This paper presents a new testing method for unsaturated, low permeability formations. Such formations are often considered as a host rock for radioactive waste disposal. As the test objectives require determination of the in-situ environmental rock properties, only gas injection can be applied as a test method. The new method is based on the classical solutions for slug test and pulse tests (modified slug test) with liquid as a testing medium. Introducing the real gas pseudofunctions - the real gas pseudopressure and pseudotime, it is possible to analyze the gas pulse injection or withdrawal test using type curve matching technique and estimate permeability and skin factors. The pressure derivative approach is implemented in order to diminish the ambiguity resulting from the matching procedure. The new method was applied during a testing campaign in the "Konrad" mine, Salzgitter, Germany, where the waste disposal is being studied in the ferruginous oolite formation. Field examples are included to illustrate the application of the method.
Increased attention is being directed toward the investigation of tight formations in relation to the storage and disposal of hazardous wastes. The first objective of investigation programs is the estimation of fluid-transmitting properties of the rock. This is usually achieved by in-situ permeability characterization by means of hydraulic packer tests. Numerous testing methods were developed and applied for investigation of acquifers and hydrocarbon reservoirs. However, their application for extremely low permeability zones results in significant test duration and consequently increases the cost of the testing program. For this reason a new test method - the pulse test - was introduced by Bredohoeft and Papadopulos in 1980. They applied the analytical solutions for the conventional slug test [Cooper et al., 1967, Ramey et al., 1975] for analyzing a "no flow" test pressure response. The test procedure is schematically shown in fig. 1. With the shut-in valve closed the fluid is removed (pulse withdrawal) from the tubing string to create underpressure. Then, the shut-in valve is opened, and shortly there after closed again and the pressure recovery is observed until a near static condition is achieved. A pulse injection test is similar except that the formation responds back towards static from an initial overpressure. As previously mentioned, the advantage of pulse tests is that the pressure response is three to four orders of magnitude faster if compared with slug tests [Grisak et al., 1985]. The pulse test is usually applied for testing of aquifers. For the purpose of hazardous waste disposal, however, the undersaturated or very low saturated formations are often being considered as a host rock. When testing such formations the usual requirement is not to change the environmental rock properties. Therefore, the gas as a testing medium can be used, since gas injection is not believed to influence strongly the initial (low) saturation conditions. The purpose of this paper is to present the modification of the pulse test theory that allows analyzing a pulse test conducted with gas.
