J. Cent. South Univ. Technol. (2008) 15(s1): 471-474

DOI: 10.1007/s11771-008-402-3

Experimental investigation of Daqing Oilfield mudstone’s creep characteristic under different water contents

HUANG Xiao-lan(黄小兰)^1,2, LIU Jian-jun(刘建军)¹, YANG Chun-he(杨春和)²,CHEN Jian-wen(陈剑文)²

(1. Institute of Porous Media Mechanics, Wuhan Polytechnic University, Wuhan 430023, China;

 2. Wuhan Institute of Rock and Soil Mechanics, the Chinese Academy of Sciences, Wuhan 430071, China)

Abstract: In order to analyze mechanism of casing damage, the uniaxial compression experiment and creep experiment of interbedded mudstone samples from Sanan development area of Daqing Oilfield under different water contents were carried out. The changes of the mudstone’s mechanical parameters and creep characteristics with the increment of water saturation were studied. The results indicate that the rock strength and elastic modulus decrease rapidly with the increment of water content, at the same time, the creep strain and creep strain rate of steady state increase with the increment of water content, and also the steady state creep strain rate is enhanced with the increment of deviatoric stress. Through the creep characteristic curves, a non-linear creep constitutive equation of mudstone considering the change of water contents is established, which will be used in future numerical analysis.

Key words: mudstone interlayer; casing damage; creep experiment; water content; non-linear creep equation

1 Introduction

Casing damage induced by water injection often happens in Daqing, Jilin and Yumen Oilfield^[1-2]. The correlative data indicate that 42.2% of casing damage is found in mudstone interlayers of Sanan and Xin development area, and 56.9% of casing damage is found in mudstone interlayers in Sanan development area, which are mainly in the form of creep extrusion failure. Moreover, it is found that all damage phenomena happen after high pressure water injection^[3].

The research shows that during the development of oilfield, especially for high pressure water injection, when water injection pressure greatly exceeds formation breakdown pressure, stratum will be fractured; when water injection is stopped, fracture is closed. Because of unsteady injection pressure, stratum fractures become open and close back and forth, which results in cement sheath of casing becoming cracked. And then, high pressure water will be injected to the surrounded mudstone interlayers along existed fractures, which makes injected water sufficiently contact with mudstone around cement sheath, and its water content increases quickly^[4]. The mudstone’s mechanical parameters and creep characteristic will change with the increment of water content. Furthermore, mudstone interlayer will produce additional creep deformation because of softening, so additional creep loading around casing becomes bigger and bigger with time going on. When stratum stress is heterogeneous, creep loading is also heterogeneous. Once heterogeneous creep loading is more than crushing strength of casing, casing damage will happen^[5].

In order to analyze the mechanism of creep extrusion failure, interlayer mudstone samples from Daqing Oilfield were chosen. After soaking, uniaxial compression experiment and creep experiment were carried out. Finally, the change laws of mudstone strength and creep characteristics were studied when the water content increases.

2 Experiment of mudstones under different water contents

Mudstone samples are taken from Sanan Development Area of Daqing Oil Field, and their embedded depth are about 1 100 m. All samples were divided into 3 groups: G891, 15 and 11, and five samples were chosen for each group. The dimension of all samples is about 25 mm in diameter and 50 mm in length. (shown in Fig.1). Firstly for each group, in light of rock test regulations for water conservancy & hydropower^[6], samples were soaked for 1, 3 and 5 d separately, and the water contents were calculated after samples were soaked in vacuum container. Finally compressive strength and creep experiments were carried out.

The basic information of mudstone samples of soaking experiment is listed in Table 1.

All the experiments were done by XTR01 automatic

Fig.1 Mudstone samples

Table 1 Basic information of mudstone samples

controlling electric-fluid serving compression machine (developed by Wuhan Institute of Soil and Rock Mechanics). The temperature was kept at 25 ℃. The errors of temperature and humidity were neglected.

2.1 Uniaxial compression experiment

Mudstones under different water saturations were selected separately in the three groups. The progress of uniaxial compression experiment is shown in Fig.2.

