SERVICE-LIFE MODEL DESIGN OF UTAH FORGE ENHANCED GEOTHERMAL MULTI-STRING CASING SYSTEMS BASED ON HPHT OIL AND GAS PRACTICES

Abstract

Geothermal well design plays an essential role while the world tries to benefit from the 24/7 clean energy originating from the core of the Earth. FORGE EGS development at the Milford, Utah site is one of these initiatives where the subsurface temperatures can reach as high as 446℉. Although these temperatures do not necessarily impose severe steel failures as much as those observed in superhot developments, the typical well delivery cost of an EGS easily exceeds that of unconventional oil and gas wells. Ignoring that high upfront CAPEX (capital expenditure) is necessary for the EGS well completions, the arising problem is whether the casing strings of these wells will perform long with integrity or not, endangering the return on investment. Since there is no globally accepted geothermal well design standard, very careful design practices need to be adopted from the HPHT (high-pressure high-temperature) oil and gas experience to ensure the long-term well integrity and success of the EGS. This study used internationally accepted API TR 5C3 and one of the most well-known geothermal standards, NZS 2403, to perform a model geothermal well design. The information sourced from the NZS 2403 confirms that the triaxial casing design may be performed for the design of geothermal well casings. However, the design factors stated by the NZS 2403 were considered in the first step as a minimum acceptable.Since the geothermal wells have a late return on investment, it must be ensured that a quality wellbore design is achieved for long-term well integrity and production objectives. While there is no internationally agreed-upon design standard for geothermal wells, this study proposes a model design using both API TR 5C3 and NZS 2403. Typical oil and gas well designs disregard the thermal profiles where the bottomhole temperatures are small. However, more critical explorations and developments in the HPHT reservoirs mandate the usage of thermal profiles during critical operations to ensure that the designed casing strings will endure the loading. For this purpose, the actual data from the completed FORGE wells 16A and 16B were used to simulate the thermal and hydraulic profiles for drilling and completion operations. After obtaining the temperature and pressure profiles using WELLCATTM software, the load cases have been applied to the installed casing strings against their API and VME triaxial envelopes using the same software. It has been accurately checked if the installed casing strings pass the working stress design, with their strengths and weaknesses being highlighted. Throughout the design process, WellPlanTM software was used to obtain the kick tolerance and casing running results. Once the current design of the casing strings and their weaknesses have been identified, their wear tolerances and rates were identified using CasingWearTM software as the second step in the design. According to the simulation results, the intermediate casings of both wells may undergo a higher amount of wear due to the directional wellpath and critical drilling parameters. This study is also unique in a way that it differs from pre-construction well design analyses; it incorporates post-wear burst, collapse, and axial/triaxial ratings to identify the weak tubular intervals under critical load cases. In addition to the need for more elaborations on the thermal profiles, the wellhead movement and annular pressure build-up concepts are typically adopted in the HPHT wells. The good practices incorporating these aspects of HPHT well designs have been considered in this study. In the third step of the design, the results on the wellhead movement have been obtained using WELLCATTM software based on the actual and planned long-term well operations’ pressure and temperature profiles. The purpose of this step was to decide on the selection of the anchor casing. It was decided that the innermost casing of the injector wells should be chosen as the anchor casing. Along with this, it would be better if the intermediate casing of the producer wells is chosen as the anchor casing to reduce the wellhead growth. Based on casing design envelopes undergoing the simulated pressure and temperature profiles, it was identified that the surface casing strings of the FORGE Wells 16A and 16B are over-designed. These surface casings showed an over-design phenomenon even under post-wear conditions. While it is appreciated that a more conservative design was adopted, a thinner or a lower-grade casing for the surface intervals may be chosen if desired. The intermediate casing of these wells showed certain under-design conditions, especially post-wear. Based on the modifications of the applied design factors and discussions, further recommendations have been made to optimize the design, where applicable. It was identified that the production casing of the producer well 16B is also over-designed in a way that it contains a sacrificial thickness up to 18% (against API burst) to eliminate the well integrity issues for the long term, should uniform metal loss occur. Further discussions in terms of the estimated corrosion rates have been made (Appendix B), however, more detailed corrosion studies are recommended to be done for future studies. Some under-design aspects were identified for the innermost injection casing of the injector well 16A, particularly under long-term cold injection and cold shut-in conditions. This injection casing has certain weaknesses against the cyclic thermal loading and should not be allowed to heat up and cool down abruptly, and has no wear allowance.