Current Science

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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.
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