Fig.2 Uniaxial compression experiment of mudstone

Figs.3 and 4 show the results of compression experiment. From the comparison curves, it is revealed that both elastic modulus and uniaxial compressive strength of mudstones decrease rapidly with the increase of water content^[7-11].

Fig.3 Elastic modulus vs water content

Fig.4 Uniaxial compressive strength vs water content

2.2 Creep experiment

In order to find out the relationship between water content and creep characteristic of mudstone, samples from group G891 and group 15 were separately used in the creep experiment under the same stress path, and during the creep experiment, the loading of deviatoric stress was kept at 10 MPa. The creep strain curves of group 15 are illustrated in Fig.5.

Fig.6 shows creep strain of group G891 under different water contents. Because samples were limited, the creep experiments were done by two steps. Firstly, deviatoric stress was kept at 5 MPa. When the creep strain became steady, deviatoric stress was updated to 10 MPa until experiment was finished.

From Figs.5 and 6, it is obvious that under the same loading pressure, the creep deformation increases with the increase of water content. Fig.7 shows the correlation curves of steady state creep rate and water contents.

Fig.5 Creep strain under different water contents and the same deviatoric stress (group 15)

Fig.6 Creep strain under different water contents (group G891)

Fig.7 Steady state creep rate vs water content

From Fig.7, it is concluded that water content has great influence on creep strain and steady state creep strain rate of mudstones. In the state of nature or when water content is below 2%, creep characteristic of mudstone is not obvious, because steady state creep rate is not more than 10×10^-6/h. But with the increase of saturation, creep strain and steady state creep rate become greater quickly, especially when water content is over 5%, steady state creep rate is bigger than 30×10^-6/h, and creep strain is close to 100×10^-4. The reason is that when clay mineral of mudstone is filled with water, internal cohesion and strength will decrease rapidly^[12], and then its strong creep characteristic is represented.

For sample 15-5, with the same water contents of 3.85%, under different deviatoric stresses, the correlation between steady state creep rate and deviatoric stress is shown in Fig.8.

Fig.8 Steady state creep rate vs deviatoric stress for sample 15-5 with water content of 3.85%

From Fig.8, it is obvious that increase of deviatoric stress will lead to the augmentation of its steady state creep rate^[13]. It can be concluded that a power exponent function between the two variables exists. When deviatoric stress is enhanced, more fissures will appear^[14].

3 Creep constitutive equation of mudstones

From the uniaxial compression experiment and creep experiment of mudstones from Daqing Oilfield, it is concluded that mudstones deformation have the following characteristics:

1) Deformation after loading is composed of two parts: one is elastic deformation; the other part is creep deformation, which comes into being gradually with time, so basically elastic component must be included in the strain equation. Form the experiment result, it is found that modulus of elasticity and compressive strength are reduced obviously when water content increases, and also the equation should embody the debilitation of water contention on elastic component.

2) When analyzing creep law, it is mainly focused on the stage of steady state creep. The creep curves reflect increscent trend of strain with time, which means there should be a viscous component in the strain equation of mudstone; at the same time, steady state creep rate is affected by water content.

3) Based on the creep characteristic curves, steady state creep rate and deviatoric stress should be non-linear exponential relationship.

According to the above explanation, a non-linear Maxwell Eqn.(1) is adopted to describe the deformation characteristic of mudstones^[15].

         (1)

where  ε₁ is axial strain, (σ₁-σ₃) is deviatoric stress; W is content of water of mudstone, %; W₀ is content of saturated water in the state of nature, %; E,  and a are rheology parameters; t is time index; N is non-linear exponent.

During a long time, compared with elastic deformation, creep deformation is the main, and steady state creep rate can be used to describe the creep characteristic. In Eqn.(1), if creep deformation produced by hydrostatic pressure and creep dilatancy are not considered, after deduction, steady state creep rate is in the form of power exponent as follows:

                                 (2)

                              (3)

where is steady state creep rate of mudstone; ; S_ij is deviatoric stress tensor.

4 Conclusions

1) In natural state, mudstone has low water content and high strength, but once water is infiltrated, with the increase of water content, mudstone’s compressive strength and elastic modulus decrease rapidly.

