Summary
Drilling wells in the pre-salt always poses a major operational challenge, directly related to the formation itself and, therefore, inherent to the scenario: fluid losses in carbonate reservoirs.
Due to secondary porosity, pores of varying sizes and distribution are found, which can be small or large, such as in vugs and caves. In addition to secondary porosity, there may be fractures or even microfractures in regions of the reservoir due to the stresses generated by the evaporitic rock structure above the reservoir.
All these characteristics of the pre-salt carbonate reservoir bring a major operational obstacle, which is the circulation loss.
To mitigate these losses or cope with them, different strategies are used, such as having large volumes of drilling fluid available, using Loss Control Materials and/or using Managed Pressure Drilling (MPD). It is important to consider that all these approaches bring additional costs to the operation and can be combined with each other to have the best performance, both in terms of time and cost.
In addition to the financial impact, drilling with losses brings some risks, such as: stuck pipe on account of poor cleaning, kick following a drop in the fluid level in the annulus, fluid exhaustion due to inefficient logistics, among others.
When drilling with MPD, the risks mentioned above are mitigated and the well is better monitored. Thus, the strategy to cure losses in wells with MPD must be reviewed, since, once the risk to well integrity is mitigated, decisions must be based on the strategy that generates the lowest cost.
This article proposes a new vision on the strategy to mitigate losses in wells with MPD, discussing the criticality of losses for well integrity, and recommends a more objective procedure of proposing strategies and decision trees for drilling with fluid losses.
