Jinkun Qin,Xueyu Png,b,*,Ashok Sntr,Guodong Cheng,Hilong Li

a School of Petroleum Engineering,China University of Petroleum (East China),Qingdao,266580,China

b Key Laboratory of Unconventional Oil &Gas Development (China University of Petroleum (East China)),Ministry of Education,Qingdao,266580,China

c Drilling Technology Team,Aramco Americas: Aramco Research Center,16300 Park Row Drive,Houston,TX,77084,USA

Keywords: High pressure and high temperature(HPHT)Strength retrogression Young’s modulus Water permeability Rietveld method

ABSTRACTIn order to investigate the problem of long-term strength retrogression in oil well cement systems exposed to high pressure and high temperature (HPHT) curing conditions,various influencing factors,including cement sources,particle sizes of silica flour,and additions of silica fume,alumina,colloidal iron oxide and nano-graphene,were investigated.To simulate the environment of cementing geothermal wells and deep wells,cement slurries were directly cured at 50 MPa and 200 °C.Mineral compositions(as determined by X-ray diffraction Rietveld refinement),water permeability,compressive strength and Young’s modulus were used to evaluate the qualities of the set cement.Short-term curing (2-30 d) test results indicated that the adoption of 6 μm ultrafine crystalline silica played the most important role in stabilizing the mechanical properties of oil well cement systems,while the addition of silica fume had a detrimental effect on strength stability.Long-term curing (2-180 d) test results indicated that nanographene could stabilize the Young’s modulus of oil well cement systems.However,none of the admixtures studied here can completely prevent the strength retrogression phenomenon due to their inability to stop the conversion of amorphous to crystalline phases.

1.Introduction

Oil well cement is a special Portland cement used to seal the annulus space in oil gas,and geothermal wells,and its main function is to provide zonal isolation inside a wellbore.With the gradual depletion of shallow oil and gas resources,deep and ultra-deep reservoirs have been playing a more and more important role in recent ten years and show great potential to meet the world’s everincreasing energy demand (Li et al.,2020a).However,the high pressure and high temperature (HPHT) environment posed significant challenges to cement deep and ultra-deep wells.For example,set cement’s physico-mechanical qualities may deteriorate significantly during high-temperature curing as a result of the cement hydration product converting from semi-crystalline C-S-H to crystalline phases at curing temperatures above 110°C(Krakowiak et al.,2015).This phenomenon,known as strength retrogression,was identified for ordinary Portland cement as early as the 1930s(Craft et al.,1935;Swayze,1954).In the petroleum industry,silica materials are commonly added to the oil well cement to prevent strength retrogression at high temperatures(Rust and Wood,1960;Eilers and Root,1976).However,most prior studies on oil well cement strength retrogression used a two-step curing regime(allowing the cement to set at low temperatures before exposing it to high temperatures) (Iverson et al.,2010,2014;Pernites et al.,2016;Costa et al.,2017;Ge et al.,2018;Krakowiak et al.,2018;Mahmoud et al.,2019;Paiva et al.,2019;Jiang et al.,2021;Liu et al.,2021;Santiago et al.,2021;Wei et al.,2021),which does not truly reflect the situation of cementing deep wells and geothermal wells.Even in studies that used a one-step curing regime(i.e.allowing the slurry to set and cure at high temperatures directly) (Meller et al.,2009;Reddy et al.,2016;Wang et al.,2017),few ensured that the slurries contained adequate amounts of retarders to prevent premature setting before the target curing temperatures were reached.To the best of authors’knowledge,Pang et al.(2021)were the first to demonstrate serious long-term strength retrogression of silicaenriched cement that was set and cured at 200°C.The goal of this study is to conduct further research on a variety of factors that may influence the long-term strength retrogression behavior of silica-enriched cement under comparable test settings as well as possible solutions to the retrogression issue.

