Effect of Weak Bedding Planes on Wellbore Stability for Shale Gas Wells

High-pH drilling fluids are often used for drilling shale gas wells. Alkali erosion of shale is one of the important factors of wellbore instability. Alkali erosion experiments of different kinds of minerals and shale were conducted in this paper. Experimental results show that the corrosion rate of kaolinite is the highest when the pH is 9, the corrosion rate of smectite is the highest when the pH is 10 or 11, and the corrosion rate of the quartz is the highest when the pH is 12. Both shale particle size and concentration of hydroxide ion all affect the reaction rate, and the former has a negative correlation with the reaction rate, and the latter has a positive correlation with the reaction rate. In addition, alkaline erosion can lead to the fracture propagation along the bedding planes of shale, which can easily result in wellbore instability. This study may offer some theoretical basis for wellbore instability induced by high-pH drilling fluids.

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Experimental Study and Stabilization Mechanisms of Silica Nanoparticles Based Brine Mud with High Temperature Resistance for Horizontal Shale Gas Wells

Journal of Nanomaterials ◽

10.1155/2015/745312 ◽

2015 ◽

Vol 2015 ◽

pp. 1-9 ◽

Cited By ~ 8

Author(s):

Xian-yu Yang ◽

Ye Yue ◽

Ji-hua Cai ◽

Yue Liu ◽

Xiao-ming Wu

Keyword(s):

Thermal Stability ◽

High Temperature ◽

Silica Nanoparticles ◽

Shale Gas ◽

Wellbore Stability ◽

Fluid Loss ◽

Gas Wells ◽

Temperature Resistance ◽

Pressure Transmission ◽

High Temperature Resistance

Previous studies showed that silica nanoparticles based fresh water drilling muds had good thermal stability up to 160°C; however its performance at high salt concentration was rather poor. Therefore, high performance silica nanoparticles based brine mud (NPBMs) with high temperature resistance for horizontal shale gas wells was proposed. Thermal stability tests from ambient temperature to 180°C, along with pressure transmission tests and rheology analysis, were performed to evaluate comprehensive properties of the NPBMs. Results show that the NPBMs embody excellent salt tolerance and thermal resistance for their rheological parameters did not suffer significant fluctuation. Fluid loss of the NPBM-1 (4% NaCl plus 3% KCl) at 180°C was only 7.6 mL while the NPBM-2 (10% NaCl plus 3% KCl) had a fluid loss of 6.6 mL at 150°C. Low water activity and good lubricity of the NPBMs were beneficial to improve wellbore stability and reduce friction resistance. Pressure transmission tests on the NPBM-1 show that it can mitigate or even prevent the transmission of drilling mud pressure into shale thus improving wellbore stability. Additionally, optimal rheological models for the NPBM-1 and the NPBM-2 were Herschel-Bulkley model and Power Law model separately.

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