Basing on the CT tomography technique, the micro structure changes of Longmaxi Shale were studied in four different drilling fluids. They are deionized water, KCl-polymer drilling fluid, SM-Shaledrill drilling fluid and oil based mud. Experiments showed that micro fracture propagation which caused by fluid invasion along the micro-fracture is the main reason of shale wellbore instability. And micro fractures propagate directively and instantaneously, fractures are mainly developed in the previous two hours when immersed in fluids. And micro fractures are mostly parallel to the bedding plane of shale. The micro structure changes obvious differently when immersed in different drilling fluids. Micro fractures propagated most and fastest in deionized water, KCl-polymer drilling fluid is the next; and shale in SM-Shaledrill drilling fluid and oil based mud has no obvious change. As for the newly developed micro fractures, width of fractures maintain constant once developed. But the width of the existing micro fractures in shale have an obviously increasing process when contact with fluid.
国家自然科学基金资助项目(51490650)
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Figure 1
SEM results of Longmaxi shale. (a) Parallel to the bedding; (b) perpendicular to the bedding.
Figure 2
(Color online)
Figure 3
2D image at different times in deionized water.
Figure 4
(Color online) 3D image at different times in deionized water.
Figure 5
(Color online) Self-absorption effect of Longmaxi shale.
Figure 6
2D image at different times in KCl-polymer drilling fluid.
Figure 7
2D image at different times in SM-Shaledrill drilling fluid.
Figure 8
2D image at different times in OBM.
Figure 9
(Color online) Width analyse of newly produced fractures.
Figure 10
(Color online) Width analyse of pre-existed fractures.
Figure 11
(Color online) Fracture quantity statistics of Longmaxi shale.
石英 (%) | 黏土 (%) | 钾长石(%) | 钠长石(%) | 方解石(%) | 白云石(%) | 黄铁矿(%) |
25.7–57.2 | 19.7–47.0 | 2.1–3.9 | 3.6–7.8 | 2.2–7.5 | 2.0–3.9 | 3.1–7.6 |
|
试样编号 |
CEC (mmol/100g) |
比表面积 (m2/g) |
|
1 |
4.1 |
28.524 |
|
2 |
3.5 |
24.628 |
|
3 |
3.8 |
16.234 |
|
4 |
5.5 |
16.925 |
|
5 |
6.1 |
35.950 |
|
6 |
6.8 |
26.610 |
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