productivity of wells and obtain a significant increase in reserves.
Unfortunately, the technology of casing wells with packer systems without cementing also has its limits of application. First of all, we are talking about the situation that has developed at many Ukrainian fields, when, in order to achieve deep promising horizons with abnormally high pressure, it is necessary to drill the main production facilities with reservoir pressures below hydrostatic. At the same time, the technology of opening and casing reservoirs with abnormally low pressure should ensure the preservation of their reservoir properties in conditions of ultra-high repression and the possibility of further commercial exploitation.
Also, the use of packer systems is risky and ineffective when separating productive layers and aquifers in the conditions of the GWC\OWC.
Development of oil-based cement mortar formulation
The task was solved within the framework of the creation of the integrated technology for the development of hard-to-recover reserves of pressed rocks of DTEK Oil&Gas's facilities. Scientists and specialists of Geosintez Engineering LLC, Tekhkor LLC and DTEK Oil&Gas took part in its solution based on the Long-Term Cooperation Program.
More than two years ago, research was started on the development of a new reversed oil emulsion cement mortar system. An activated slag-sand mixture is used as the ONECR main cementing material. Barite or hematite is used for weighting. The dispersion medium is diesel fuel-based emulsion, containing a complex of surfactants - emulsifiers and water repellents of the solid phase. Depending on pressure-and-temperature conditions in the cementing interval, the aqueous phase of the emulsion may contain electrolyte reagents (salts, acids or alkalis). To regulate filtration losses and the rate of hydration of the cementing material, substances are used that increase the viscosity of the dispersion hydrocarbon medium, in particular, oxidized bitumen. Considering the first experience of the ONECR compounding in the 70s, the system was named ONECR-2.
The density of ONECR-2, its consistency, filtration, indicators of electrical and sedimentation stability, the beginning and end of setting can be adjusted within wide limits while being compounded.
Technological properties of ONECR-2 depend on many factors to large extent. Even a slight change in the concentration of solid components of the system, their composition or ratio results in the need to adjust the activity of the cementing material, change the type and amount of surfactants, the concentration of electrolytes, etc. That is, the composition and properties of the system change directionally depending on its density, permissible setting time and temperature at the bottom of the well.
ONECR-2 is characterized by a higher initial consistency value than usually water-based cement mortars have. However, due to the delayed nature of the hydration of the cementing material, such values are completely safe for the technological process, since their value remains unchanged during the entire period of cementing. In this case, the consistency value is an indirect rheological characteristic of the grouting system, while the lack of its growth shows that the intensity of the thickening process meets the temperature and pressure conditions of the well and the estimated time of the cementing process (Fig. 2).
Figure 2 – Results of the ONECR-2 consistometry at temperatures up to 150°С
The differential characteristic of the ONECR-2 system is the preservation, and even a slight increase, of the value of the electrical stability indicator, which is determined based on the results of testing using a thermobaric consistometer. This fact confirms the lack of signs of the destruction of the emulsion or the reversal of its phases under conditions of temperature exposure at all stages of the process of thickening and stone formation. Thus, direct contact of the water phase of the grouting system with rocks and hydrocarbon drilling fluid is completely excluded.
At the beginning of the process of forming a stone made from ONECR-2, its strength abates that of a stone made from water-based cements. However, these indicators level out within a few days, and over time, the strength of a stone made from ONECR-2 can even exceed the strength of a stone from water-based cements. The process of gaining strength under conditions of temperature exposure takes a long time, observations have not established the stop of its growth within 14 days, although in the first 3 days the intensity of strength growth is the highest. It should also be noted that the strength of a stone made from the ONECR-2 is greatly affected by the conditions of its formation at the initial stage of strength gain. In case of destruction, for any reason, the primary crystallization structure of a stone, its strength significantly decreases. Accordingly, the final determination of the strength of a stone is to be carried out no earlier than after 3 days of its autoclave formation, which corresponds to the minimum duration of the WPC time (Fig. 3).
Figure 3 – Dependence of the strength of a stone made from ONECR-2 on the time and temperature of formation: a – stone formation temperature is 150°С, pressure is 2 MPa; b – stone formation time is 3 days, pressure is 2 MPa
Gas permeability made from the ONECR-2 stone is at the level of 0.03 - 0.06 mD and decreases with increasing pressure at which the stone was formed. The strength of the contact of its adhesion to steel, including in the case of its wetting with a hydrocarbon liquid, is high and exceeds the strength of the stone. The strength of adhesion increases with time and temperature rise.
Corrosion resistance is high, even after 5 days of stay in hydrochloric acid with a concentration of 15%, the stone made from the ONECR-2 retains almost half of its original strength. For comparison, a stone of a similar composition formed from a water-based cement mortar cracks under such conditions and completely loses its strength.
