Introduction

Portland cement additive chemistry is fundamentally important to oil and gas well construction and to ensure zonal isolation throughout the life of an oil well. When sealing the casing to the wellbore formation with cement, oil well cement additives regulate many important slurry properties including rheology, suspension stability, cement slurry fluid loss control, set time, and mechanical properties of set cement.^{[Reference Nelson and Guillot1]} Poor engineering of oil well cement slurries can result in the loss of zonal isolation in a well potentially leading to the loss of a well and considerable hazard to field personnel.^{[Reference Plank2]} The appropriate selection of the additives in cement slurries depends on the temperature and pressure profiles down the length of the well, the depth of the well, the geologic formations that the slurry contacts, as well as the presence of gas, water, and oil in the different zones of the well.^{[Reference Hunter, Tahmourpour and Faul3–Reference Kutchko, Strazisar, Dzombak, Lowry and Thaulow5]} The understanding of cement chemistry and cement additive chemistry is critically important for the appropriate design of oil well cements used in wells with these well-specific parameters.^{[Reference Plank, Lummer and Dugonjić-Bil6]}

There are many kinds of cement additives used in well cementing. The additives can be classified by the function that they impart into the cement for oil and gas well cementing operations. For example, fluid loss control additives are typically polymeric species that prevent the loss of water from the cement slurry into the rock formations surrounding the cement where it is placed. Some of the additives which are among the most used in well cementing are accelerators, fluid loss control additives, suspension aid additives, and retarders.^{[Reference Fink7]} Retarding additives are added to cement slurries to delay the setting (hydration) time of the cement.^{[Reference Cheung, Jeknavorian, Roberts and Silva8]} This is very important in well cements where the different thermal conditions in the well can give correspondingly different set times. Additionally, since the cement in slurry form is pumped into the well from the surface, the time required for accurate placements varies from well to well and zone to zone.

To understand the effect of how additives control cement properties, the composition of cement must be regarded. Cement is composed of multiple different components, or phases. These phases are responsible for different rheologic and mechanical characteristics of the cement before and after it hydrates. In order to appreciate the benefits of certain cementing additives, it is useful to review some essential cement chemistry. Cement chemistry is expressed for convenience in the abbreviated standard shorthand. In cement shorthand, calcium oxide (CaO) is referred to as C, silicon oxide (SiO₂) is labeled S, water is H, aluminum oxide (Al₂O₃) is A, sulfate (SO₃) is ${\bar{\rm S}}$, and iron oxide (Fe₂O₃) is F. Calcium hydroxide [Ca(OH)₂], for example, through cement shorthand is written as CH and gypsum (CaSO₄) is written as C${\bar{\rm S}}$. The primary phases of cement are tricalcium silicate (C₃S), dicalcium silicate (C₂S), tricalcium aluminate (C₃A), tetracalcium aluminoferrite (C₄AF), and gypsum (C${\bar{\rm S}}$). The primary phases of Portland cement are presented in Table S1 in the ESI. Fig. S1 illustrates the organization of these phases on a cement grain.

The mechanism for cement curing is highly complex with many aspects of the hydration process still under debate.^{[Reference Bullard, Jennnings, Livingston, Nonat, Scherer, Schweitzer, Scrivener and Thomas9]} When a cement grain is placed in contact with water, the different phases of cement react at different rates and with different enthalpies. The relative reaction rates follow this relation C₃A > C₃S > C₄AF > C₂S. However, gypsum is added to the cement to slow the reaction of tricalcium aluminate with water. This is essential since the hydration of C₃A in the absence of gypsum produces a false set. The early mechanical strength of the cement comes almost exclusively from the hydration reaction of tricalcium silicate (C₃S). At later times, C₂S also provides mechanical strength to the set cement.^{[Reference Gani10]}

Under ambient conditions, the hydration of C₃S and C₂S produces amorphous calcium-silicate-hydrate (C-S-H) and calcium hydroxide (or Portlandite/CH). Standard Portland cement maintains average calcium to silicon ratio of 1.7 in C-S-H, but when temperatures exceed 120 °C, the hydration products of Portland cement transform from amorphous C-S-H into crystallites.^{[Reference Krakowiak, Thomas, Musso, James, Akono and Ulm11]} The mineralogy of the cement roughly depends on the relative concentrations of CaO and SiO₂ in the cement. In standard Portland cements, the CaO : SiO₂ ratio lends itself to the formation of α-C₂SH and β-C₃SH. These minerals are higher density than the amorphous C-S-H. The transformation to these high-density products introduces porosity into the cured cement which corresponds to a significant reduction in strength. The phenomenon is known in the industry as “strength retrogression.” Cements which undergo strength retrogression are more likely to contain breaches which result in zonal isolation loss. To minimize the impact of retrogression, it is standard practice to add crystalline silica to the cement to shift the mineralogy of the crystalline products to tobermorite [Ca₅(OH)₃Si₆O₁₆·4H₂O] and xonotlite [Ca₆(Si₆O₁₇)(OH)₂]. These products are known not to effect cement in the same way as C₂SH and C₃SH mineral compositions and Portland cement does not display the same strength retrogression behavior when the mineralogy of cement is dominated by tobermorite and xonotlite over the higher calcium:silicon ratio mineralogies.^{[Reference Meller, Hall, Kyritsis and Giriat12]}

