A Practical Approach for Determining Permeability From Laboratory Pressure-Pulse Decay Measurements

Abstract

Abstract A new pressure-pulse decay method is presented to measure permeabilities of tight rock samples. Existing pressure-pulse methods yield pressure decays which are either slow or difficult to analyse. Often approximations are made which may yield considerable errors in the derived permeability values. The new method is simple and enables very fast and accurate permeability measurements. The exact solution of the differential equation describing the decay curve resulting from a pressure-pulse measurement was analysed in detail. This showed how the decay curve is influenced by various parameters in the design of the pressure-pulse permeameter. In this way various existing designs could be compared. Furthermore, the analysis showed how a pressure-pulse permeameter should be designed such that the measured decay is fast and simple to analyse. Also an approximate solution was obtained which is accurate to within 0.3% of the exact solution which is much more difficult to handle. Using this solution a new and simple analysis procedure was developed. Combining the new design with the new analysis method results in an elegant method for fast and accurate pressure-pulse measurements. Introduction Permeabilities of reservoir rock samples are often measured using constant-flow equipment. However, for tight rock with permeabilities lower than 1 mD such experiments become very time consuming and inaccurate. Therefore, the pressure-pulse technique was developed in the past to enable measurement of permeabilities lower than 1 mD (ref. 1). In a pressure-pulse permeameter a cylindrical core sample is placed between two vessels (Fig. 1). The gas in the pore space of the sample is kept initially at the same pressure as the gas in the down stream vessel. The gas pressure in the upstream vessel is initially slightly higher. upon opening the valve connected at the upstream vessel, the pressure difference over the sample will decay as the gas flows from the upstream vessel through the sample to the downstream vessel. The measured decay curve (pressure difference versus time) is indicative for the permeability of the sample, as the decay will be slower for samples with lower permeability. The method is suitable for measuring effective permeability to gas in the presence of a liquid (ref. 2) and absolute permeability to both liquid and gas. Hydrostatic net confining stress is applied to the rock sample to simulate in situ conditions. Potential gas slippage effects (ref. 3) are reduced, because an elevated pore pressure is used. The pressure decay curve is described by a differential equation for the flow of the gas through the sample. To enable calculation of the permeability from the measured pressure decay, the solution of this differential equation should be known for the boundary and initial conditions involved. An exact solution of the problem was given in ref. 4. This solution is, however, difficult to evaluate. Several researchers, therefore, have proposed approximate solutions to the problem which are valid for certain ranges of the volumes of the upstream, downstream and pore volumes. These solutions have, however, several disadvantages. Some solutions are only valid when the porosity of the sample is negligibly small. Other methods are valid only for certain experimental conditions which are unfortunately not always fully mentioned. In most proposed methods the experiment either takes a long time, or the permeability value obtained is inaccurate. In the present paper a new method is proposed in which the experiment is carried out such that the resulting pressure decay curve is easy to analyse. This method allows fast experiments, resulting in accurate values of the permeability. P. 285^