Pulse Dampener for Coreflood Experiments
Authors: Kerig Philip D. , Watson A. Ted 
Published: December 2007

A pulse dampener that significantly reduces the fluctuations frequently present in the flow from reciprocating piston pumps is described. Frequency-response methods are used to design the pulse dampener. A design procedure is presented to enable one to design the pulse dampener for the particular experimental situation of interest. This pulse dampener is effective when the pressure drop across the experimental system changes with time as well as when it is constant. The pulse dampener is also compact inexpensive and easy to construct.

"The first step in the design is to examine the pump to determine the frequency and amplitude of the fluctuations in the flow."

Introduction

Positive-displacement pumps are used extensively in research laboratories. Typical applications include liquid chromatography, kinetic experiments, filter design, and flow experiments on core samples from petroleum reservoirs. A constant flow rate is usually desired for purposes of analysis of experimental data. Syringe-type pumps are generally able to achieve this constant flow rate. One disadvantage of syringe pumps is that the cumulative injection volume is limited by the volume of the syringe and for many applications it is undesirable to stop the experiment to refill the syringe. For this and other reasons, pumps of the reciprocating-piston type are often used. Mechanical features of such pumps typically cause cyclic fluctuations in the flow from the pumps. For pumps with small pistons, and thus frequent reversals, these fluctuations can be as much as several percent of the nominal flow rate and can be an important error source when parameters in a mathematical model of the experiment are estimated from experimental data. An example is the estimation of relative permeability curves from laboratory displacement data on a porous rock sample. At a microscopic level, the displacement of one fluid by another in a porous medium is a function of local pore geometry and fluid pressures. Oscillations in the injection flow rate will cause local pressure oscillations. In turn, fluid may be displaced from individual pores at times that would be different from those for pulse-free injection. Furthermore, pressure oscillations may give rise to hysteresis effects. Such local phenomena are not represented in the macroscopic model used to represent the experiment in the process of estimating the relative permeability parameters. Thus, pulsing flow could be a source of error in estimating those parameters, the magnitude of which is not easily estimated. In situations for which derivatives of measured data are used in the estimation process, flow fluctuations may be a significant error source in parameter estimates because measurement noise is amplified in the differentiation process. Flow fluctuations can be reduced by use of a pulse dampener. We consider here a pulse dampener that is constructed of a fluid capacitance in series with a fluid resistance, as shown in Fig. 1. The accumulator initially filled with gas) acts as the fluid capacitance, with the coil of capillary tubing and the experimental system providing the fluid resistance. An expandable diaphragm, or bellows, separates the gas from the liquid in the accumulator. A backpressure regulator (which can be omitted) is included in this diagram and in the subsequent analysis as well. Because the accumulator may be operating at high pressure, it should be equipped with a pressure-relief valve and constructed of suitable materials. This pulse dampener works in the following manner. Changes in the pump flow rate correspond to changes in pressure. As the pressure of the stream coming out of the pump changes, the gas volume inside the accumulator expands and contracts, accepting fluid when the pump flow rate (and hence the pressure) increases and releasing fluid when the pump flow rate decreases. If the flow characteristics (i.e., the relationship between pressure drop and flow rate) of the experimental system do not change with time, then the gas volume will oscillate about a constant value. However, if the pressure drop across the experimental system changes with time (as in most coreflood experiments) and if this change significantly affects the pressure on the accumulator, then the gas volume will also show bulk changes over time. In this case, the flow rate through the experimental system will be smooth but not constant, an undesirable result. This problem is solved in the present system by using a sufficient length of capillary tubing and/or a sufficiently high backpressure so that the variation of the pressure drop across the experimental system is negligible compared with the pressure on the accumulator. In this paper, we show that the performance of this pulse dampener can be described mathematically by a nonlinear, first-order ordinary differential equation. A frequency-response approach is used to develop design criteria for the pulse dampener, taking into account the specific characteristics of the pump and the experimental apparatus.

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