Mechanisms causing injectivity decline in geothermal wells

The Upper Pannonian (UP) sandstone formation in Hungary has been utilized for thermal water production without reinjection since the 1960s. However, there is an increasing need for setting up geothermal doublets or triplets, where used water is expected to be reinjected into the same formation. Sustainable injection into porous sandstone rock formation is not straightforward. Thus, the Hungarian research and development project “Development of a well completion technology for sustainable and cost-effective reinjection of thermal water” aims at designing a methodology for sustaining the injectivity of geothermal wells in UP sandstone reservoirs. In this case study, we present an intervention approach of an old geothermal production well for reinjection at the Szentes Geothermal Field. Based on the evaluation of well conditions, the injectivity decline may be associated with reservoir characteristics, i.e., low transmissibility, mineral precipitation or local particle migration. Thus, a stimulation program was designed, where various techniques such as hydraulic fracturing, acid treatment and skin frac experiment were conducted. The novelty of this study is that to the best of our knowledge, the skin frac technique has been applied for the first time in a geothermal injection well worldwide. Based on the comparison of the various stimulation experiments, the injectivity reduction is related to phenomena in the near-wellbore area, i.e., mechanical clogging due to fines migration in the reservoir. The largest injectivity enhancement was observed due to skin frac experiment and pump lift, which was not part of the original stimulation program. The skin frac method has the potential for providing injectivity enhancement of recompleted wells in poorly cemented sandstone formations, but further field demonstration with optimized design is required. The results of this well treatment, such as regular conduction of pump lift and recommendations for proper design of skin frac treatment can be used in designing long-term reinjection tests in loose sandstone formations in geothermal reservoirs. Furthermore, future study should focus on the investigation of the link between pressure depletion due to overexploitation, subsequent compaction and land subsidence as well as injectivity problems.

Drilling and completing wells at minimum initial cost may not be the most cost effective way to exploit the geothermal resource. The impacts of completion practices on production and maintenance costs must also be considered. To evaluate alternative completion and workover technologies, a simple model has been developed that compares total well cost to total production or injection. This paper briefly discusses the model and focuses on results from its application to different completion and workover strategies. The model development project had three aspects: 1) the establishment of a data base for the cost and effectiveness of various geothermal completion and workover activities; 2) the development of a computer model built around these data; and 3) application of the model to specific cases. The data collected include geothermal production characteristics; initial costs and effectiveness of common workover equipment and operations; the frequencies of and times required to perform work-overs; etc. The model facilitates comparisons of completion and workover alternatives. Its primary purpose is the evaluation of potential impacts of proposed geothermal completion technologies in order to aid the management and direction of the DOE/Sandia Geothermal Technology Development Program. The results discussed include an analysis of the impact of variations in well lifetime. Additional lifetime beyond 10 to 15 years adds little value to the well. A comparison of mechanical descaling of geothermal wells to chemical scale inhibition indicates that for certain conditions chemical inhibition is more cost effective. Results of an analysis of injectivity decline are also presented, as are studies of original well cost, initial flow rate, and productivity decline for production wells. Other results involving underreaming, changing casing profiles, perforating, and hydraulic fracturing are also discussed.