Effects of Drilling Fluid Soaking on Surface Morphology of Rocks at Different Temperatures

Soaking of rocks in drilling fluids often causes the surface morphology of the rocks to be damaged, which is probably related to the collapse of borehole walls. To reveal the relationship between the two phenomena, sandstone and shale samples were selected to test using 3D optical microscope for their surface morphology prior to and after soaking in drilling fluids at different temperatures. The characteristic parameters of the rocks were quantitatively analyzed and the effects on and damage to the surface morphology of the rocks by temperature investigated. It was found that as temperature increases, the characteristic parameters S_a, S_q and S_k also increase, while the characteristic parameter S_r decreases, resulting in increases in the roughness, discreteness and volatility of the morphologic contour of the rocks, reduction in the relief of the rocks and better symmetry of the rocks after soaking than before. In the temperature range of 30-120 ℃, the degree of morphology damage is gradually increasing with temperature, in the temperature range of 120-150 ℃, the degree of morphology damage is gradually decreasing with temperature. The Ts, which is the extreme value of the degree of morphology damage, lies between 120 ℃ and 150 ℃. Compared with sandstones, shale has a bigger relative degree of damage. Compared with the degree of damage of the rocks measured with texture characteristic parameters, degree of damage of the rocks measured with height characteristic parameters is bigger.