The mineral composition of shale is complex. In addition to common clay minerals such as kaolinite, montmorillonite, and illite, it is also mixed with minerals such as quartz, feldspar, mica calcite, dolomite, pyrite, and apatite. Clay minerals are easily swelled when exposed to water, which affects normal drilling and causes downhole complexities such as the collapse of the borehole wall. At the same time, it causes drilling fluid pollution and reduces productivity during later production. This is the main reason for the use of oil-based mud.
If it is a vertical well, you can try to use water-based mud, but for shale gas, the development benefit of vertical wells is far less than that of horizontal wells. The development of shale gas mainly uses horizontal wells and most of them are horizontal wells along the direction of minimum principal stress. Rock hydration and expansion is easy to collapse, and it is easier to collapse in this direction. If it is a horizontal well, the anti-collapse performance of oil-based mud is indeed better than that of water-based drilling fluid.
1. Water-based mud is used in the drilling process. The shale itself is a brittle mineral, which swells in contact with water, which will easily cause the borehole to collapse and affect the quality of the drilling;
2. The non-organic pores of shale may be water-wet facies, and the gas in shale pores is easily displaced by water, which affects the quality of subsequent logging;
3. Water-based mud is easy to cause blockage, and the permeability of the shale interval itself is very low, causing changes in productivity;
4. Oil-based mud can support shale. Because the pores are small and blocked by capillary pressure, it is difficult for oil molecules to enter the organic and inorganic pores of shale.
Therefore, for drilling, oil-based mud equipped with a certain proportion of proppant is used as the drilling fluid, and for horizontal wells, rotary steering, well trajectory, and later cementing are all very critical, and it should be said to be a comprehensive package. Technology integration.