OPERATOR ERRORS AND GAS PERMEABILITY MEASUREMENTS

Gas minipermeameters are a commonly used to measure the permeability heterogeneity of rock and sediment outcrops. These instruments have a tip seal, which is manually applied to the outcrop surface for gas injection. Some syringe based permeameter designs require that the tip seal be removed from the outcrop surface between measurements. The quality of the seal made at the gas/material interface and the force applied to the tip seal influence the precision and accuracy of the data collected. To assess and control instrument precision it is common practice to average 3-5 measurements at a point. Removing and replacing the tip between each of these measurements changes the volume of sediment/rock sampled. Measurement precision decreases on outcrops with small spatial scale variations of permeability (eg, due to sandstone lamina). The manual application of the tip makes it difficult to align and use a constant force on the seal. The seal is not necessarily consistent from measurement to measurement and the flow field varies, perhaps even including some leakage. The result is inaccurate measurements, where the measured permeabilities are usually biased high. Previous investigators developed a pistol grip mounted tip seal, with a force gauge (to ensure consistent application force) and automatic controls (to alleviate the problem of removing the tip seal between measurements). To evaluate these instrument improvements this study uses a syringe-based gas minipermeameter to address two questions. 1) For a variety of sample types, is there a significant difference between measurements averaged with and without removing the tip seal between measurements? 2) Is there a significant difference between measurements made at constant application force and measurements in which hand application force is controlled subjectively and is unknown? Suites of data were collected by varying the method of tip seal application with and without removing the seal between measurements. Preliminary results indicate a significant increase in bias and noise when the tip seal is removed between measurements, while the results for different methods of controlling application force indicate no difference for skilled operators.