A variety of factors can impact the strength retrogression behavior of oil well cement.First of all,the performance of Portland cement can vary widely from one manufacturer to another due to differences in the raw materials as well as in the production process;hence it is worth studying the effect of cement source on test results to make sure the previous finding is not an isolated case.Secondly,in our previous study (Qin et al.,2021a),both silica dosage and silica particle size were found to play important roles in preventing cement strength retrogression at high temperature,because they can alter the composition of hydration products;decreasing silica particle size and increasing silica dosage seem to mitigate strength retrogression,at least in the short term (Pang et al.,2021;Qin et al.,2021b).Thirdly,the substitutability of the crystalline silica by highly reactive amorphous silica (silica fume)seems to be significantly affected by temperature.Noik et al.(1998)discovered that oil well cement systems made with silica flour and silica fume were resistant to strength retrogression at temperatures as high as 140°C.On the other hand,Pernites and Santra (2016)noticed that oil well cement systems prepared by partial substitution of silica flour with silica fume completely failed when exposed to 288°C.Based on our short-term curing(14 d)test results under 200°C,the partial replacement of crystalline silica with silica fume can lead to reductions in set cement permeability (Qin et al.,2021a).Similarly,the addition of various nanomaterials to oil well cement systems also results in set cement with lower permeability;among them,incorporation of colloidal iron oxide nanomaterials in the amount of 5% by weight of cement (BWOC) decreased water permeability by about 60% compared with the control (Qin et al.,2021a).However,the long-term effects of silica fume and nanomaterials are still unclear.Lastly,the incorporation of aluminum into calcium silicate hydrates was previously found to prevent the transformation of tobermorite by aluminum substitution into the tobermorite crystal lattice structure at high temperature(>150°C)(Kalousek and Chaw,1976;Shaw et al.,2000),which is beneficial for maintaining good mechanical properties.The substitution occurs primarily at silicon sites and possibly at OH-and O2-sites as well to maintain charge balance(Kalousek et al.,1957).Meller et al.(2009) found that alumina enhanced the physico-mechanical performance of CaO-SiO2hydroceramic system by stabilizing tobermorite above 200°C;hence it would be interesting to study the effect of alumina on the strength stability of silica-enriched oil well cement systems as well.

Oil well cement slurries with appropriate fluid characteristics(thickening time,rheology and fluid loss,for example) were designed for field applications in deep and ultra-deep wells in this work,and a one-step curing regime similar to that used in earlier studies was used to obtain set cement(Pang et al.,2021;Qin et al.,2021a,b).Unlike our recent study (Qin et al.,2021a),which was focused on the 14-d performance of different systems,set cement property change over time is also evaluated here.The first section of this article investigated several parameters affecting the strength retrogression behavior of oil well cement during a 30-d curing period,including cement source,silica size,and application of various admixtures such as alumina,silica fume and nanomaterials.In particular,the performance of an internationally widely used Dyckerhoff Class G cement was evaluated and compared with local cements.The performance of a high purity ultrafine silica with a median particle size of 6 μm was compared with two types of low purity coarse silica.The second part examined the long-term performance of several optimized slurries for curing periods up to 180 d.The effect of alumina and nanomaterials (including nanoiron oxide and nano-graphene) on long-term strength retrogression behavior was investigated for the first time.The mineral compositions of set cement were evaluated by X-ray diffraction(XRD) method accompanied by Rietveld refinement to study the driving factor of long-term strength change.

2.Experiment

2.1.Materials

Aksu class G oil well cement was produced in Xinjiang,China,by the Aksu cement factory.Dyckerhoff class G oil well cement was produced by Dyckerhoff AG and imported from Germany.Jiahua class G oil well cement was manufactured by Jiahua special cement Co.,Ltd.,Sichuan,China.The physico-chemical characterizations of the three types of cement are shown in Table 1.The XRD profiles of the cement are used to estimate the compound compositions of three different types of cement using quantitative Rietveld refinement shown in Table 2.Apparently,Aksu-G oil well cement has a slightly higher C3S,and Jiahua class G oil well cement has a lower median particle size(D50).

