Gasified Liquids
Limitations of Gasified Liquid Drilling
Formation Pressure and Productivity
Gasified liquid drilling is used to drill higher pressure and productivity formations due to its high drilling fluid density. Most of the time, the limiting factor becomes the pressure capacity of surface equipment.
If the reservoir pressure is high enough, even a single-phase liquid drilling fluid could be used for underbalanced drilling.
Wellbore Instability
Gasified liquids reduce the effective stress adjacent to the borehole wall due to higher pressure gradients. Many horizontal wells are drilled with gasified-liquids in weak formations. Wellbore pressure puts another limitation on the underbalanced drilling program. Using hydrocarbon-based liquids may reduce the chances of having chemically induced wellbore instability problems in shales.
Pressure Control
Downhole pressures typically fluctuate when tripping and making connections. Most of the time, these fluctuations do not cause any problems. If the borehole pressure increases above the formation pressure then damage is probable. Therefore, formation pressure sets the upper limit, whereas production potential, wellbore instability and surface facilities determine the lower limit.
Produced Water
While drilling with gasified liquids, the well can potentially be overbalanced near the bottomhole but underbalanced up the hole due to the variation in gas volume fraction with depth. Under these conditions, if a zone up-hole is producing large volumes of water, it can become very difficult to establish complete circulation.
If drilling continues very long after the inflow starts, water disposal may be a problem. If water production stops, re-injection is usually an option. Normally, produced water cannot be separated from the water based gasified liquid. Therefore, water based drilling fluids should not be used if water production is expected. Otherwise, the cost of maintaining gasified mud properties can be very expensive.
Corrosion
Corrosion is a serious problem when drilling with gasified liquids. Corrosion inhibitors have to be used to reduce the conductivity of the liquid phase that causes the corrosion to occur. Some inhibitors are ineffective above 100° F. They might even promote corrosion at temperatures of 300° F or above. Potential corrosion problems can be eliminated if natural gas, cryogenic nitrogen or hydrocarbon based liquid phase is used.
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