| Title | Pressure Transient Analysis of an Injection/Fall-off Test Using Analytical and Numerical Reservoir Modelling |
|---|---|
| Authors | Renan Jhon GUERRA, John O'SULLIVAN, Jericho OMAGBON |
| Year | 2020 |
| Conference | World Geothermal Congress |
| Keywords | pressure transient analysis, SAPHIR, PEST |
| Abstract | Pressure transient data obtained from an injection/fall-off test are analyzed using pressure transient analysis (PTA) to derive reservoir permeability (k) and skin factor (S). Geothermal pressure transients are commonly interpreted using analytical models which assumes that the fluid flow in the reservoir is isothermal. However, for geothermal reservoirs analysis of pressure transient data is complicated by non-isothermal reservoir conditions. Numerical models to simulate geothermal pressure transients are therefore needed in place of analytical model which was developed mainly for oil and gas reservoirs. In this study the numerical reservoir simulator TOUGH2 was used to simulate the non-isothermal effects during injection/fall-off tests. The numerical PyTOUGH/TOUGH2 model set-up is demonstrated to represent homogeneous reservoirs, fissured or fractured reservoirs, and layered reservoirs. Pressure transient data generated by these models were used as simulated field data and analyzed using PTA software SAPHIR to derive k and S. A TOUGH2 model setup to run with the non-linear parameter estimation software PEST was also made to demonstrate an automated numerical pressure transient analysis method of the simulated pressure data. The results from SAPHIR (analytical) and TOUGH2 (numerical) were compared to the model values to quantify the difference in k and S. The PTA result shows a good fit on the simulated pressure data for both analytical and numerical models. The numerical models correctly estimate the k and S but the analytical models resulted to a high k and S compared to the model values. The numerical models were also used on a case study using field data from Leyte geothermal field, Philippines. Field data used are pressure fall-off transients from an injection/fall-off tests performed before and after acidizing of a well. The results of pressure transient analysis are examined to determine the effect of acidizing on the permeability and skin of the reservoir. The results of PTA on the actual field data shows a good fit on the analytical model but poor on the numerical model, both for the pre and post-acidizing injection/fall-off dataset. Numerical results indicate an increase in k but no change in k was obtained from analytical results. Skin factor indicate an increase for both analytical and numerical results. The numerical result is not considered reliable for this PTA due to the poor model fit especially on the fall-off dataset. A number of improvements in the numerical PTA method were identified and is subject for future work. |