Pressure gradients are routinely used to determine fluid contacts. The accuracy of static formation-pressure measurements directly affects the estimation of original oil in place (OOIP). Depth errors, pressure gauge accuracy, gauge temperature sensitivity, gradient-fitting errors, capillary pressure, and compositional gradients are among the most prevalent sources of uncertainty. Most of them are well documented at lower temperatures and pressures, but, until now, fluid contact uncertainty in high-pressure, high-temperature (HPHT) environments has received little attention.
Formation testing pressure gauges are subject to a constantly changing temperature environment. Most gauges are temperature calibrated, but they often struggle to account for changes in temperature. This is particularly true in HPHT environments, where pressure and temperature are currently measured by different sensors. This leads to large and unrecognized measurement error. Additionally, the gauges’ specified accuracy is relatively low because the calibration covers a wide range of temperature and pressure. Such errors accumulate when multiple gauges are used in multiwell gradient extrapolations. Reduced dynamic response, repeatability, and stability further decrease the performance of today’s HPHT gauges. The resulting errors in fluid contact measurements in HPHT environments, which could be hundreds of feet, lead to billions of dollars of variation in reserves.
A new gauge technology addresses many of these potential errors, with specific application to HPHT environments. When pressure and temperature are measured by a single crystal, we will refer to as single-crystal dual-frequency-mode or dual-mode. The simple design of the new single-crystal dual-frequency-mode gauge increases the maximum pressure and temperature limits. The overall size of the gauge is reduced, which shortens its thermal dynamic equilibrium time. The stress-balanced and temperature-compensated dual-rotation cut angle of the resonator design concurrently allows operations at high pressure and temperature. In a temperature range between 200° F and 392° F at 15,000 psi or pressure up to 30,000 psi at 365° F, the gauge achieves 2 psi sensor accuracy, and 0.008 psi resolution.
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