| Abstract |
Geothermal and hydrocarbons well construction is similar, although the primer may experience higher loads. Several papers have documented failures within the cement layer but, some situations remain unclear. It is accepted that the cement layer is used as a mechanical and hydraulic barrier to support casing and to prevent vertical and horizontal fluid migration. However, the main role of the cement is to create a seal for fluid movement behind the casing. Therefore, the wellbore cementing operation represents one of the most important operations while drilling for geothermal resources or oil and gas. Long-term wellbore integrity strongly depends on the quality of the cementing job, slurry quality, as well as the annular clearance between casing and formation - usually called standoff, with 100% standoff being the best case (concentric case) and 0% standoff meaning the casing is touching the borehole or the previous casing. With the advancements of Enhanced Geothermal Systems and the use of modern tools enabling directional drilling and formation permeability increase (via hydraulic stimulation), the construction of geothermal wells becomes a challenge. Cementing directional wells implies more difficulty because of increased frictional pressure losses when circulating the slurry, difficulties while running casing in hole and improper centralization. Both in vertical and directional wells, centralization of casing in the wellbore or in another casing is expressed as standoff (%). Through Finite Element Analysis, different scenarios have been handled in this paper covering increase of temperature (production phase) or change in pressure (fracturing job). A conventional vertical well and the curved segment of a directional well. Each scenario has multiple sets associated with different standoff percentages. The modeling was performed using the Finite Element Method and covers the casing and cement interaction as a function of standoff. We have been able to demonstrate that the lack of centralization may increase the stress on cement with up to 25% which could be the key to understand some reported failures in geothermal wells. |