Abstract
A cascade casing collapse occurred in a well in the Santos Basin was caused by the thermal transient effect, as discussed on Moreira Jr. et al, 2019, leading to operational restrictions that might be mitigated by using ROV-drilled holes near the wellhead. This paper proposes a simplified conservative methodology to calculate structural loads in wells considering the annuli communication by ROV intervention. Hence the objective is to assess technical viability, as regards well integrity, aiming life extension of susceptible wells. A practical and simplified analysis method was sought using the WellcatTM thermal-structural simulator, due to a potential large number of wells to be evaluated. Steady-state loads were selected that conservatively represent the transient analysis for the initial moments of injection (using high flow and low injection temperature) in the four main proposed scenarios: ROV-drilled relief hole in surface casing, relief holes in surface and intermediate casings, relief holes in surface and intermediate casings plugging the surface casing at geothermal temperature, and relief holes in surface and intermediate casings, plugging the surface casing afterwards while injecting at cold temperature. It was possible to successfully apply the simplified casing design structural analysis method in five injection wells at Santos Basin pre-salt so far. It was possible to state that the adopted conservative simplifications did not make the operation of the wells unfeasible, since all of them were approved without restrictions in at least two of the four main evaluation scenarios. In two of the injection wells, the study brought the restriction to manage annulus pressure. In another well, due to a less resistant surface casing, plugging the surface casing was only approved if done at cold injection, because it would bring a casing cascade collapse threat if the plugging operation was done at a higher temperature (geothermal). It is also important to state that this ROV-drilled relief hole technology also requires attention concerning the type of mud on the trapped annulus (as synthetic mud must be recovered if not plugged to avoid pollution) and non-identified failures on the cement that are acting as a well safety barrier. Besides, the scenario of not plugging the surface casing may incur in external corrosion of intermediate and production casing strings. This work presents a new methodology for casing design applied to wells with ROV-drilled holes and brings example cases where the method was successfully used. Before, this technique was seen as viable, but there was no formal method to evaluate casing integrity in this condition. It is an enabling solution for injection wells where transient analysis indicates the possibility of cascade collapse failure, and a ROV drilling campaign is already being based on the proposed method.