Cement mineral admixtures used in this study include three types of silica flour with different particle sizes,the names of the three different types of silica were distinguished by their median particle sizes (rounded to 1 μm).The particle size distributions(PSDs) are shown in Fig.1.Kuche drilling mud material facility in Xinjiang,China supplied 83 μm and 53 μm silica;Tongbai factory in Henan,China supplied 6 μm silica.Alumina(α-Al2O3)was supplied by Zhengzhou Huarun raw materials Co.,LTD.Silica fume wasprovided by Kuche drilling mud material factory,Xinjiang,China.Colloidal iron oxide was obtained from Beijing Deke Daojin Science and Technology Co.,Ltd.with a 30%activity.The colloidal iron oxide was dried in the oven to remove the powder for density and X-ray fluorescence(XRF)measurements.Nano-Graphene(with a density of 2.24 g/cm3) was provided by Chengdu Organic Chemicals Co.,Ltd.More details of the liquid admixture,such as retarder (BCR-300L),fluid loss agent (BXF-200L),dispersant (BCD-210L),defoaming agent (G603),and high-temperature suspension agent(BDJ-300S),can be found in our previous study (Pang et al.,2021;Qin et al.,2021a).The oxide compositions of different materials were determined using X-ray fluorescence(PANalytical AxiosMAX)in accordance with ASTM C114-18 (2019),the results of which are provided in Table 1.The powder materials indicated in Table 1 have their densities determined using a pycnometer (Quantachrome Instrument UltraPYC 1200e) in accordance with API RP 10B-2(2013).

Table 1 Characterization of cement,silica mineral and nano-Fe2O3 admixtures.

Table 2 Compound compositions (%) of three different types of cement estimated by the Rietveld method.

Fig.1.PSD frequency curve of raw materials.

2.2.Formulation design and preparation of cured HPHT samples

Several test series were designed to investigate the effect of admixtures on strength retrogression of silica-enriched cement systems under 200°C and 50 MPa.All the formulations were designed to have a density of 1.9 g/cm3.Test series I was designed with 70%silica addition of different particle sizes(Table 3).Because 6 μm silica has a significantly higher surface area,the dosages of additives were adjusted in slurry S3 to obtain similar slurry properties as slurries S1 and S2.Based on the test results of the first series in Section 3.1,the 6 μm silica (D50=6 μm) showed the best performance to slow down the strength retrogression during the first 30 d.Therefore,6 μm silica was used in the following slurries as the primary admixture.Series II,III,IV and V were designed to explore the influence of the dosage of alumina and colloidal iron oxide,cement sources,and silica fume on the strength retrogression of cement slurries under HPHTconditions(Tables 4-6).Finally,three optimized slurries,as shown in Table 7,were used for 2-180 d long-term curing evaluation.These slurries had an initial consistency of about 20 Bc and thickening time in the range of 300-400 min (test condition of 180°C/120 MPa and ramp time of 90 min).The HPHT consistometer was manufactured by Liaoning Bassrett petroleum equipment manufacturing Co.,Ltd.The previous studies suggest that microstructure coarsening is the primary driving force of cement strength retrogression (Pang et al.,2021).Nano-graphene was added to slurry S3 with the intention of providing bridging inside the microstructure to resist the coarsening process physically.

The dosages of defoamer(G603),fluid loss reducer(BXF-200L),retarder(BCR-300L)and dispersant(BCD-210L)in Tables 4-7 were 0.5% BWOC,6% BWOC,4.5% BWOC,and 5.5% BWOC,respectively,which is consistent with our previous study(Pang et al.,2021).The dosage of suspension agent varied for each slurry as shown in the tables.

Table 3 Test series I -different silica sizes (% BWOC).

Table 4 Test series II-different cement sources (% BWOC).

Table 5 Test series III -effect of alumina and colloidal Fe2O3 (% BWOC).

Table 6 Test series IV -effect of silica fume (% BWOC).

Table 7 Optimized formulation design for long-term tests(% BWOC).

All cement slurries were prepared following the standard procedures specified in API RP 10B-2(2013)using a laboratory blender(manufactured by Tianjin Nithons Technology Co.,Ltd.).In a stainless-steel mixer cup,the mixtures were first blended at a rotation speed of 4000 r/min for about 15 s,then at 12,000 r/min for 35 s.After preparation,the slurry was poured into the self-designed cylindrical molds (25 mm × 70 mm) and placed in autoclaves for HPHTcuring(200°C and 50 MPa),as shown in Fig.2.Slurries S1-S3 were cured for different durations from 2 d to 30 d,while slurries C1-C3 were only cured for 30 d.Pang et al.(2021) found that the characteristics of set cement cured under HPHT conditions for 7 d and 14 d had little change,thus the durations of slurries A1-A3,slurries D1-D3,slurry W1 and slurry W2 were 2 d,7 d and 30 d;the curing durations of slurries A4-A8 were 2 d,14 d and 30 d.The curing durations of the optimized slurries Z1-Z3 were 2 d,60 d,90 d and 180 d.Samples of different curing durations were cured in separate autoclaves,which avoided unwanted sample damage during the cooling and resealing process.Our previous study has confirmed that good reproducibility can be obtained between separate curing batches when tests were properly conducted (Qin et al.,2021a).At the end of curing,the natural cooling method was adopted to release the pressure and temperature of the autoclave to room condition.After cooling for 40 h,the set cement slurries were extracted from the molds.The end surfaces of demolded specimens were cut and ground according to ASTM D4543-19 (2019),and the final tested samples were 25 mm×50 mm cylinders.A more detailed curing process can be found in our recent publications (Pang et al.,2021;Qin et al.,2021a).

