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Nikolay I. Nikolaev, Liu Haoya
Results of Cement-to-Rock Contact Study
Oil and Gas
UDC 622.244; 622.245
RESULTS OF CEMENT-TO-ROCK CONTACT STUDY
Nikolay I NIKOLAEV1, Liu HAOYA2
1 Saint-Petersburg Mining University, Saint-Petersburg, Russia 2Jilin University, Changchun, China
The paper focuses on the problems of oil and gas well cementation. It has been established that the main reason why formation fluids leak behind the casing is due to poor isolation of the annulus caused by incomplete cleaning of the filter cake, which results in weak or absent adhesion between the cement and the rock. The authors demonstrate that adding polymer modifier GM-II to the spacer fluid strengthens the cement-to-rock contact by several times. Moreover, mixing spacer with cement slurry does not degrade such important properties as spreadability and consistency of the latter. The paper contains investigation results on the content of polymer spacers that improve cementation quality of oil and gas wells. Spectral infrared and X-ray analysis allowed to define the impact of phase composition and mineral structure of the system «cement stone-filter cake-formation rock» on improving leak tightness of the annulus. Electron microscopic study showed that the use of polymer spacers facilitates formation of felted net-like structures between hydrated minerals. The composition of these structures, binding cement grains and clay minerals, has been studied by means of energy dispersive X-ray microanalysis. Obtained results allow to come to the conclusion that these structures mostly consist of hydrated calcium silicate (more than 77% by mass), which agrees well with the findings of infrared and X-ray analysis. Pilot tests, performed at the Shengli oil field in China, have confirmed efficiency of developed spacer fluids.
Key words: well cementation, spacer fluid, filter cake, cement stone, adhesion, clay and cement minerals
How to cite this article: Nikolaev N.I., Haoya L. Results of Cement-to-Rock Contact Study. Zapiski Gor-nogo instituta. 2017. Vol. 226. P. 428-434. DOI: 10.25515/PMI.2017.4.428
Introduction. Casing and cementation belong to the key processes of wellbore construction, and are characterized by high technological complexity. Over 25% of oil and gas wells in the world have inter-reservoir communications of varying intensity, leading to great economic losses [14]. Behind-the-casing leakages usually follow the cement-to-rock contact, as in many cases complete removal of the filter cake is impossible. As a result, there is no adhesive connection between the cement stone and the formation rock. With this in mind, it is evidently a highly relevant task to develop new polymer spacers which will increase cement-to-rock adhesion.
Authors present results of experimental research on contact strength (adhesion) in the system «cement stone - filter cake - formation rock» with varying concentrations of polymer modified clay GM-II and on its changes in the cement setting process (temperature 90 °C) [6].
Research methods and analysis of obtained results. The impact that developed compositions of spacer fluids have on the filter cake and cement slurry can be divided into several stages [8]. At the first stage, loose clay particles decompose in the spacer solution and form active ions that implement into the filter cake; shattered clay particles bind together into stable polymer chains.
At the second stage, a part of spacer solution diffuses into cement slurry. Hydrate ions of the latter are absorbed by the polymer grains of the solution, cement slurry and filter cake exchange their ions. After the annulus is filled with grouting mortar, the system «cement stone -filter cake - formation rock» is established.
The third stage is marked with the setting of the above mentioned system. Binding substances, responsible for peculiar net-like structures between the filter cake and cement stone, form on the surface of polymer spacer grains. These minerals play an important role in the improvement of well cementation quality.
It has been identified [5] that polymer spacer fluids with various concentrations of GM-II have significantly higher adhesion between the cement stone and rock formation. As GM-II concentration rises from 15 to 25 %, in two days adhesion increases by factor of 6-18, in 60 days - roughly by factor of 14.
In order to test the possibility of using polymer spacers in wellbore cementation, experimental research has been carried out, where drilling mud and cement slurry were mixed with spacer fluids [15].
Results of spreadability tests for clay and grouting mortars with varying content of polymer spacer are presented in Table 1. The data clearly demonstrates that the growth of spacer concentration directly correlates with increasing spreadability of the mixture.
