Modern wireline pressure data can have resolution and reproducibility sufficient to detect small fluid-density changes and pressure barriers, yet these features are commonly overlooked on conventional pressure-depth plots. The large pressure variation caused by weight of subsurface fluids hides these subtle features. Excess pressure is the pressure left after subtracting the weight of a fluid from the total pressure. This concept is applied to wireline pressure data to remove effects of weight and emphasize subtle pressure differences caused by density variations and pressure barriers. Fluid-density changes of 0.02 g/cm3 or less can be resolved, and within-well pressure barriers in the order of 5 kPa (0.7 psi) can be detected. Using good-quality data, effects of reservoir capillary-displacement pressure can be detected by offset of the free-water level from the petroleum-water contact. This effect can be used to estimate reservoir wettability. Subsurface fluid-density measurements can also be used to evaluate oil or gas quality on a bed-by-bed scale in traps having variable oil or gas composition, to detect compartmentalization by small petroleum density differences, to verify quality of samples for PVT (pressure, volume, temperature) analysis, and estimate salinity or temperature of unsampled water zones.
Data quality limits barrier and fluid-contact resolution; thus, quality control is essential. Pressure measurement errors on the 3-kPa (0.5-psi) scale can be detected from behavior of the buildup pressure. Tests having the potential for small amounts of supercharge are identified from the overbalance and formation mobility. Examples illustrate identification of free-water levels and fluid contacts, fluid identification, supercharge identification, and water-zone compartmentalization.