2.3.Experimental methods

2.3.1.Compressive strength and Young’s modulus tests

The testing methods of compressive strength and Young’s modulus can be found in our previous study (Qin et al.,2021a).Young’s modulus and failure strain are important parameters determining the long-term sealing capacity of different cement systems based on finite element analysis (Cuello Jimenez et al.,2016,2017).A loading rate of 0.3 mm/min was conducted to perform the test.Three specimens were prepared for each formulation at a specified curing time (except for samples 2dA1,30dA4,30dA7 and 30dA8,where only two complete specimens were obtained).The coefficient of variations (COVs) of compressive strength are shown in Tables A1-A5 in Appendix.The average COV of all test sets is 6%,which suggests the number of specimens used is appropriate for the purpose of this study.

2.3.2.Water permeability test

There were two methods mainly used to measure the permeability of the set cement: water and gas permeability tests.The sample had to be saturated for the water permeability test,whereas the sample had to be dried for the gas permeability test.The drying conditions may significantly influence the gas permeability results(Pang et al.,2021).Therefore,water permeability was used to assess the permeability of set cement.The specific experimental procedures can be found in Pang et al.(2021) and Qin et al.(2021a).The water permeability is calculated by

Fig.2.Process of HPHT curing sample preparation.

Fig.3.Effects of silica sizes on the properties of oil well cement: (a) Compressive strength,and (b) Water permeability.

Fig.4.Compressive strength of Slurries A1-A3 and D1-D3 with short-term curing.

whereKwdenotes the permeability (mD);Qdenotes the flow rate(cm3/s);μwis the water viscosity,which is assumed to be 1 mPa s;Lis the length of the sample (cm);Arepresents the cross-sectional area of the sample (cm2);and ΔPdenotes the pressure difference for water permeability test (atm,or～0.1 MPa).

2.3.3.XRD tests

The XRD testing was carried out following ASTM C1365-18(2018).For precise quantitative mineral composition analysis of this specific test series,a Malvern PANalytical X-ray diffractometer(Model AerisX) was used.Details about sample preparation and application of internal standard(99.9%purity TiO2)can be found in Qin et al.(2021a).Rietveld analysis was done using Highscore Plus 5.0 software to quantify the principal crystal phases from the Crystallography Open Database (COD-2019),such as alumina(Al2O3,Ref.code: 96-900-8082),rutile (TiO2,Ref.code: 96-900-7532),quartz (Ref.code: 96-500-0035),tobermorite (C4.5S6H5,Ref.code: 96-900-5498) and xonotlite (C6S6H,Ref.code: 96-900-8440),as well as the amorphous C-S-H phase.

Fig.5.Young’s modulus of Slurries A1-A3 and D1-D3 with short-term curing.

Fig.6.Water permeability of Slurries A1-A3 and D1-D3 with short-term curing.

3.Influence of various factors on short-term curing set cement

3.1.Influence of particle size of silica flour

It can be seen from Fig.3a and b that the particle size of silica flour had a significant influence on the mechanical properties of set cement.The error bars in all the test results presented in this study are the standard deviation between replicated samples.Silica with a smaller size can increase the compressive strength and reduce the severity of strength retrogression during the 30-d curing period(decline rates of Slurry S1,S2 and S3 were 45%,38% and 12%,respectively).The strength decline rate of Slurries S1 and S2 were comparable to previous studies of similar test conditions,where 27%-44% reductions were observed within 30 d (Li et al.,2020b;Pang et al.,2021).However,the strength decline rate of Slurry S3 with 6 μm silica was significantly reduced (only 12%).In addition,Slurry S3 exhibited the highest 30-d compressive strength(72.76 ± 2.12 MPa),200% higher than Slurry S1 and 100% higher than Slurry S2.The 2-d water permeability of Slurry S3 was two orders of magnitude lower than Slurry S1,while its 30-d water permeability was one order of magnitude lower than Slurry S1;thus,it seemed that the ability of ultrafine silica to reduce water permeability was weakened with increasing curing durations.This test series showed that reducing silica particle size could improve the set cement properties at HPHT conditions,at least in the short term.The reason may be due to optimized PSD as well as increased reaction extent of the crystalline silica.It has been found in previous studies that the reaction depth of large silica particles is only a few micrometers(Pang et al.,2021).The 6 μm silica was used in all the following test series.