The interaction of grouting mortars with the porous medium of reservoir formations was modeled by filtering them through ceramic disks with a hole size of 50 μm at a temperature of 130°C and a pressure drop of 3.5 MPa. The filtration properties of the ONECR-2 and a weighted water-based grouting mortar were compared, which is used for cementing wells in abnormal high pressure zones (density - 2100 kg/m3, w/c - 0.38, water trapping - 0, spreadability - 25 cm, heat resistance - 150°C). Prior to measurements, ceramic discs were saturated with kerosene. Technogenic pollution of the porous medium was simulated by preliminary filtration through oil-based drilling mud ceramic disk – ІЕР Witer II.
According to the research results given in Table 1, it was found that the values of the filtration indicators of the ONECR-2 and Witer II drilling mud are close in their values. ONECR-2 filtrate is a homogeneous hydrocarbon fluid. At that, mixing of the filtrates does not result in the formation of clots or sediment, which can negatively affect the collector (Fig. 4, experiments 1 and 2).
A different situation occurs when filtering water-based cement mortar. Even with the use of filtration limiting reagents, its value is more than three times higher than the filtration of the ONECR-2. The filtrate is represented by an aqueous medium of cement mortar. As expected, mixing the hydrocarbon and aqueous filtrates results in a stable and highly viscous emulsion, which is characterized by high adhesion to the glass of the laboratory cylinder. This indirectly evidences the high strength of the contact of the emulsion with rocks, the creation of conditions for blocking the porous space and reduction of the permeability of the reservoirs (Fig. 4, experiments 3 and 4).
Table 1 – Filtration of hydrocarbon-based and water-based process fluids through ceramic discs saturated with kerosene
Figure 4 – Oil-based (ONECR-2) and water-based filtrates of cement mortars after filtration through ceramic disks saturated with kerosene and hydrocarbon filtrate of the Witer II drilling mud:
1 – ONECR-2 filtrate;
2 – mixture of the ONECR-2 and Witer II filtrates,
3 – water-based cement mortar filtrate,
4 – mixture of water-based cement mortar and Witer II filtrates
The studies made it possible to formulate the basic technical requirements for oil-based grouting mortars and the conditions for their laboratory studies that ensure reliable isolation and preservation of the productive characteristics of reservoirs.
Commercial tests
The ONECR-2 system was first tested in production conditions at the well No. 55 of the Machukhske gas condensate field. The well was drilled in the productive interval of 5,161 – 5,472 m using oil-based drilling fluid IEP Witer II. According to the project, the temperature at a depth of 5,472 m is 147°C, the pressure is 109 MPa. According to logging data, an aquifer that must be isolated prior to the well development was found in the lower part of the drilled interval.
Since the use of water-based cement could lead to contamination of fractured and compacted carbonate reservoirs overlying the aquifer, it was decided to use ONECR-2. Along with solving the main problem of isolating the aquifer in the interval of 5,220-5,472 m, the tasks were set to work out the industrial technology for compounding and injecting cement mortar; studying the processes that occur during the contact of drilling and cement mortars; determining the insulating properties of the ONECR-2 stone and its strength.
Grouting fluid was separated from the IEP Witer II with two packs of buffer fluid, which was supposed to completely replace the drilling fluid at the bottom hole. The buffer fluid was similar in composition to the ONECR-2 grouting fluid, which excluded its negative effect on cement thickening and stone formation. Considering the actual time spent on cementing, the ONECR-2 formulation was adjusted towards accelerating thickening and setting.
During the well test, the inflow of formation water from the interval overlapped was not observed. The cement plug is found sealed.
The commercial introduction of the ONECR-2 system made it possible to develop a reliable technology for isolating and maintaining the productive characteristics of reservoirs opened with oil-based drilling fluids, which in turn creates the basis for increasing the drilling depth and obtaining commercial inflows from oil and gas deposits at great depth.
Findings
The use of hydrocarbon drilling fluids when opening deep-lying productive horizons with a complex type of porous space, often compacted, requires the creation of a single complex of drilling and casing in order to avoid a collision of mixing various types of filtrates in the near-wellbore space and worsening of its filtration properties, which provides for the use of similar in nature and properties of systems of drilling and grouting mortars.
Laboratory research and industrial tests have proven that Ukraine has revived the technology of well casing with reversed oil-based emulsion cement mortar, which, together with oil-based drilling fluid, contributes to the preservation of reservoir productivity. A new formulation of the ONECR-2 cement mortar, a method for adjusting its properties and laboratory tests and a technology for compounding it in an industrial environment were developed. The technology undergoes production improvement and replication at the facilities of DTEK Oil&Gas.
Literature
1. A.S. 502111 USSR, M.Kl. 2 Е 21В 33/138, В 28С 5/00. Method for compounding of reversed oil-based emulsion cement mortar / Ogolikhin E.A., Mukhin L.K., Luban V.Z. (USSR). - No. 1795191 / 22-3; declared 06/09/72; published 02/05/76, Bul. No. 5.
2. Using of reversed oil-based emulsion cement mortar for cementing wells drilled with lime-bitumen mud / [Ogolikhin E.A., Mukhin L.K., Luban V.Z. et al.] // Drilling: RNTS / VNIIOENG. - M., 1974. - No. 2. - P. 22 - 24.
