Influence of components properties on sealing ability of cementing isolation system in deep-water shallow formation

Influence of components properties on sealing ability of cementing isolation system in deep-water shallow formation

Construction and Building Materials 204 (2019) 50–57 Contents lists available at ScienceDirect Construction and Building Materials journal homepage:...

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Construction and Building Materials 204 (2019) 50–57

Contents lists available at ScienceDirect

Construction and Building Materials journal homepage: www.elsevier.com/locate/conbuildmat

Influence of components properties on sealing ability of cementing isolation system in deep-water shallow formation Bailing Zhang a, Jin Yang a, Huajie Liu b,c,⇑, Ting Sun a, Shujie Liu d a

China University of Petroleum, Beijing 102249, China Shandong Key Laboratory of Oilfield Chemistry, Key Laboratory of Unconventional Oil & Gas Development (China University of Petroleum (East China)), Ministry of Education, Qingdao 266580, PR China c School of Petroleum Engineering, China University of Petroleum (East China), Qingdao 266580, PR China d CNOOC Research Institute, Beijing 100027, PR China b

h i g h l i g h t s  Cementing isolation system in deep-water shallow formation is investigated.  The sealing ability of cementing isolation system is not related to the casing size.  Increasing the cement strength just can improve the sealing ability very slightly.  Solidifying and enhancing the loose formation is the most effective method.

a r t i c l e

i n f o

Article history: Received 12 September 2018 Received in revised form 22 January 2019 Accepted 25 January 2019 Available online 2 February 2019 Keywords: Sealing ability Cementing isolation system Deep-water shallow formation Casing size Cement strength Soil strength

a b s t r a c t In order to guarantee the safety of oil and gas exploration and development and ocean environment, the influence of components properties comprised of casing size, cement strength and formation strength on sealing ability of cementing isolation system in deep-water shallow formation was investigated by using the simulated cementing isolation system device in this paper. Results show that casing size has little effect on the sealing ability of cementing isolation system, and the sealing ability is not related to the casing size. When the cement strength is lower than formation, the cement strength decides the sealing ability, and the maximum sealing pressure of highest strength cement (2.15 MPa) is about 2 MPa. When the cement strength is higher than formation, the unconsolidated formation is the weakness and sealing failure area, and the maximum sealing pressure of highest strength formation (2.46 MPa) is about 3 MPa. Increasing the cement strength grade and extending the curing time can improve the sealing ability of cementing isolation system to a certain extent, however, solidifying and enhancing the loose formation is the most effective method to improve the sealing ability of cementing isolation system in deep-water shallow formation. Ó 2019 Elsevier Ltd. All rights reserved.

1. Introduction After well drilling, well cementing operation is conducted as follows. The high-quality steel pipe named casing is run into the well, and then cement slurry is pumped into the annular space between the casing and formation via the inner of casing to form cement sheath to isolate the formation [1]. The combination of formation, cement and casing is just the cementing isolation system. The main function of cementing isolation system is to seal the ⇑ Corresponding author at: Shandong Key Laboratory of Oilfield Chemistry, Key Laboratory of Unconventional Oil & Gas Development (China University of Petroleum (East China)), Ministry of Education, Qingdao 266580, PR China. E-mail address: [email protected] (H. Liu). https://doi.org/10.1016/j.conbuildmat.2019.01.143 0950-0618/Ó 2019 Elsevier Ltd. All rights reserved.

wellbore to ensure the safe production of oil and gas. And with the development of oil and gas resources in deepwater [2,3], because the sealing failure of cementing isolation system would also seriously threaten the ocean environment and safety, the sealing ability of cementing isolation system is more important [4–6]. Because the joint between formation and casing is cement sheath which is the weak part in cementing isolation system, the sealing of cementing isolation system is normally destroyed by the failure of cement sheath [7,8]. In addition, the cement sheath with high permeability could also lead to seal failure. In view of the permeability of cement sheath is related to the hydration degree and compactness of cement, furthermore, the cement strength which is easily measured could indirectly reflect the

