Description

The PTT700 is used to evaluate how a drilling fluid's filtrate penetrates shale formations over time under overbalanced pressure conditions. A cylindrical shale sample is placed in a core holder and exposed to a drilling fluid (test fluid) applied at an overbalanced pressure, meaning the pressure is intentionally set higher than the native pore pressure within the shale. As the test progresses, the downstream pressure is continuously measured to track how quickly the fluid filtrate penetrates through the shale matrix. The observed pressure build-up on the downstream side simulates the increase in pore pressure that occurs in the near-wellbore region during actual drilling operations. This dynamic is crucial, as it reflects the shale's response to fluid invasion over time. A mathematical model is used to analyze the pressure data and compute the shale’s hydraulic conductivity. From this, a delay factor is derived, indicating how much more slowly the test fluid infiltrates the shale compared to the native pore fluid, thus providing key insights into the fluid’s ability to maintain borehole stability.