2) Under the same pressure, the bigger water content of mudstone is, the bigger creep deformation and creep rate are.

3) Through the creep characteristic curves, a non-linear creep equation of mudstone considering the changes of water contents is established, which can describe how water influences creep characteristic, and this equation will be used to analyze casing damage.

References

[1] WANG Zhong-mao, LU Wan-heng, HU Jiang-ming. Casing failure mechanisms and protection measures of oil and water wells in oilfield [M]. Beijing: Petroleum Industry Press, 1994: 123-127. (in Chinese)

[2] HU Bo-zhong, XU Zhi-liang. Casing failure mechanisms and protection measures of oil and water wells in Daqing Oilfield [J]. Drilling & Production Technology, 1998, 20(5): 95-99. (in Chinese)

[3] LI Shi-bin, AI Chi. Study on water invasion law of shale in section of casing damage [J]. Natural Gas Industry, 2004, 24(4): 47-49. (in Chinese)

[4] LIU Zi-jin, WANG Xi-mao, ZHAO Fu-qiang. Analysis of casing failure protection measures in Daqing Oilfield [R]. Daqing: CNPC Daqing Petroleum, 1990: 3-8. (in Chinese)

[5] YIN Zhong-min, LIU Zhong-chun. Changes of rock structure after water flooding in Sanan Oilfield [J]. Scientia Geologica Sinica, 1999, 8(3): 321-330

[6] MAO Hai-jun, YANG Chun-he, HUANG Xiao-lan. Experiment study and theoretical analysis of slate under different saturated conditions [J]. Rock and Soil Mechanics, 2006, 27(9): 3253-3259. (in Chinese)

[7] LIU He, WANG Xiu-xi. Viscoelastic constitutive equation of mudstone and calculation of pressure on casing in Daqing Oilfield [J]. Journal of University of Science and Technology of China, 2005, 35(1): 118-123. (in Chinese)

[8] HE De-cai, ZHANG Hong, ZHANG Lai-bin. Investigation on the creep induced casing failure in water injection wells [J]. China Petroleum Machinery, 2005, 33(6): 17-22. (in Chinese)

[9] LU Bao-ping, LIN Yong-xue, ZhANG Chuan-jin. Laboratory study on effect of hydration to shale mechanics [J]. Journal of Geomechanics, 1999, 5(1): 65-70. (in Chinese)

[10] TAN Yin-jie. Elasto-plastic analysis of mechanism of dilatation of mudrock [D]. Harbin: Architectural and Civil Engineering Institute, Harbin Engineering University, 2005: 23-30. (in Chinese)

[11] YAN Xiang-zhen, YANG Heng-lin, YANG Xiu-Juan. The reason analysis of mudstone creep on casing damage [J]. Drilling & Production Technology, 2003, 26(2): 65-68. (in Chinese)

[12] HAN Xiu-ting. Casing damage mechanism in Daqing Oilfield [D]. Harbin: Architectural and Civil Engineering Institute, Harbin Engineering University, 2001: 56-58. (in Chinese)

[13] YANG Chun-he, DAEMAN J J K, YIN Jian-hua. Experimental investigation of creep behavior of salt rock [J]. International Journal of Rock Mechanics and Mining Science, 1999, 36(2): 233-242.

[14] GAO Xiao-ping, YANG Chun-he, WU Wen. Experimental studies on time dependent properties of rock salt [J]. Chinese Journal of Geotechnical Engineering, 2005, 27(5): 558-561. (in Chinese)

[15] HUANG Rong-zun, DENG Jin-gen. The viscosity coefficient of rheological formation and its influential factors [J]. Chinese Journal of Rock Mechanics and Engineering, 2000, 19(S): 836-839. (in Chinese)

(Edited by ZHAO Jun)

Foundation item: Project(2002CB412704) supported by the Major State Basic Research Development Program of China

Received date: 2008-06-25; Accepted date: 2008-08-05

Corresponding author: HUANG Xiao-lan, Doctor; Tel: +86-13886071061; E-mail: hxl_huang0226@sina.com