3.2.Influence of cement sources

Figs.4 and 5 illustrate the mechanical property test results for oil well cement provided by different sources.It can be seen that the properties of Aksu class G oil well cement slurries(A1-A3)and Dyckerhoff class G oil well cement slurries (D1-D3) had similar development trends with increasing alumina dosage and curing time.However,at the same curing time,Aksu class G oil well cement slurries had better mechanical properties than Dyckerhoff class G oil well cement slurries when the additives dosages were the same.On average,the Young’s modulus and compressive strength of Akesu class G oil well cement were higher than those of Dyckerhoff class G cement by 14%and 16%,respectively.As shown in Figs.6 and 2-d Aksu cement samples also showed lower permeability than Dyckerhoff cement samples.However,slightly faster growth in permeability during the first 7 d was observed with the Aksu cement sample.After 30-d curing,Aksu cement still exhibited slightly lower permeability than Dyckerhoff cement.As shown in Tables 1 and 2,Aksu and Dyckerhoff cements have similar chemical compositions;therefore,the slightly better physicomechanical performance of Aksu cement may be attributed to its finer particle size (see Fig.1).

Table 8 Strength and modulus changes of Slurries A1-A8 from 2 d to 30 d.

Fig.7.Properties of Slurries C1-C3 with 30-d curing: (a) Compressive strength and Young’s modulus,and (b) Water permeability.

Fig.8.Compressive strength of Slurries A1-A8 with short-term curing.

Fig.9.Young’s modulus of slurries A1-A8 with short-term curing.

To further investigate the influence of cement source,another local cement called Jiahua cement (D50=12.7 μm),which has similar chemical compositions as Aksu and Dyckerhoff cements,was employed.The same slurry formulation prepared with three different cements,as shown in Table 4 (Slurries C1-C3) were studied.As shown in Fig.7,all three slurries maintained excellent physico-mechanical properties after 30-d curing at 200°C and 50 MPa;the performance of Aksu class G cement (Slurry C1) was similar to Jiahua class G cement (Slurry C3),and better than Dyckerhoff class G cement (Slurry C2).These test results further confirm the previous observations for Slurries A1-A3 and D1-D3(see Figs.4-6).Given that Aksu and Jiahua cements had lower particle sizes than Dyckerhoff cement,it can be concluded that the particle size of the cement had a significant impact on the ultimate physico-mechanical properties of the cement cured under HPHT conditions.Apparently,the effect of reducing cement particle size is similar to the effect of reducing silica size,both resulting in higher compressive strength,higher Young’s modulus and lower permeability within the 30-d curing period.

3.3.Influence of alumina and colloidal iron oxide

The mechanical properties of Slurries A1-A8 are given in Fig.8(compressive strength) and Fig.9 (Young’s modulus).It showed weakening mechanical properties of set cement with increasing alumina dosage from 7.5% BWOC to 56.3% BWOC,probably caused by the dilution effect and the low reactivity of alumina in the hydration reaction of cement.Similar to compressive strength,Young’s modulus also showed a decreasing trend with increasing alumina addition.As shown in Table 8,Slurry A3 with 15% BWOC alumina addition had only 5% strength retrogression from 2 d to 30 d.However,by comparing the alumina addition and the decline rate of ultimate strength and Young’s modulus,it can be seen that alumina’s ability to alleviate the strength retrogression in the first 30 d was not stable.For all eight slurries presented here,the average decline in compressive strength was 17%,while the average decline in Young’s modulus was 5%,significantly better than previous systems prepared with coarser silica (Li et al.,2020b;Pang et al.,2021).When the alumina addition was fixed at 15%,the increase of colloidal iron oxide(Slurries A3-A5)will slightly decrease the strength of the set cement.However,when the alumina addition was 33.3%,the increase of colloidal iron oxide had a positive effect on the mechanical properties of the set cement: the compressive strength of the 30-d Slurry A7 increased by about 12%compared with that of Slurry A6.Overall,the influence of colloidal iron oxide dosage on the mechanical properties of cement was insignificant from a statistical viewpoint.