Table 1
Results of spreadability tests for grouting mortar (water-to-cement ratio 0.5) and polymer spacer
Composition of the grouting mortar and polymer spacer (by volume) Spreadability, cm Composition of the grouting mortar and polymer spacer (by volume) Spreadability, cm
Grouting mortar 17 Grouting mortar: polymer spacer = 4:1 >25
Grouting mortar: polymer spacer = 9.5:0.5 18 Grouting mortar: polymer spacer = 7:2 >25
Grouting mortar: polymer spacer = 9 : 1 21 Grouting mortar: polymer spacer = 5:5 >25
Results of research on the consistency of clay and grouting mortars with varying content of polymer spacer are presented in Fig.1 [16]. The graphs demonstrate that the time needed to reach technological conditions that would satisfy regulatory requirements of the cementing process is limited to two hours.
At the next stage, the authors investigated the impact of phase composition and mineral structure of the system «cement stone-filter cake-formation rock» on improving leak tightness of the annulus [3, 4].
Infrared spectroscopy of samples with polymer spacer allowed to identify binding hydrosilicates C-H-S(I) and zeolites (Fig.2). Peaks on the infrared spectrum of the control sample in the area of 3623 and 718.48 cm1 correspond to stretching vibrations of O-H and Si-O in montmorillonite [7]. Infrared spectrum of polymer spacer sample does not contain these peaks, which points to the fact that mont-morillonites in the filter cake were modified as a result of interaction with the spacer fluid. The peaks, corresponding to stretching vibrations of H2O, are displaced in the spacer sample (wavelength
m
Ö o O
70 60 50 40 30 20 10
50 100 150 200 Time, min
250
300
50 45 40 35 30 25
20
Ratio of grouting mortar to polymer spacer (by volume) 1:1 -A-7:3 -•— 4: 1 -A-8.5:1.5 -0-9:1 -»-9.5:0.5
Fig.1. Dependency between grouting mortar consistency and polymer spacer composition by volume (t = 90 °C)
-1-1-1-1-1-1-14000 3500 3000 2500 2000 1500 1000 500
y, cm-1
Fig.2. Infrared spectrogram of the cement-to-rock system composition 1 - control sample; 2 - polymer spacer sample
0
3438.43 cm-1) as compared to the control sample (wavelength 3438.45 cm-1), which points to significant differences in their structures. The absorption spectrum at wavelength 3462.04 cm-1 corresponds to hydrosilicate C-H-S(I), and the band at 669.71 cm-1 is characteristic of zeolites [9].
It is an established fact [2, 13] that hydrosilicates C-H-S(I) and zeolites can bind grains of cement and clay minerals and reduce tension on their contact, which plays an important role in increasing adhesion on the boundary «cement stone - filter cake - formation rock».
Mineral composition of the filter cake can be examined by means of X-ray structural analysis, results of which are presented in Tables 2 and 3. X-ray structural analysis of filter cake composition revealed the prevalence of inert minerals (quartz, calcites, feldspars, montmorillonites, chlorides etc.) in control samples.
A the same time, polymer spacer samples were observed to contain binding materials: hydrated calcium silicate, ettringite, hydrated calcium alluminate, hydrosulphoaluminate, which increased cement-to-rock adhesion [1, 18].
