Effect of the Drilling Fluid on Hardness Characteristics of Tight Sandstone

Abstract

To understand the effect of the drilling fluid on hardness characteristics of tight sandstone, indentation hardness tests were conducted on tight sandstone which were soaked in a water- or oil-based drilling fluid at different temperatures and time. The results of the study indicate that: (1) the hardness of tight sandstone decreased rapidly after being soaked in the water-based or oil-based drilling fluid and it decreased by 22.9% and 10.1% respectively after two hours; (2) with the increase in soaking time, the hardness remained almost constant when soaked in the oil-based drilling fluid. However, the reduction in hardness reached 33.1% after soaking in the water-based drilling fluid for 15 days; (3) there was little change in the hardness with the increase in temperature in the oil-based drilling fluid, but in the water-based drilling fluid, the hardness decreased at a temperature above 50°C; (4) high temperature would cause mineral expansion and hydration, resulting in hardness reduction with increase in soaking time in the water-based drilling fluid which would lead to the softening of the tight sandstone’s surface structure.