Numerical analysis of pressure increase in injection wells within stochastically heterogeneous reservoirs

Geothermal conditions in the Pannonian Basin are among the most favorable in continental Europe, thus increasingly more geothermal projects are being launched in Hungary. However, reinjection is often burdened with practical problems, which can seriously decrease its efficiency, and endanger the entire geothermal project.

While most research tends to concentrate on local scale biological-mechanical-chemical problems (e.g. pore-scale clogging), we argue that reservoir (regional) scale heterogeneity of siliciclastic aquifers can pose an equally important risk to reinjection well efficiency. Significant reservoir heterogeneity affects (i) the size of the effective reservoir volume, (ii) the degree of connectivity between zones of high permeability, and (iii) the communication between production and reinjection wells, as a consequence, it can influence the success of reinjection.

Stochastic permeability distributions were generated using Sequential Gaussian (SGS) and Sequential Indicator Simulations (SIS) to represent the heterogeneity of a siliciclastic porous reservoir. The effect of lateral permeability heterogeneity was quantified on head increase in the injection well screened within the reservoir. The propagation of the front of the pressure increase is also monitored in the numerical simulations to make a comparison between the pressure evolution in homogeneous and heterogeneous reservoirs. The effect of the mean, variance and correlation length (i.e. heterogeneity scale) of the permeability distribution was investigated on the pressure increase and calculated using 2D transient numerical flow modelling with COMSOL Multiphysics finite element software package.

Model results show a pressure increase of several bars over the lifetime of the injection well, depending on all three statistical parameters characterizing the permeability heterogeneity. When the well is placed into a high-permeability zone (common operational practice), anomalous pressure rise occurs when the pressure front reaches low-permeability rock. Therefore, the success of the reinjection could also depend on the permeability distribution of areas farther away from the well. The onset time of the head increase observed in the well depends on the actual permeability realization, but, in general, greatly exceeds the time period of a typical pumping test. Insufficient preliminary data and/or disregarding the macroscale hydrogeological variations can lead to relevant overestimations of the injection well capacity. On the other hand, understanding the influence of aquifer heterogeneity on reinjection efficiency can contribute to more proper positioning of reinjection wells and to more successful injection projects.