| Title | In-depth Investigations of Casing-Cement System Failure Modes in Geothermal Wells Considering Cement Voids and Improper Centralization |
|---|---|
| Authors | Michael F. MENDEZ, Lihong HAN, Shangyu YANG, Catalin TEODORIU, Xingru WU, |
| Year | 2019 |
| Conference | Stanford Geothermal Workshop |
| Keywords | centralization, wellbore integrity, finite element analysis |
| Abstract | Hydraulic fracturing has been used extensively to enhance geothermal reservoirs. Well construction techniques are similar between geothermal, and oil and gas wells, although the former considers additional parameters due to the high temperatures. Failures in the casing and cement have been reported during hydraulic fracturing but their causes remain unclear. The cement sheath is used as a mechanical and hydraulic barrier to support the casing and to prevent premature failures that reduce well life. Hence, cementing operations represent one of the most vital steps while drilling for geothermal or oil and gas resources. Drilling fluid voids in the cement sheath can be formed during wellbore cementing operations. The chances of cement channeling will increase if the annular clearance between casing and formation, frequently called standoff, is reduced. Therefore, centralization is an important parameter in the long-term wellbore integrity considering that Enhanced Geothermal Systems implement directional drilling in the construction phase. The well-known annular pressure buildup (APB) problem, commonly experienced in wells subjected to thermal heating is caused primarily because of the expansion of trapped annular fluids that cannot be relieved. In this paper, a similar concept considering the contraction of these fluids during hydraulic fracturing will be discussed. Our work presents a Finite Element Analysis to evaluate the effects of differential temperatures and standoff values at a constant casing internal pressure and in-situ stresses. The modeling involves casing and cement interaction as a function of wellbore eccentricity (standoff). We were been able to demonstrate that an improper centralization, coupled with the temperature differences experienced during hydraulic fracturing, might increase the stress on the casing by more than 100%. These results might explain casing failures reported in geothermal wells. |