Harmonic Pulse Testing for Well Monitoring: Application to a Fractured Geothermal Reservoir

Abstract

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.

Aquifer Characterization and Uncertainty in Multi‐Frequency Oscillatory Flow Tests: Approach and Insights

“…Field studies employing oscillatory flow interference testing described in the literature use multiple approaches to generate the pressure stimulation. One approach uses a pump in a water storage tank at the surface and another in the borehole to alternate pumping and injection flow rates in a periodic manner, creating a square signal centered around either a flow rate of zero or a constant background pumping rate (Rasmussen et al 2003 ; Renner and Messar 2006 ; Salina Borello et al 2019 ). Using this approach, the amplitude of the observation signal is controlled by the chosen pumping and injection flow rates.…”

Period‐ and Pressure‐Dependent Hydraulic Properties of a Fractured Granite Inferred From Periodic Pumping Tests

“…Indirect hydromechanical coupling effects, which tend to be most important in fractured rocks, involve changes in the hydraulic or mechanical properties of the system in response to variations in applied stress or fluid pressure (e.g., stress‐dependent effective permeability and stiffness). In the context of the hydraulic characterization of fractured rocks, hydromechanical coupling effects, which depend on the structural, hydraulic, and elastic properties of the fractures and their embedding background, can range from almost imperceptible pressure variations to induced seismicity due to pressure‐induced fracturing or frictional failure along preexisting faults (e.g., Rutqvist & Stephansson, 2003; Salina Borello et al., 2019). Understanding and recognizing the role of hydromechanical coupling for fluid flow through fractures is, thus, crucial for improving the currently employed modeling and interpretation methods, and for reducing uncertainties in the analysis of experimental data related to fluid‐driven processes in fractured media (e.g., Ghassemi, 2012).…”

“…Time lapse monitoring is greatly facilitated by the periodic nature of the induced flow rates, which allows for the isolation of the probed formation's response from other hydraulic processes. Periodic pumping can even be combined with other injection operations such as, for example, hydraulic stimulation experiments and be used to monitor changes in the hydraulic response at specific oscillation periods (Salina Borello et al., 2019). The periodicity is an important characteristic of these methods, as varying the oscillation period, and thus the penetration depth L d of the pressure diffusion process ($${L}_{d}\sim \sqrt{DT}$$, with D and T being the diffusivity of the probed medium and the period of the PHT, respectively), allows for the assessment of the hydraulic heterogeneity in the probed formation (e.g., Cardiff et al., 2013; Fokker et al., 2013).…”

Stiff, Smooth, and Solid? Complex Fracture Hydraulics' Imprint on Oscillatory Hydraulic Testing

“…In contrast to constant‐rate pumping tests, oscillatory hydraulic tests can be designed in a mass‐conservative manner such that there is no net water extraction or injection into tested fractures, thereby minimizing alterations to the ambient stress and flow fields along the fracture (Cardiff & Barrash, 2015; Rabinovich et al., 2015). In applications that require continuous pumping (e.g., geothermal production), a periodic pressure signal can be superimposed at the pumping well by systematically varying flow rates above and below a long‐term pumping rate, allowing reservoir characterization to occur without interrupting production operations and minimizing revenue losses (Fischer et al., 2018; Fokker et al., 2021; Salina Borello et al., 2019). Finally, because the frequency of the input signal is known, recorded observation signals are readily extracted from instrument noise, instrument drift, and/or hydrologic noise (e.g., evapotranspiration and recharge events) using standard linear signal processing techniques (Bakhos et al., 2014).…”