Fig.10.Representative stress-strain curves of Slurries A1-A8 with short-term curing.

Fig.11.Water permeability of Slurries A1-A8 with short-term curing.

Representative stress-strain curves for eight formulations,including alumina and colloidal iron oxide at varying dosages,are shown in Fig.10.The ranges ofxandyaxes of the subplots were kept the same to show the trend of variation more clearly.With the increasing dosage of alumina from slurry A1 to slurry A8,the ultimate strength was reduced,while no significant changes in axial and lateral failure strains (i.e.strain at maximum stress) were observed.Similarly,for the same slurry,increasing curing time led to slight reductions in ultimate strength and Young’s modulus within the 30-d curing period,but no significant changes in axial and lateral failure strains.These test results are consistent with previous studies(Pang et al.,2021;Qin et al.,2021a).

Water permeability test results of Slurries A1-A8 are shown in Fig.11.For most slurries,the water permeability of the set cement increased by about an order of magnitude from 2-d to 30-d curing.Slurries A5-A8 showed relatively small percentage increases due to higher initial values.Increases in alumina dosage resulted in a modest rise in water permeability,but the addition of colloidal iron oxide led to reductions in permeability,particularly for 30-d samples.This is consistent with our previous observations at 14-d curing(Qin et al.,2021a).The permeabilities of Slurries A4-A8 with colloidal iron oxide were stable over the curing duration from 14 d to 30 d.

Fig.12.XRD analysis of Slurries A1-A8 with short-term curing.

In summary,using 6 μm ultrafine crystalline silica at high dosage (70% BWOC) can mitigate the strength retrogression of oil well cement systems within the 30-d curing period.Adding appropriate dosages of alumina and colloidal iron oxide might help to further stabilize the stability of the oil well cement during the 30-d test period studied.However,almost all eight slurries of this test series showed trends of slow deterioration in physicomechanical properties.Therefore,it is necessary to perform further studies to evaluate the long-term performance of these optimized slurries under HPHT conditions.

The compound compositions of the set cement were analyzed by XRD method.As shown in Fig.12,the xonotlite peaks commonly observed in other studies (Li et al.,2020b;Pang et al.,2021) had almost disappeared regardless whether alumina was added.This is indicated by the absence of its unique peak at around 12.6°.As xonotlite has been previously associated with poor mechanical performances of set cement(Li et al.,2020b;Pang et al.,2021),the results here helped explain the improved strength stability of the set cement due to the addition of high dosage ultrafine silica.The main changes from Slurry A1 to Slurry A8 were the increased tobermorite peak at 7.8°and 28.9°with the increasing dosage of alumina.These test results suggested that the addition of alumina favored the formation of tobermorite,possibly at the expense of amorphous C-S-H gel(hump peak at 28.5°-30°).The formation of tobermorite hydration product is generally conducive to set cement properties (high strength and low permeability) at high temperatures.However,the increase in tobermorite concentration did not result in more stable macroscopic characteristics over time,probably owing to the diluting impact of unreacted alumina.Consistent with earlier test results(Qin et al.,2021a),the addition of colloidal iron oxide had no discernible effect on the mineral composition of the set cement.In general,no significant changes in the XRD profiles were found over time;hence,the reported changes in the mechanical characteristics and permeability of the set cement were most likely driven by physical factors rather than mineral composition changes.

3.4.Influence of silica fume

The results of physico-mechanical property tests on Slurries W1 and W2 are shown in Fig.13.Silica fume was supposed to enhance the mechanical properties of cement-based materials due to wellknown pozzolanic reactivity at lower temperatures (<90°C),while its effect under ultrahigh temperature (>200°C) was not clear.By comparing the test results of Slurry W1 with Slurry A1,and those of Slurry W2 with Slurry D1(see Section 3.2),it appears that silica fume improved the mechanical characteristics of 2-d samples.However,the decline rates in compressive strength were dramatically increased with silica fume addition.Slurries W1 and W2 exhibited about 46% decline from 2 d to 30 d,compared to about 15% decline in the control slurries (Slurries A1 and A2).A similar trend was observed in a previous study (Li et al.,2020b).Meanwhile,the increase in water permeability was about 600%,which is comparable to the control slurries with about 200% increase.The mechanical properties of Slurry W1 with Aksu cement were higher than that of Slurry W2 with Dyckerhoff cement,consistent with the results in Section 3.2.Slurries W1 and W2 exhibited similar deformation behavior,and the latter had higher water permeability from 2 d to 30 d.

