Proceedings World Geothermal Congress 2020+1 Reykjavik, Iceland, April - October 2021 1 Numerical Pressure Transient Analysis of a Low Permeability Well with Water Hammer Effects during Injection Fall-off Test: Belgium Case Study Azhari S. Adiputro 1 , Sadiq J. Zarrouk 2 *, Richard J. Clarke 2 , Virginie Harcouët -Menou 3 and Stijn Bos 3 1 Ulubelu Field, PT. Pertamina Geothermal Energy, Jakarta, 10340, Indonesia 2 Department of Engineering Science, The University of Auckland, Private Bag 92019, Auckland, New Zealand 3 VETO NV, Boeretang 200, 2400 Mol, Belgium *s.zarrouk@auckland.ac.nz Keywords: Deep geothermal well, fractured reservoir, PTA, numerical model, injection fall-off test, fractional dimension, water hammer. ABSTRACT Deep geothermal wells and geothermal wells targeting naturally fractured/fissured reservoirs often encounter low matrix permeability in the target reservoirs (pay zone). In some cases, extended pressure fall-off data of such wells display oscillations that cannot be easily interpreted by using the existing analytical pressure transient analysis (PTA) methods. Additionally, water hammer effects resulting from the shutting off the well are sometimes observed immediately after valve closure. A new numerical PTA framework using the TOUGH2 simulator (Pruess, Oldenburg, & Moridis, 1999) was developed to analyse wells with such a typical behaviour. The approach was applied to a case study in Belgium (well MOL-GT-03) for which extended injection-fall-off well test data were analysed. A 1-D numerical radial model with a fractional dimension (fractal) grid structure was set up. A PyTOUGH script (Croucher, 2011) was written for correcting the measured wellhead pressure field data to reflect reservoir conditions and for automating the models running and historical data matching (during the injection and fall-off stages). For well MOL-GT-03 the good match between the fractional dimension numerical model and the field data clearly indicated that the low reservoir permeability is dominated by small fractures and not rock matrix permeability. In addition, the water hammer effect (appearing as a sinusoidal wave) visible during the first 90 seconds after shutting down the injection test master-valve (very beginning of the pressure fall-off) was investigated by solving the wave equation. This new approach helps identify the extent of the well reach and provides a new estimate to the distance between the well and the reservoir boundary. 1. INTRODUCTION Pressure Transient Analysis (PTA) is a standard tool in the petroleum and ground water industries to analyse a well test dataset. Estimated permeability, wellbore storage, and wellbore skin can be obtained. PTA can also be used to identify reservoir boundaries. PTA is commonly used in geothermal well test analysis of injection fall-off tests. However, the conventional PTA, which based on analytical models of the diffusion equation, does not commonly match geothermal well test datasets (McLean & Zarrouk, 2015a). Numerical PTA is then developed to give more understandings of the geothermal well test (McLean & Zarrouk, 2017b). Table 1 lists few numerical model implementations when analysing several well test data. Table 1. Summary of numerical model implementation for different phenomena Phenomena Numerical model implementation Results of the numerical model References Presence of linear impermeable boundary in the reservoir 1-D radial model. Modification of volume and contact area between blocks. Similar response to analytical model equivalent. McLean & Zarrouk (2015c) Presence of a channel in the reservoir 1-D radial model. Modification of volume and contact area between blocks. Produced the same plot as analytical PTA but different result. Analytical PTA overestimates the skin. McLean & Zarrouk (2017a) Good match with a narrow channel reservoir model — approximate 5m width. McLean et al. (2018) Fractured formation during the injection-fall-off test 3-D grid model with horizontal and vertical fracture model. Vertical fracture model gives a perfect match with data. Bakar & Zarrouk (2018) In this work, an unusual injection/fall-off pressure response data from a deep geothermal well are analysed. The pressure fall-off data shows multiple oscillations (Figure 1 left), which cannot be interpreted using analytical PTA. Furthermore, the sinusoidal pressure wave (water hammer effects) shows in the very early fall-off time (Figure 1 right). In order to analyse this deep geothermal well behavior during the injection fall-off test, new numerical PTA framework is applied (McLean and Zarrouk, 2017b). The injection fall-off and water hammer will be analysed separately. Injection/fall-off pressure data oscillation will be analysed using a 1-D radial model combine with the fractal dimension model, to represent changing fracture