A new method for measuring the transmissivity of a single fracture subject to variable normal stress conditions is described. The method utilizes a flow pump to generate very low constant flow rates while the hydraulic gradient is measured with an electronic pressure transducer. The method is well suited to measure the transmissivity of core samples with natural or artificial fractures under radial flow conditions.
Measurements of low permeability materials have presented difficulties in the past to researchers because of the low flow rates resulting from gradients of interest or, alternatively, because of the high gradients necessary to produce measurable flow rates. Traditional methods involve applying a known hydraulic gradient across a sample and measuring the induced flow rate by monitoring the movement of a meniscus or an air bubble within a capillary tube. Problems arise with this technique due to the long time periods necessary for measurable flow rates and also the variation of capillary forces associated with the meniscus or air bubble caused by contamination in the tubing. Calibrated flow meters consisting of a ball inside a glass tube have also been used. However, their accuracy decreases to +10% for flow rates on the order of 10-3cc/sec.
An alternative technique was devised by Olsen (1966) in which a controlled flow rate is imposed through a sample and the hydraulic gradient across the sample is measured with an electronic pressure transducer. The flow is generated by an actuator consisting of a water-tight syringe driven by a variable speed gear motor. Infusion-withdrawal pumps available from the medical supply industry are commonly used. This technique allows one to generate flow rates of less than 10-Scc/sec with great accuracy. This method eliminates the need for direct measurement of flow rates and allows the time required for each test to be reduced substantially. Testing can be performed using low gradients similar to those which often occur in nature and in engineering applications. A flow pump combined with a sensitive pressure transducer improve the accuracy of permeability measurements in comparison to methods requiring measurement of very low flow rates.