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degree of hydration and compactness of cement [9], therefore, the strength property is usually considered to be an important indicator to evaluate the sealing ability of well cement and the highstrength cement is generally used for well cementing. However, the researchers have different views about whether the compressive strength decides the sealing effect of cement or not. Some studies proposed that there was no significant correlation between compressive strength and cementing sealing ability [10]. Some studies suggested that the cementing sealing effect depended on the composition of the cement and the time of cement solidification [11–13]. Furthermore, because the main components are unformed-rock silt and sand, the structure of shallow formation in deep water is very loose [14,15]. The loose stratigraphic structure, which can even reach hundreds of meters below the mud line, also increases the sealing-failure risk of the cementing isolation system. At present, the relationship between the cement compressive strength and the sealing ability of cementing isolation system remains uncertain, and there is less research on the sealing ability of cementing isolation system in deep-water shallow formation. Therefore, the sealing ability of cementing isolation system is investigated under the condition of simulated deep-water shallow formation in this paper. The influence of casing with different sizes, cement with different compressive strength and simulated shallow formation with different compressive strength on the sealing ability of cementing isolation system is revealed by experiments. This research could provide basis for the design of cementing isolation system in deep-water shallow formation to guarantee the safe exploration and development of oil and gas resources and ocean environment security.

2. Experimental 2.1. Materials Portland cement which has been used as the well cement for many years [16] is used in our research. Class 325 cement (Compressive strength 32.5 MPa), Class 425 cement (Compressive strength 42.5 MPa) and Class 525 cement (Compressive strength

Table 1 Physical property of experimental soil. Parameter

Water content/%

Density/g/cm3

Specific gravity

Value

84

1.4

2.7

51

52.5 MPa) which were provided by the International Oilfield Services Cementing Company were chose to research the influence of cement with different compressive strength. The cement class is determined according to GB/T 4131-2014 of the People’s Republic of China. The plain cement sample was designed as follows: cement + 44% water (by the weight of cement) + 2% fluid loss additive (by the weight of cement) + 0.5% dispersant (by the weight of cement) + 0.5% defoamer (by the weight of cement). The percentage of different ingredients are commonly used in cementing industry. The fluid loss additive and dispersant are polycarboxylate polymers and sulfonated ketone/aldehyde polycondensates respectively. In order to simulate the submarine shallow soil properties, experimental soil was gathered from the shallow water area of Bohai Sea in China. The typical mineral composition and physical properties of experimental soil are given in Table 1. The simulated casing was produced according to the API standard pipe processing. Other instrument and pressure pumps are commercially available. 2.2. Method 2.2.1. Experimental device for evaluating the sealing ability The sealing ability of cementing sealing system was characterized by maximum gas pressure that can be sealed. The experiment was completed at the Education Ministry Key Laboratory of Petroleum Engineering. The experimental device are mainly comprises of simulated cementing sealing system and gas injection and pressure measuring system, which as shown in Fig. 1. 2.2.2. Simulated cementing isolation system 2.2.2.1. Simulated shallow formation in deep water. The test tank with length of 3 m, width of 3 m and height of 1.5 m was filled with experimental soil which had been compacted by electric hammers to simulate the shallow formation. The compressive strength of experimental soil was adjusted by the frequency and amplitude of electric hammer. After curing for 48 h at room temperature, the compressive strength was tested. A total of four soil models were produced and the compressive strength and porosity ratio are shown in Table 2. Porosity ratio is the ratio of pore volume to solid particle volume in experimental soil sample. The smaller the value of porosity ratio, the denser the soil sample is, and the higher the strength is. 2.2.2.2. The preparation of experimental casing. Because the depth of the shallow cementing section reaches several hundred meters, it

Fig. 1. Schematic diagram of the experimental device.

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Table 2 Compressive strength of experimental soil sample.

Table 4 The density and gas permeability of different cement samples after curing for 48 h.

