Borehole stability in stratified shale

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Authors:

Yuan Cao, State Key Laboratory of Petroleum Resource and Prospecting, China University of Petroleum-Beijing, Beijing, China

Jingen Deng, State Key Laboratory of Petroleum Resource and Prospecting, China University of Petroleum-Beijing, Beijing, China

Baohua Yu, State Key Laboratory of Petroleum Resource and Prospecting, China University of Petroleum-Beijing, Beijing,  China

Chao Ma, State Key Laboratory of Petroleum Resource and Prospecting, China University of Petroleum-Beijing, Beijing, China

Abstract:

Purpose. Stratified shale formation is prone to collapse in drilling. A method of precise calculation of collapse pressure should be developed to avoid severe wellbore instability of stratified shale.

Methodology. Clay mineral components of stratified shale were measured by X-ray diffraction, and micro-structures of stratified shale were observed by scanning electron microscope. Shale cores had been drilled under different dip angles and then immersed in drilling fluids. Strength, cohesion and internal friction angle of stratifications and rock mass (vertical to the stratifications) were measured over different immersion time. The method suitable for calculating stratified shale strength has been developed. Based on the strength calculation method and circumferential stress distribution equation, the collapse pressure of stratified shale has been calculated.

Findings. The stratified shale is mainly composed of illite and kaolinite; and stratifications are actually 500 nm–30μm wide microcracks (stratified shale of Weizhou formation, Beibuwan basin, South China Sea). Cohesion and internal friction angle of stratification decrease in exponential rule with immersion time, while that of the rock mass decrease in linear rule with immersion time. The rock strength decreases firstly and then increases with the dip angle (0–90°); and the lowest value occurs at the dip angle 50–60; and single weak plane criterion is suitable for calculation of the strength of stratified shale. Contrasted to the drilling fluid density and wellbore stability situation while drilling stratified shale formation, it can be obtained that the numerical results of collapse pressure are quite precise.

Originality. Stratifications are microcracks developed in shale, and free water could seep inward shale through stratifications under capillary force. The method of calculating collapse pressure of stratified shale taking into account the seepage effect has been developed.

Practical value. The research results allow us to calculate precisely the collapse pressure of stratified shale under different deviation angles and azimuth angles. This contributes to optimization of drilling fluids density, wellbore trajectory and configuration while drilling in stratified shale.

References:

1. Deng, Jingen, Guo, Dongxu, Zhou Jianliang (2003), “Mechanics-chemistry coupling calculation model of borehole stress in shale formation and its numerical solving method”, Chinese Journal of Rock Mechanics and Engineering, no.1, pp. 2250–2253.

2. Oleas, A.M., Osuji, C.E., Chenevert, M.E. (2010), “Entrance pressure of oil-based mud into shale: effect of shale, water activity, and mud properties”, SPE Drilling & Completion, no.3, pp. 39–44.

3. Oort, E.V., Hale, A.H., Mody, F.K., Roy S. (1999), “Transport in shale and the design of improved water-based shale drilling Fluid”, SPE Drilling & Completion, no. 9, pp. 137.

4. Zhang, Jianguo, Al-Bazali, T.M., Chenevert, M.E., Sharma, M.M. (2006), “Compressive strength and acoustic properties changes in shale with exposure to water-based fluids”, ARMA Conference Paper, no. 06-900, pp. 1–12.

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7. Yang, Chunhe, Mao Haijun, Wang Xuechao (2006), “Study on variation of microstructure and mechanical properties of water-weakening slates”, Rock and Soil Mechanics, no. 12, pp. 2090–2098.

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9. Jin, Yan, Chen, Mian, Chen Zhixi. (1999), “Straight wells drilled through weakly consolidated formations”, Drilling & Production Technology, no. 3, pp. 13–14.

 

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