Table 2
Results of X-ray structural analysis of the mineral composition for control samples
Characteristic peak n x 1000 Hydration product Characteristic peak n x 1000 Hydration product
3.0945; 2.1167; 1.5330 etc. Calcite 3.7593; 3.3089; 1.8540 etc. Feldspar
2.8289; 2.4751; 1.4895 etc. Quartz 4.4719; 2.1010; 1.6711 etc. Montmorillonite
4.4240; 1.8170; 1.5334 etc. Iddingsite 4.3207; 3.0945; 1.6370 etc. Chloride
Table 3
Results of X-ray structural analysis of the mineral composition for polymer spacer samples
Characteristic peak n x 1000 Hydration product Characteristic peak n x 1000 Hydration product
3.0292; 2.8303 1.5261 etc. Calcite 2.2792; 1.5330 1.4562 etc. Mg(OH)2
3.8895; 3.3131 1.8170 etc. Quartz 3.6774; 2.4785 1.9282 etc. Al2SiO4(OH)2
4.4313; 1.8170 1.5334 etc. Iddingsite 4.2455; 3.7619 1.9284 etc. Hydrated calcium silicate
3.7619; 3.2810 1.8558 etc. Feldspar 3.3601; 2.5281 1.8558 etc. Hydrosulphoaluminate
4.5907; 2.8303 1.6354 etc. Chloride 3.3601; 2.4785 1.8558 etc. 3CaO Al2O3CaSO4- nH2O
4.3276; 3.3130 1.5334 etc. CaSO42H2O
Microphotographs demonstrate that the use of polymer spacer fluid facilitates formation of felted net-like structures between hydrated minerals. These structures bind cement grains and loose particles of clay minerals (Fig.3). The structure of the minerals is dense and solid. In con-
Nikolayi. Nikoiaev, LiuHaoya DOi: 10.25515/PMI.2017.4.428
Results of Cement-to-Rock Contact Study
trol samples, the intermediate zone between the cement and the filter cake has a lot of fractures and pores.
Analysis of the mechanism and process of interaction between the spacer fluid, grouting mortar and the filter cake revealed that polymer spacer is highly alkaline, which leads to degradation of filter cake particles. This process can be formulated as follows [11]: montmorillonite ((Ca, Na) (Mg, Al, Fe)2[(Si, Al)4O10] (OH)2 «H2O)
Na2SiO3 = 2Na+ + SiO2" ;
Al2O3 + 2NaOH = 2NaAlO2 + H2O;
NaAlO2 = Na+ + AlO"; kaolinite (A^^S^^O)
Al2Si2O5(OH)4 + 5H2O = 2Al(OH)3 +2Si(OH)4;
A12Si2O5(OH)4 + 2Na+ + 2OH- + 4Si(OH)4 = 2NaAlSi3O3 + 11H2O;
A12Si2O5(OH)4 + 2Na+ + 2OH- + 2Si(OH)4 = 2NaA1Si2O6'H2O + 5H2O.
Illite, chlorite and hydromuscovite, as well as montmorillonite and kaolinite share similar oxides, hence the products of their reaction with polymer spacer are identical.
As a result of clay mineral degradation, the alkaline solution contains free anions SiO2", AlO" and cations K+, Na+, Ca2+, Mg2+. This encourages their participation in consequent setting reaction and creates a possibility of their implantation into the filter cake through the destruction of clay particles in the alkaline media. Most polymer spacer minerals do not bind solely to the surface of the filter cake, but get implanted within its structure.
Apart from that, polymer spacer solution contains numerous hydroxyls R-OH and salts R-COO+, which bind to negative surface charges of clay mineral particles. As a result, primary loose and shattered grains form integral polymer chains with solid bonds (Fig.4).
Due to the presence of latex in spacer composition, its ions get absorbed by hydrated formations of setting cement [10, 12]. Its negative charge and carboxyl bond -O-O- intensify interaction with cement ions, such as Ca2+, Na+, Fe3+, Al3+, Mg2+, AlO", SiOf", OH-, SiO^" etc.
These active compositions, absorbed by the surface of polymer grains, react with each other in the following manner:
Ca2+ + SiOf" + 2OH- ^ *CaO-roSiO2-wH2O + H2O; xCa2+ + yHf SiO" + zOH- ^ C-S-H;
5Ca2+ + 2H3 SiO" + 8OH- ^ Ca5(SiO4)2(OH)2 + 6H2O;
3Ca2+ + 2H3 SiO" + 4OH- ^ CafSi2OrH2O + 4H2O;
Ca2+ + 2H3 AlO4" + 6OH- + 4H2O ^ CaAl2O4 10H2O;
1,74Na+ + 2HfAlO4" + 3HfSiO" ^Na1,74(Al2SifO10)(H2O)1,5 + 6H2O.
