Shale has a special fracture-pore structure,most of them are microcapillary pores with small pore throat and low permeability. It is difficult to form dense filter cake on the borehole wall, which leads to the increase of filtration loss and the easy hydration of rock, resulting in the collapse of the borehole wall.
Adding nanomaterials to drilling fluid can effectively seal nanoscale pore throat, so as to stabilize well wall and protect reservoir.
The size and concentration of the material used for sealing are the key factors, which should ensure no penetration ,and also can form thin and tough mud cakes.
Conventional plugging agents, such as bentonite and barite,their particle sizes way larger than the average pore throat size of shale, which can only solve the problem of micron-scale pores. They are difficult to prevent the filtration loss, and usually accumulate on the surface without going deep into the leakage layer, they neither achieve the sealing effect nor form a compact filter cake to reduce the filtration lose.
Nanomaterials have special properties of small size effect and surface effect, less consumption, in line with the development trend of drilling fluid to low solid phase or no solid phase, also have excellent lubricating property and rheological characteristic, which is beneficial to improve the dispersion performance and wetting performance of drilling fluid, The sealing effect is better, forming the mud cake thickness less than 1 mm.
(The difference of mud cake before and after adding rigid nanometer plugging agent）
Nanomaterials has extremely high surface activity , easy to combine with other atoms, stable with high chemical activity.
In addition, the particles of nanomaterials are generally 1-100 nanometers with evenly distribution, which are easy to enter the micro-fractures of shale, play the role of filling and stabilizing,can seal the fractures better and form a dense sealing layer to prevent filtration loss.
Core before adding plugging agent Core after adding 1%plugging agent
Due to their physical blocking effect, nanomaterials can improve the rheological of drilling fluids, improve their thermal stability, mud cake compactness, and lubricity.
Therefore, nanomaterials play an important role in drilling fluid, which can reduce filtration loss, enhance lubrication performance, improve formation bearing capacity, stabilize wellbore, protect reservoir.