Soil sample

Compressive strength/MPa

Porosity ratio

Cement sample

Density/g/cm3

Gas permeability/10

S1 S2 S3 S4

1.14 1.68 2.05 2.46

3.8 3.1 2.5 2.1

Class 325 cement Class 425 cement Class 525 cement

1.880 1.895 1.905

5.29 4.91 4.26

is difficult to carry out on-site field experiments. In addition, due to the big size of casing used for shallow cementing, it is also difficult to conduct full-scale simulation experiments. The casing size has three specifications, namely diameter 340, 508 and 762 mm. In order to study the effect of casing size on the sealing performance of cementing isolation system, a total of three sizes of casing were used for comparative experiments. According to the geometric size similarity principle, considering the experimental casing selection and site condition, the diameter of the experimental casing is reduced by a similarity ratio of 10:1. Three kinds of casing diameters of 34.0, 50.8 and 76.2 mm respectively were selected as the casing model for this experiment. The wall thickness of experimental casing is 2.54 mm, and the length of experimental casing is 100 cm. 2.2.2.3. Well cement. Cement slurry was mixed based on API Spec. 10B-3-2013 [17]. After being prepared, the slurry was poured in moulds and kept in chamber with water at room temperature for curing. The compressive strength of the same sample under the same condition was tested three times. The average value are shown in Table 3. After curing for 48 h, the density of different cement samples were tested by drainage method at room temperature and the permeability of different cement samples were measured by gas with confining pressure of 10 MPa at room temperature. The results are given in Table 4. As we can see, after curing for 48 h, the strength of different cement samples are obviously different, however, there is little difference in density and permeability between different cement samples. Furthermore, the permeability of cement samples are very low. 2.2.2.4. Simulated well locations. In order to reduce the interference of the boundary effect, and considering the size of the test tank as mentioned before, a total of nine simulated cementing locations were designed in the tank, which are shown in Fig. 2. In order to avoid the interference between two closed simulated well locations and the boundary of tank, the distance between two closed simulated well locations is 0.5 m and simulated well location and the wall of tank is greater than 0.5 m. The sizes of the nine wellbore which were drilled vertically were set as follows: The diameter of the wellbore at the 1#, 5# and 9# positions is 49.24 mm and is used to insert the 34.0 mm casing model. The diameter of the wellbore at the 3#, 4# and 8# positions is 66.04 mm and is used to insert the 50.8 mm casing model. The diameter of the wellbore at the 2#, 6# and 7# positions is 91.44 mm and is used to insert the 76.2 mm casing model.

3

mD

After the casing model was placed in the wellbore, cement with different compressive strength was injected into the annual space between the simulated shallow formation and the experimental casing. The compressive strength of 32.5 MPa cement was injected to the 1#, 2#, 3# simulated cementing position. The compressive strength of 42.5 MPa cement was injected to the 4#, 5#, 6# simulated cementing position. The compressive strength of 52.5 MPa cement was injected to the 7#, 8#, 9# simulated cementing position. After the cementing operation was completed, seawater was injected into the whole experimental pool and the depth of the seawater was 10 mm.

2.2.3. Gas injection and pressure measuring system The gas injection and pressure measuring system consists of air compressor, gas supercharger, pipeline and pressure sensor. Gas injection was achieved by air compressor and gas supercharger. The maximum injection pressure is 12 MPa. The pressure sensor set on the pipeline was used to test the instantaneous pressure of the gas breaking through cementing sealing system, and the measuring accuracy of the pressure sensor is 0.01 MPa. After curing the cement for a certain time, the air was injected into the experimental casing slowly by air compressor and gas supercharger. When the first bubble overflowed, observe the location of the bubble escape and record the pressure value of pressure sensor as the maximum gas pressure that can be sealed by the cementing sealing system, which as shown in Fig. 3.

2.2.4. The selection of experimental time point and sample Comparing the compressive strength of cement and soil sample, which as shown in Fig. 4. As we can see, after curing for 8 h, the compressive strength of all cement samples are lower than soil sample 4. When the curing time is 10 h, the compressive strength of two kinds of cement is greater than soil sample 4. After curing for 12 h, the compressive strength of all cement samples are greater than all the soil sample. Therefore, after curing for 8 h, soil sample 4 was choice to research the influence of cement strength which was lower than soil sample on the sealing ability of cementing isolation system. When the curing time reached and exceeded 12 h, the influence of soil strength and the cement strength which is higher than soil sample on the sealing ability of cementing isolation system were tested. Because the well test operation is usually conducted at 48 h after cementing, the experimental time was also extended to 48 h.