The XRD test results of slurries with and without silica fume are presented in Fig.14.Similar to previous systems(Fig.12),xonotlite phase was nearly absent in these samples.Slurries A1 and D1 were control systems without silica fume.In previous studies,it has been observed that silica fume can inhibit the formation of tobermorite while favoring the formation of amorphous phase(Li et al.,2020b;Pang et al.,2021;Qin et al.,2021a).The inhibition of tobermorite due to the addition of silica fume can be seen in Fig.14b and c,as suggested by the reduced peaks at 7.8°.The reductions of tobermorite peaks due to silica fume addition seem to be more significant for Dyckerhoff cement than for Aksu cement.The tobermorite peaks at 7.8°and 28.9°were both observed to grow over time in all systems,consistent with previous observations (Pang et al.,2021).It should be mentioned that the analysis of the qualitative results discussed here is based on the overall trend,and there will be some natural variations from sample to sample,as only one XRD profile is obtained per sample.

4.Long-term properties of optimized slurries (from 2 d to 180 d)

4.1.Long-term evolution of mechanical property and permeability

Based on the above analysis of the influences of silica particle sizes,cement sources and admixtures,it appeared that the silica flour and cement with smaller particle sizes had better strength stability within 30 d.The addition of alumina at 15% further improves the strength stability of the set cement,while the addition of colloidal iron oxide can reduce its water permeability.However,the long-term effect of these admixtures is still not clear.Fig.15 showed long-term property change of Slurries Z1,Z2 and Z3,which contained these various admixtures.From 2 d to 90 d curing,the compressive strength of Slurries Z1,Z2 and Z3 decreased by 47%,47% and 42%,respectively,while permeability of all slurries increased by approximately one order of magnitude;all slurries still maintained relatively high compressive strength of approximately 35 MPa.From 90 d to 180-d curing,the compressive strength of Slurries Z1,Z2 and Z3 decreased by 52%,47%and 37%,respectively,while permeability of all slurries increased by another order of magnitude.Interestingly,permeability showed almost no change from 60 d to 90 d curing,suggesting that the decline rate is unlikely to be constant.Overall,the properties of Slurry Z3 with nanographene were more stable than those of Slurries Z1 and Z2;in particular,its Young’s modulus showed almost no change from 2 d to 180 d.Test results shown here indicate that,the use of 6 μm ultrafine silica,as well as the addition of alumina and colloidal iron oxide,can only mitigate the early-stage property deterioration while the long-term strength retrogression was still difficult to avoid.Nano-graphene was somewhat effective in mitigating longterm property deterioration,but the underlying mechanism and optimum dosage need to be further explored.Fig.16 shows representative stress-strain curves of three optimized slurries with long-term curing.With increasing curing time,the stress-strain curve changed from a nearly perfect linear-elastic behavior to an elastic-plastic behavior with a clear yield point.Very little changes in failure strain(strain at maximum stress)were observed with the retrogression of compressive strength from 2 d to 90 d.When curing time was further increased to 180 d,the failure strains of all samples were significantly lower.These test results suggest that long-term curing under HPHT conditions significantly reduced the ability of set cement to absorb energy before failure,which is in good agreement with our previous study (Pang et al.,2021).

4.2.Long-term phase evolution based on XRD analysis

Fig.13.Properties of Slurries W1 and W2 with short-term curing: (a) Compressive strength,(b) Young’s modulus,(c) Representative stress-strain curves,and (d) Water permeability.

Fig.14.XRD profiles of(a) Influences of silica fume and cement type on set cement,(b)Tobermorite region of samples with Aksu cement,(c)Tobermorite region of samples with Dyckerhoff cement,(d) C-S-H region of samples with Dyckerhoff cement,and (e) C-S-H region of samples with Dyckerhoff cement.