2
3
Fig.4. Pattern of interaction between polymer spacer and clay minerals 1 - polymer grain of spacer fluid; 2 - small mineral particles; 3 - clay minerals; 4 - active ions
1
4
V
VA
b
m4
+
I" - *+ Uf \
262 25.0kV x20000 1pm
Fig.5. Results of energy dispersive X-ray microanalysis: light spots a and b
The reaction leads to formation of products observed in the mineral structure of the system «cement stone - filter cake - formation rock», including kCaO mSiO2 nH2O and hydrated calcium silicate; wollastonite; oceanite; natrolite. These minerals have either strong binding properties, or high adhesion [15].
In the course of hydration process, solidification of cement slurry and concrete strength development, active ions produce a sufficient amount of binding and solid minerals (hydrated calcium silicate, wollastonite, oceanite etc.) on the polymer grain surface of spacer fluid. This process is accompanied by the formation of a net-like structure, which can be observed in the photographs.
By means of energy dispersive X-ray microanalysis [7], the composition of felted net-like structures has been studied (light spot positions a and b in Fig.5)
Numerical values for the results of energy dispersive X-ray microanalysis are presented in Table 4. Performed examinations allow to come to the conclusion that the structure mostly consists of hydrated calcium silicate (over 77 % by mass), which is in good agreement with the findings of performed infrared and X-ray structural analysis.
A part of this structure - white dendrite formation (Fig.5 and 6) - has a strong bond with the cement stone, another part is linked to filter cake particles. Furthermore, a contact between separate mineral particles has been observed in the filter cake (Fig.6).
As visible from Fig.5 and 6, mineral particles in the system «cement stone - filter cake - formation rock» bind together by means of solid net-like 3D-structures. These structures intertwine and form long polyanionic chains of high hydration, as well as strong crystallohydrate coating on the surface of clay and cement grains.
New formations, characterized by felted structure, high strength and adhesion to clay and cement grains, fill fractures and pores between the cement and the filter cake, facilitate cement setting, and provoke thickening of minerals in the intermediate zone, which enhances adhesion in the system «cement stone - filter cake - formation rock» [17].
Table 4
Investigation results of the net-like structure mineral composition, %
Light spot a Light spot b
Oxides Mass Mole Mass Mole
fraction fraction fraction fraction
SiO2 34.65 33.68 32.22 32.91
K2O 4.13 2.56 3.12 2.03
CaO 61.22 63.76 45.06 49.32
Na2O - - 4.75 4.70
MgO - - 3.88 5.91
Al2O3 - - 4.20 2.53
Fe2O3 - - 6.77 2.60
Nikoiay /. Nikoiae v, Liu Haoya
Results of Cement-to-Rock Contact Study
-v-
Intermediate zone
—V—
Filter cake
Cement column
Fig.6. Binding pattern for the products of cement hydration and the filter cake
Industrial tests of the developed polymer spacer fluid were performed at the Shengli oil field in China, where vertical wells have been drilled to the depth of 3728 m (bottom-hole temperature 91.8 °C). Acoustic control of the cementation process and electrical logging have demonstrated high adhesion between the cement stone and the filter cake; the quality of well casing and cementation was 30 % higher than that in the neighbouring wellbores.
1. Review and analysis of modern methods and technological means have shown that development of new polymer spacers can enhance adhesion between the cement stone and the rock, which makes it a promising way to reduce behind-the-casing leakages.
2. Performed investigations of mineral composition changes in the system «cement stone - filter cake - formation rock» have confirmed the presence of binding hydrosilicates C-H-S(I) and zeolites in polymer spacer samples. Visual examination of microphotographs allowed to indentify that polymer spacer fluid facilitates formation of felted net-like structure between hydrated cement minerals and the filter cake; this structure binds the cement stone and the filter cake together.
3. Developed polymer spacer enhances adhesion between the cement and the filter cake: after 2 days - by factor of 20, after 30 days - by factor of 11, after 60 days - by factor of 15. Technological parameters of these mixtures satisfy regulatory requirements of the cementing process.
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Conclusions
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Authors: Nikolai I Nikolaev, Doctor of Engineering Sciences, Professor, nikinik@mail.ru (Saint-Petersburg Mining University, Saint-Petersburg, Russia), Liu Haoya, Candidate of Engineering Sciences, Associate Professor, lhy091575@163.com (Jilin University, Changchun, China).
The paper was accepted for publication on 25 October, 2016.
