Underbalanced interventions in gas wells belonging to mature fields.
Argentina Tătaru, Sándor Balázs, Ion Foidaş
Vol. 34, no. 2 (2017), s. 553-566, 
The exploitation of natural gas fields from Transylvanian Basin started a century ago. The majority of these fields were discovered and developed in the last century, from the 1920s through the 1980s. So, these reservoirs have almost 100 years of production historical. Even if at their discovery, the initial reservoir pressures were relative high (around 100–150 bar), nowadays these mature gas fields have a very low reservoir pressure. For some of these reservoirs the pressure is 10–20% of initial values. With the rehabilitation projects ongoing, and where the challenge is to maintain or even better to increase by just a little the base production, the greatest challenge is to make certain interventions in depleted reservoirs wells. At the beginning it was not a problem to do workover for these wells because the completion fluids were lost in the reservoir and the reservoir pressures helped the wells to clean up. Now, because of the low reservoir pressures, after the interventions it would take a long time for the wells to be cleaned, or in the worst cases the wells could be abandoned. From time to time, some net pays have to be bypassed because the fluid can be lost in the reservoir. So the best method is to do workovers jobs in these wells underbalance or even with live well, and interventions using snnubing unit or the coiled tubing unit. This paper presents some of the technologies used by Romgaz to accomplish this goal and also some case studies from different types of interventions in these gas wells.