Application Of The Pulse-Decay Technique To The Measurement Of Heavy On Core Fluid Mobilities And Porosity

Abstract

Abstract The pressure pulse-decay method is a very useful technique for estimating injectivity and assessing permeability damage in preserved heavy oil cores in the 10 °C to 50 °C range. At present, there is no pressure decay model in the literature which is specifically designed for heavy oil displacement experiments. This paper addresses this need, dealing with the fundamentals a new test apparatus and the theory for measuring both fluid mobilities and porosity from a single pressure pulse decay test. The resulting model is valid for all values of time and has no limitation on the pore volume to upstream test apparatus volume ratio, R. The effects of small hold-up volumes are also addressed. The validity of this method is verified by conducting pulse tests on a sandpack and two preserved heavy oil cores. In these tests, the average difference between interpreted and extracted permeabilities was 10%. The average difference between interpreted and extracted porosities was 6%. The accuracy of this technique improves with increasing values of R. Introduction The pressure pulse-decay technique has so far been restricted to the testing of tight rocks or tight gas sand core samples(1–6). However, this method offers several attractive features in heavy oil core analysis. First, it is ideal for assessing core damage because only very small volumetric flows typically of about 0.1 cc are involved. Second, this method is capable of measuring total fluid mobilities in cores containing heavy oil at temperatures in the 10 °C to 50 °C range. This provides estimates of injectivity in unsteamed heavy oil reservoirs which can in turn be used to evaluate steaming strategies. In short, there exists a strong need to develop a pressure pulse decay model for heavy oil applications. The essence of this method is to deduce core data from the pressure drop vs time history of a fluid-filled core that received a pressure pulse at one end. Three configurations are possible in pulse-decay testing. First, a constant upstream pressure can be used to increase the pressure in a downstream vessel through a core. Second, a downstream constant pressure sink can be used to draw down the pressure of a closed upstream vessel through acore. In a third configuration, both upstream and downstream pressures can be allowed to vary. The above systems are defined as types I, II and III, respectively. The pulse-decay models in the literature are restricted to gas or water flow. An early model, due to Brace et al.(1), assumes that core pore volume is negligible compared to the downstream volume in a type I system or compared to the upstream volume in a type II system. Rock compressibility is assumed to be small compared to that of the pore fluid. Subsequent work has been aimed at removing one or both of these restrictions through analytical(2,4) and parametric studies(3,5). Chen and Stagg(6) have developed a theory which is suitable for testing tight gas sand samples and is valid for all times and which has no restriction on the pore volume.