Anti-caving mechanism analysis of saturated saltwater drilling fluid for high expansion tuff

To solve the problems with drill pipe sticking caused by high-expansion and collapse tuff, the drilling fluid has been developed in early stage research, and the anti-caving mechanism of the saturated saltwater drilling fluid is discussed in this paper. Here, by discussing the functions of the soluble starch, NaCl, NaOH, tuff extracting solution and the wall film components, the anti-water and anti-caving function of the drilling fluid is studied. By designing incremental solvent anti-sloughing experiments and comparing the dissolution ion state, the anti-dissolving function is studied too. Researches show that, the soluble starch of the formula flocculates along the crack when it loses water, and the flocculation may block the channels along which the solution go into the tuff, and, soluble starch provides the basis for NaCl crystallization; the adding of NaOH contributes to the NaCl crystallization and NaOH can prevent the salt solution; saturated NaCl can block the solution of the salt in tuff and NaCl crystallization can block the channels; the tuff extracting solution plays an important role in preventing dissolution of the salt solution, and in helping the crystal formation. After the well tuff absorbs water, under the common action of NaCl and other components, the hybrid film which contained NaCl crystallization is formed on the well wall. All the flocculation, crystallization and film prevent water infiltration into the tuff. The quantitative and qualitative analysis shows that the leaching of soluble matters plays an important part in tuff breakdown and collapse. The quantitative analysis of the leached material shows that, the drilling fluid not only prevents collapse and obstructs water obstructing physically, but also it prevents dissolution and collapse chemically.