Fig.15.Properties of Slurries Z1,Z2 and Z3 with long-term curing: (a) Compressive strength,(b)Young’s modulus,and (c) Water permeability.

Fig.16.Representative stress-strain curves of slurries Z1,Z2 and Z3 with long-term curing.

As all slurries exhibited similar changes in physico-mechanical properties over the long term,which is also consistent with our previous study (Pang et al.,2021),one representative slurry (Z1)was selected for further XRD analysis.XRD test results of Slurry Z1 both with and without TiO2internal standard are given in Fig.17.It can be seen that the addition of internal standard reduced the original peak intensities due to the dilution effect.The inset in Fig.17 illustrates the details around the quartz peak without internal standard: clearly,unreacted quartz content decreased significantly from 2 d to 60 d,and then stabilized from 60 d to 180 d.

As discussed in Section 2.3.3,the phase quantities of the set cement are determined by XRD Rietveld refinement using the test data with internal standard method(Fig.17,bottom).Test results of Slurry Z1 are provided in Table 9.There was no trace of anhydrous cement in the XRD profile,suggesting that the cement was completely hydrated after 2-d HPHT curing.Compared with the quartz(silica)content in the original cement mix composition,the quartz consumption had reached about 82% after 180-d curing,indicating that the quartz addition was excessive.In other words,quartz addition in the amount of 60%(70%×0.82)BWOC appeared to be enough to support the reaction process within 180 d.Alumina seems to be not reacted at all during the entire curing process.Xonotlite content was very small from 2 d to 90 d;the main hydration product phase composition changes from 2 d to 90 d were the slight decrease of tobermorite content and the slight increase of amorphous content (mainly C-S-H gel).Xonotlite content increased suddenly from 90 d to 180 d,probably deriving from the phase transformation of tobermorite.Despite the uncertainties in quantitative XRD analysis of hydrated cement,it can be concluded that there is at least no apparent reduction in amorphous phase content during the entire curing period from 2 d to 180 d.Therefore,although phase change (for example,C-S-H to tobermorite and xonotlite) may still be a significant factor on the long-term strength retrogression of silica-enriched oil well cement under HPHT conditions,it may not be the only cause.The coarsening of the amorphous C-S-H structure itself,similar to that caused by temperature increase and drying (Wyrzykowski et al.,2017;Zhou et al.,2017),may also contribute to the strength retrogression process.Further studies are needed to validate this conjecture.

Table 9 Phase change analysis for long-term cured sample Z1 (wt%).

5.Conclusions

A comprehensive evaluation of various influence factors on the high-temperature strength retrogression behavior of oil well cement is presented in this study,and the following conclusions can be drawn:

(1) For silica-enriched oil well cement systems directly set and cured under HPHT conditions simulating deep well and geothermal well conditions(at 200°C and 50 MPa),the longterm strength retrogression phenomenon that had been revealed recently seemed to be universal for different cement and silica sources.

(2) In the short term(within 30 d),reducing the particle size of silica and cement can help to mitigate strength retrogression of silica-cement systems;adding proper dosages of alumina and colloidal iron oxide to system can also reduce the rate of strength decline or permeability increase.

(3) The addition of silica fume can improve the 2-d compressive strength of silica-cement systems but result in a more significant strength decline in the long term.Therefore,great caution should be taken when adding silica fume to the cement slurry under HPHT conditions.

(4) Within 30 d,the compressive strength of most formulations prepared with 6 μm ultrafine silica shows an inconspicuous decline,but a significant increase in water permeability occurred,suggesting that the latter is a better metrics to evaluate the long-term stability of set-cement at HPHT conditions.

(5) Nano-graphene can significantly improve the Young’s modulus stability of the silica-cement systems over longterm curing up to 180 d,but its effect in mitigating strength decline and permeability increase is relatively small.

(6) Long-term strength retrogression of silica-cement systems cured under HPHT condition may be caused by both hydration product phase transformations and structural coarsening of amorphous C-S-H gel.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

Financial support comes from China National Natural Science Foundation (Grant No.51974352) as well as from China University of Petroleum (East China) (Grant Nos.2018000025 and 2019000011).The authors would like to also thank Petro China Tarim Oilfield Company for providing raw test materials for this study.

Appendix A.Supplementary data

Supplementary data to this article can be found online at https://doi.org/10.1016/j.jrmge.2022.02.005.