Table 3 The compressive strength of different cement samples. Cement sample

Class 325 cement Class 425 cement Class 525 cement

Compressive strength/MPa Curing 8 h

Curing 10 h

Curing 12 h

Curing 24 h

Curing 48 h

1.89 2.00 2.15

2.37 2.53 2.65

4.22 4.72 5.32

10.04 14.95 17.45

15.99 22.02 26.82

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Fig. 2. Schematic diagram of simulated cementing position.

Fig. 3. Schematic diagram of the sealing ability test.

Fig. 5. The location of bubble overflow after curing for 8 h.

Fig. 4. The comparison of compressive strength between soil sample and cement.

3. Results and discussion

the casing until bubbles leaking out from the cementing isolation system. Record the location of the bubble which as shown in Fig. 5. One can observe that, the bubble overflow position of each cementing isolation system appears at the sealing interface between cement and soil, and the bubble position distribution is random which can prove that the simulated cementing quality could meet the demand of experimental operation. And during the experimental process, there are no cracks within the soil, which means that the prepare method of simulated formation is reliable. Therefore, it can be inferred that the experimental data are scientific and credible. The value of maximum sealing pressure of different cementing isolation systems are given in Table 5.

3.1. Experimental result 3.1.1. The test results after curing for 8 h According to the selection of experimental sample, soil sample 4 was used in this test. After curing for 8 h, the air was injected into

3.1.2. The test results after curing for 12, 24 and 48 h After curing for a certain time, the air was injected into the casing and the locations of bubbles leaking out from the cementing isolation system were recorded as shown in Fig. 6. It can be seen

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Table 5 The maximum sealing pressure after curing for 8 h. Well number

Cement class

Casing diameter / mm

Maximum sealing pressure/ MPa

1# 2# 3# 4# 5# 6# 7# 8# 9#

325 325 325 425 425 425 525 525 525

34.0 76.2 50.8 50.8 34.0 76.2 76.2 50.8 34.0

1.89 1.87 1.86 1.94 1.95 1.96 2.06 2.06 2.05

that, when the curing time reached and exceeded 12 h, no matter what class of cement and what size of casing, the location of the bubble overflow just occurs in the soil surrounding the cement sheath, and is away from the sealing interface between the cement and the soil. The specific value of maximum sealing pressure of different cementing isolation systems are provided in Table 6. 3.2. Discussion 3.2.1. Influence of casing size on sealing ability of cementing isolation system The experimental conditions consisted of cement class, soil strength, curing time and casing size. When cement class, soil strength and curing time remained unchanged, the maximum sealing pressure data of cementing isolation system with different sizes of casing were extracted and compared. The comparable results are shown in Fig. 7. As we can see, some experimental results are quite different under different combinations of cement class/curing time/soil sample. However, in the same group of cement class/curing time/soil sample, the maximum sealing pressure of cementing isolation system with different sizes of casing are almost the same, which means that casing size has little effect on the sealing ability of cementing isolation system. And though the values of maximum sealing pressure are slightly different with the casing size increasing, the change of the value is not regular. For example, under the combination of Class 525 cement/curing for 12 h/soil sample 4, comparing all three kinds of casing size, the maximum sealing pressure of cementing isolation system with the casing diameter of 50.8 mm is the highest; however, for the combination of class 525 cement/curing for 48 h/soil sample 3, the lowest value was produced by the casing with the diameter of 50.8 mm. Therefore, it can be inferred that there is no direct relationship between the casing size and the sealing ability of cementing isolation system. 3.2.2. Influence of cement strength on sealing ability of cementing isolation system The main factors affecting cement strength include cement slurry formulation, cement class, curing temperature and curing time. In our research, only one cement slurry formulation was determined, and due to the limitation of experimental equipment, the experimental temperature was set as room temperature. Therefore, just the cement class and curing time were choice to make different cement strength. Considering the different stages of cement strength, the strength which is lower and greater than the formation were studied separately.

Fig. 6. The location of bubble overflow after curing for 12, 24 and 48 h.

