Mechanical stability analyses are mandatory to identify suitable candidates for openhole completions. These analyses should comprehend the effects of production drawdown and reservoir depletion state anticipated by the production plan. The understanding and modeling of the physico-chemical interactions between rock and flown fluids and their impacts on the rock mechanical properties is imperative in the openhole stability analysis of wellbores drilled in carbonate reservoir rocks, especially in the wells subjected to acid stimulation.
This paper presents a methodology for determining the impact of stimulation jobs in the confined rock mechanical response and thus on the acidized wellbore stability. The authors briefly discuss the main rock dissolution patterns, ranging from compact dissolution to homogeneous, passing through the wormhole generation. The homogeneous dissolution pattern has been pursuit in the laboratory because of: i) absence of confident methods to anticipate which other pattern should be more dominant in a given scenario; and ii) Its highest impact on the mechanical properties, leading to safe analyses.
As a result, 24 samples from an off-shore carbonate reservoir have been mechanically tested. 13 of these have been tested after a massive dissolution. The initial effective pore volume and its evolution due to dissolution have been registered and were applied as a geomechanics indicator. The triaxial test results show that the rock softens during injection with impacts on its kinematic parameters. The ultimate rock strength, otherwise, remained almost unchanged. An analytical expression has been devised to map the porosity variations in the wellbore surroundings based on the acid job operational parameters. It indicates that the main failure behavior of a uncased wellbore subjected to acidizing jobs or non-native fluid flows should be related to excessive near wellbore rock straining rather than brittle failures. In this scenario, a standalone completion scheme would be preferable against the barefoot because of its ability to provide mechanical support to the altered rock at the borehole walls.
The current dissolution-induced rock weakening model presents a significant increase in the understanding on how the acid job affects the carbonate rock mechanical response, building a solid base for the well integrity analyses in the production phase.