The impact of wellbore cooling on reducing in-situ stresses is a well-recognized phenomenon in the oil and gas industry with emphasis on water flood activities or other operations where fluids are injected for secondary or tertiary recovery applications. Another aspect regarding wellbore cooling is the unintentional reduction of near-wellbore effective stresses that can occur in a shorter timeframe (hours, days or weeks) and lead to downhole losses during drilling and completions. Wellbore cooling and its associated risks to operations is greater in deepwater wells where 1) the drilling fluid travels through deeper water columns, 2) the in-situ formation temperatures are higher, 3) depletion is observed, and 4) some combination of the three.
During the drilling phase of a deepwater infill development well (Well A), downhole losses occurred 13 hours after well total depth (TD) was reached and required a bypass. Well A is in a field with all three of the above conditions with highest measured depletion of ~4,200 psi. Understanding the downhole losses at Well A was critical for Chevron U.S.A. Inc. ("Chevron") because four (4) upcoming development wells in analogue Field B would also encounter all of the conditions and would be the highest depleted sands in the target reservoirs that Chevron has drilled in the Gulf of Mexico (depletion in excess of 8,500 psi). Field B has been on production for years and reservoir heterogeneity adds additional complexity with some sands at virgin conditions. To reduce operational risks during the execution, a mechanical earth model (MEM) was used with analytical and numerical thermo-poroelastic simulations to assess thermal stress effects from wellbore cooling. Thermal stress effects were quantified to model temperature, overpressure and stress perturbations at and away from the wellbore wall in the depleted reservoirs. Operational constraints for degree of cooling, allowable circulation time while below cooling threshold, and amount of overpressure were provided to the Integrated Drilling Team (IDT) to help guide well planning, mitigation strategies, and execution. Mitigations included use of Managed Pressure Drilling (MPD) and incorporating MEM outputs into engineering work to help refine wellbore strengthening material, drilling strategy to reduce wellbore cooling, and reduce completions risks. With the use of a hydraulics software, wellbore temperature and exposure time to wellbore cooling were predicted for different sets of operational parameters to provide guidance to the operations team to stay within the operational constraints for wellbore cooling during drilling execution. Thermal stress effects were updated after the initial well was drilled to improve drilling strategies for the remaining three wells.
With strict adherence to drilling and circulation guidelines, all four of the development wells in Field B were successfully executed with no wellbore cooling related downhole losses, with the wells now on production and performing above expectations. This paper provides the methodology used during planning and execution to successfully mitigate the critical risks with 8,500 psi depletion.