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Summary Gas reservoirs are generally subject to non-Darcy effects, especially in the near-wellbore zone. In fact, the assumption of Darcy-flow regime is no longer valid because of inertial phenomena and/or turbulence. These could significantly reduce the peak performance of a gas well. Therefore, characterization and monitoring of the non-Darcy effects is key for defining an optimal reservoir-exploitation strategy. This is particularly true in the case of storage fields, where withdrawal- and injection-gas rates are typically very high (hundreds of thousands of m3/d) and determining and monitoring well performance is key to ensuring that deliverability meets demand and/or contract obligations. Pulse testing, which is dependent on a periodic variation of produced/injected rate, is an effective methodology to test a well during ongoing field operations without stopping production. Although pulse testing is very promising for monitoring well performance, it has never been exploited for this purpose. In this paper, the development of a method for pulse testing high-performing gas wells is presented and discussed. The pressure response to the imposed rates is analyzed in the frequency domain to evaluate reservoir and well properties. An analytical solution in the frequency domain taking into account wellbore-storage effects was derived. The method was applied to test a real gas well of a storage reservoir under two different pressure conditions to assess the effect of turbulence on deliverability. Although the pulse-testing technique might not replace traditional well testing for determining reservoir properties, it can be successfully applied to monitor well performance as a function of reservoir pressure.

Harmonic Pulse Testing (HPT) has been developed as a type of well testing applicable during ongoing field operations because a pulsed signal is superimposed on background pressure trend. Its purpose is to determine well and formation parameters such as wellbore storage, skin, permeability, and boundaries within the investigated volume. Compared to conventional well testing, HPT requires more time to investigate the same reservoir volume. The advantage is that it does not require the interruption of well and reservoir injection/production before and/or during the test because it allows the extraction of an interpretable periodic signal from measured pressure potentially affected by interference. This makes it an ideal monitoring tool. Interpretation is streamlined through diagnostic plots mimicking conventional well test interpretation methods. To this end, analytical solutions in the frequency domain are available. The methodology was applied to monitor stimulation operations performed at an Enhanced Geothermal System site in Pohang, Korea. The activities were divided into two steps: first, a preliminary sequence of tests, injection/fall-off, and two HPTs, characterized by low injection rates and dedicated to estimate permeability prior to stimulation operations, and then stimulation sequence characterized by a higher injection rate. During the stimulation operations other HPT were performed to monitor formation properties behavior. The interpretation of HPT data through the derivative approach implemented in the frequency domain provided reliable results in agreement with the injection test. Moreover, it provided an estimation of hydraulic properties without cessation of stimulation operations, thus confirming the effectiveness of HPT application for monitoring purposes.