3.2.2.1. When the cement strength is lower than formation. As we designed previous, after curing for 8 h, the maximum sealing pressure data of cementing isolation system with soil sample 4 were extracted and compared. The comparable results are shown in

Table 6 The maximum sealing pressure after curing for 12, 24 and 48 h. Well number

Cement Class

Casing/mm

Maximum sealing pressure/MPa Soil sample 1

1 2 3 4 5 6 7 8 9

325 325 325 425 425 425 525 525 525

34.0 76.2 50.8 50.8 34.0 76.2 76.2 50.8 34.0

Soil sample 2

Soil sample 3

Soil sample 4

12 h

24 h

48 h

12 h

24 h

48 h

12 h

24 h

48 h

12 h

24 h

48 h

1.26 1.26 1.27 1.29 1.29 1.30 1.36 1.35 1.36

1.26 1.27 1.27 1.30 1.31 1.32 1.38 1.39 1.39

1.27 1.27 1.27 1.32 1.33 1.33 1.40 1.39 1.40

1.75 1.75 1.76 1.78 1.77 1.79 1.78 1.82 1.79

1.76 1.77 1.77 1.78 1.79 1.78 1.82 1.79 1.81

1.77 1.75 1.76 1.82 1.83 1.83 1.84 1.85 1.84

2.15 2.16 2.14 2.15 2.13 2.17 2.17 2.20 2.19

2.19 2.23 2.22 2.23 2.25 2.25 2.20 2.30 2.25

2.25 2.24 2.28 2.30 2.32 2.28 2.34 2.28 2.35

2.59 2.59 2.56 2.74 2.80 2.80 2.86 2.91 2.85

2.66 2.72 2.67 2.82 2.81 2.80 2.95 2.96 2.93

2.79 2.79 2.74 2.86 2.90 2.87 3.00 3.01 3.00

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Fig. 8. The influence of cement strength on the maximum sealing pressure (curing for 8 h).

Fig. 9. The influence of cement class on the maximum sealing pressure.

Fig. 7. The influence of casing size on the maximum sealing pressure.

Fig. 8. Due to the casing size has little effect on the sealing ability of cementing isolation system, the average maximum sealing pressure value of different casing size in the same group of soil sample/cement class was calculated. It can be seen that, with the cement strength increase, the maximum sealing pressure increases. Comparing Class 525 and 325 cement, the cement strength increases by 13.76%, which causes the maximum sealing pressure increases by 9.82%. Furthermore, the bubble overflow positions are located on the interface between cement and formation (Fig. 5), which means that, due to the low strength, cement couldn’t form good cementation with formation. Therefore, when it is lower than formation, the cement strength decides the sealing ability of cementing isolation system. 3.2.2.2. When the cement strength is higher than formation. After curing for 12, 24 and 48 h, keeping the curing time and soil samples consistent, the maximum sealing pressure data of cementing isolation system with different cement class were extracted. Because the casing size has little effect on the sealing ability of cementing isolation system, the average maximum sealing pressure value of different casing size in the same group of curing time/soil sample was calculated and compared. The comparable results are shown in Fig. 9. One can observe that, with the cement class changing from 325 to 525, the maximum sealing pressure

increases, and the biggest increasing degree is 11.63%. However, the increasing degree of maximum sealing pressure caused by cement class is much lower than that of cement strength. For example, after curing for 48 h, the compressive strength of Class 525 cement is 67.73% higher than Class 325 cement (Table 3); however, the maximum sealing pressure of cementing isolation system under the combination of class 525 cement/curing for 48 h/soil sample 4 is just 8.30% higher than the combination of class 325 cement/curing for 48 h/soil sample 4. Therefore, it can be indicated that, when the cement strength is greater than formation, high-strength grade of cement can improve the sealing ability of cementing isolation system to a certain extent, however, the effect is not obvious. Keeping the cement class and soil samples consistent, the maximum sealing pressure data of cementing isolation system with different curing time were extracted. The average maximum sealing pressure value of different casing size in the same group of cement class/soil sample was calculated and compared. The comparable results are shown in Fig. 10. It can be seen that, except the combination of class 325 cement/soil sample 2, with the curing time increase, the maximum sealing pressure increases, and the biggest increasing degree is 7.36%. Just like the cement class, the increasing degree of maximum sealing pressure caused by curing time is also much lower than that of cement strength. For example, the compressive strength of Class 525 cement curing for 48 h is 404.14% higher than curing for 12 h (Table 3); however, the maximum sealing pressure of cementing isolation system under the combination of class 525 cement/soil sample 4/curing for 48 h is

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Fig. 10. The influence of curing time on the maximum sealing pressure.