The pozzolan reaction for silica with Portlandite^{[Reference John, Matschei and Stephan13,Reference Jeong, Youm and Yun14]} :

$$\eqalign{x {\rm Ca}({\rm OH})_2 + y {\rm SiO}_2 + z {\rm H}_{\rm 2}{\rm O}\; \to x{\rm CaO}\cdot y {\rm SiO}_2\cdot (x + z)\, {\rm H}_{\rm 2}{\rm O}{\rm.}} $$

In addition to the use of crystalline silica to mitigate strength retrogression at these higher temperatures, specialized retarders are used which are active and effective at these higher temperatures. The introduction of crystalline silica to the cement for high-temperature cementing introduces a new series of hydration reactions with different kinetics and mechanisms and so the corresponding hydration reactions are observed as distinct multi-tiered processes. The cement appears to build mechanical strength through a two-tiered process. The chemical reactions responsible for these two steps are proposed to include (1) the reaction of C₃S with water to produce calcium-silicate-hydrates and (2) reaction of the crystalline silica with Portlandite to render calcium-silicate-hydrates (the Pozzolan reaction shown above). We have observed that two-tiered transition can become more distinct with powerful high-temperature cement set retarders. When the cement slurries are modified appropriately with nanosilicas, however, the two-tiered strength development is replaced by a single transition to the highest compressive strength.

Nanosilicas show distinct advantages in their application as cement additives.^{[Reference Bu, Hou, Wang and Du15–Reference Santra, Boul and Pang17]} They are known to display a distinct advantage to commonly used accelerators^{[Reference Björnström, Martinelli, Matic, Börjesson and Panas18]} because they decrease the set cement porosity and have been observed to increase the mechanical strength of set cement by up to 35%.^{[Reference Nili and Ehsani19]} Nanosilicas and C-S-H nuclei have also been shown to reduce the risk of gas migration in oil well cements by shortening the time (transition time) between when the cement is a weak gel to when it is a strong set structural material.^{[Reference Bu, Hou, Wang and Du15–Reference Björnström, Martinelli, Matic, Börjesson and Panas18,Reference Choolaei, Rashidi, Ardjmand, Yadegari and Soltanian20–Reference Madani, Bagheri and Parhizkar22]}

In this paper, we introduce a retarder package of three additive chemicals which display a pronounced synergetic effect on the hydration kinetics of Portland cement and on the underlying inorganic structure of the cement. The additive package overcomes high-temperature gelation phenomena characteristic of Saudi class G oil well cements. The package exploits the synergetics of diethylenetriamine penta(methylene phosphonic acid) (DTPMP), zinc oxide, and an acrylamido-tert-butyl-sulfonate/acrylate (ATBS/AA) copolymer [chemical structures illustrated in Figs. 1(a) and 1(b)]. These three components work synergetically to provide set hydration delay in Portland cement at temperatures up to 290 °C. The results obtained with the new retarding system are compared with DTPMP in the absence of other synergetic retarding additives and with ATBS/Itaconic acid copolymer (ITA). ATBS/ITA and DTPMP are known high-temperature synthetic retarders used for temperatures up to 260 °C; the structure for this additive is shown in Fig. 1(c).

Figure 1.

The chemical structures of (a) DTPMP, (b) ATBS/acrylic acid (AA) copolymer, and (c) ATBS/Itaconic acid (ITA) copolymer.

Zinc oxide is a known retarder in Portland cement.^{[Reference Berke and Caldarone23]} It has also been demonstrated for use in supplementary cementitious materials (SCMs) such as metakaolin in cement offering the possibility of increasing the reactivity of metakaolin as an SCM in Portland cement. Zinc oxide is believed to retard cement hydration through a nucleation and/or growth poisoning mechanism.^{[Reference Ataie, Juenger, Taylor-Lange and Riding24,Reference Taylor-Lange, Riding and Juenger25]} Since the ZnO is known to accelerate the reaction of siliceous SCMs with Portland cement and water, it was thought that this additive while retarding C3S hydration could accelerate the pozzolan reaction of Portlandite with nanosilica and quartz (from silica our). In this way, the strength development of cement may occur in one stage as opposed to two as is often observed in high temperature cement when crystalline silica is added to cement as a strength retrogression mitigation additive.

DTPMP is an organophosphonate retarder which likely follows the same mechanism of cement hydration as nitrilotrismethylene phosphonate (NTMP). NTMP is believed to inhibit cement hydration through a nucleation and/or growth poisoning mechanism in tandem with a second mechanism. The second mechanism that has been proposed is that the NTMP coordinates to calcium in the cement slurry and the calcium complexes nucleate and grow until their solubility limit is reached in the cement pore solution. They then precipitate on the surface of the Portland cement grains.^{[Reference Bishop, Bott and Barron26–Reference Sangodkar, Smith, Gajan, Rossini, Roberts, Funkhouser, Lesage, Emsley and Chmelka28]} The combination of the aforementioned retarding additives with each other renders a retarding package which has a remarkable high-temperature performance with a tunable influence on the kinetics of the cement's high-temperature mineralogic transformations.