Fig. 11. The influence of soil strength on the maximum sealing pressure.

just 4.17% higher than the combination of class 525 cement/soil sample 4/curing for 12 h. Therefore, it also can be indicated that, when the cement strength is higher than formation, extending the curing time can improve the sealing ability of cementing isolation system to a certain extent, however, the improvements are very slight. Furthermore, considering the little difference of maximum sealing pressure at different curing time, the result of having exception on class 325 cement/soil sample 2 might be caused by measurement error. According to the previous findings, the casing size doesn’t affect the sealing ability, and when the cement strength is higher than formation, increasing cement strength just can improve the sealing ability of cementing isolation system very slightly. However, the highest and the lowest values of sealing pressure provided in Table 5 are 1.26 and 3.01 MPa respectively. Therefore, the influence of soil strength on the sealing ability of cementing isolation system was researched continuously.

time can improve the sealing ability of cementing isolation system to a certain extent, however, solidifying and enhancing the loose formation is the most effective method to improve the sealing ability essentially.

3.2.3. Influence of formation on sealing ability of cementing isolation system In view of the slight effect of casing size and cement strength, during the process of studying soil strength, the factors of casing and cement were ignored. The maximum sealing pressure data of cementing isolation system with different soil sample were extracted. The sealing pressure distribution of each soil sample was compared. The comparable results are shown in Fig. 11. As one can observe that, under the influence of soil, the distribution regularity of the sealing pressure is notable, and with the soil strength increasing, the maximum sealing pressure increases, and the maximum sealing pressure value of each soil sample is relatively concentrated. Comparing with soil sample 1, the soil strength of sample 4 is increased by 115.79%, and the maximum sealing pressure of cementing isolation system with soil sample is increased by 115.00%. The increasing degree of soil strength and sealing pressure are almost the same. Therefore, it can be inferred that, when the cement strength is greater than formation, soil strength is the most important factor affecting sealing ability of cementing isolation system. Furthermore, when the curing time reached and exceeded 12 h, the location of the bubble overflow, which just occurs in the soil as shown in Fig. 6, also can prove that soil strength determines the sealing ability of cementing isolation system. In summary, the unconsolidated formation is the weakness of cementing isolation system in deep-water shallow formation. Increasing the cement strength grade and extending the curing

4. Conclusions In this paper, the influence of casing size, cement strength and soil strength on the sealing ability of cementing isolation system in simulated deep-water shallow formation were investigated, which could provide guidance and support for the design of cementing isolation system to ensure the safe exploration and development of oil and gas resources and ocean environment security. Based on the experimental results, the following conclusions can be derived: Casing size has little effect on the sealing ability of cementing isolation system, and there is no direct relationship between the casing size and the sealing ability. When the cement strength is lower than formation, the cement strength decides the sealing ability, and with the cement strength increasing, the maximum sealing pressure increases, and the sealing failure points are located on the interface between cement and formation. When the cement strength is higher than formation, increasing the cement strength grade and extending the curing time just can improve the sealing ability of cementing isolation system very slightly, and soil strength determines the sealing ability of cementing isolation system, and the sealing failure points are located in the soil. Therefore, solidifying and enhancing the loose formation is the most effective method to improve the sealing ability of cementing isolation system in deep-water shallow formation. Conflict of interest There are no conflicts of interest. Acknowledgments We would like to thank to Innovative Research Team in University (IRT1086), National Basic Research Program of China (2015CB251202), the Natural Science Foundation (51804332, 51434009), Shandong Provincial Natural Science Foundation (ZR2017LEE005), Shandong Provincial Postdoctoral Science Foundation (201702025), the Ministry of Science and Technology major projects (2016ZX05033004-006), the Fundamental Research Funds for the Central Universities (18CX02161A) and Qingdao Postdoctoral Applied Research Project (2016219) for funding this project.

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