Horizontal wells – Underbalanced

Horizontal wells – Underbalanced

•Under balanced drilling can reduce lost circulation, minimize differential sticking, increase drilling rates, and, most importantly, create higher productivity completions because formation damage is minimized. •To maximize benefits, extreme care must be taken to keep drilling and completion operations underbalanced at all times. •The rate of return from wells drilled underbalanced is typically higher. •Techniques for drilling underbalanced include –Lightweight drilling fluids, –Gas injection down the drill pipe, –Gas injection through a parasite string –Foam injection. •Nitrogen is typically used because of its relatively low generation cost, scale control and low potential for downhole fires. •Nitrogen injection down the drill pipe is the most cost effective when electromagnetic measurement-while-drilling (MWD) is used. •Despite added cost and time, parasite injection of nitrogen is the preferred method when electromagnetic MWD is not possible. •Foams are more stable than aerated systems, but they are more costly

Horizontal wells – Underbalanced Mud Systems

•Lightweight Drilling Fluids. –The simplest mechanism to reduce hydrostatic pressure in the wellbore is the use of lightweight drilling fluids, such as fresh water, diesel or lease crude. –The primary problem with this approach is that hydrostatic pressure can not be reduced enough to remain underbalanced in many reservoirs. Gas Injection Down Drillpipe. –With this technique, air or nitrogen is added to the drilling fluid and it is pumped directly down the drill pipe. –Advantages of this technique include: •Hydrostatic advantage gained over entire vertical depth, •Wellbore does not have to be specifically designed for underbalanced condition, •Less gas is required to achieve given pressure compared to parasite injection •Penetration rate may be improved. •Gas Injection Down Drillpipe Cont –Disadvantages of this technique include: •an overbalanced condition may occur if the well is shut down •exotic MWD systems are required. •Gas Injection Via Parasite String. –With this technique, a second pipe is run outside of the intermediate casing. –Advantages of this technique include: •No operational differences, •Constant bottom hole pressure is achieved, and •Standard MWD equipment can be used. –Disadvantages of this technique include: •Additional costs are incurred, •Additional time is required, •Larger diameter surface casing is required. •Foam Versus Two Phase Flow. –A nitrogen foam system is less damaging to water sensitive formations and has been used on a limited basis. –The additional nitrogen requirements to generate stable foam have made this cost prohibitive in most cases. –Aerated systems with gas/liquid ratios varying from 10-to-1 to 50-to-1 are simple and flexible, but pressure control/gas surging can be a problem. –The margin of safety for aerated systems is typically larger than for more stable systems, such as foams. –Foams also exhibit some sensitivity to hydrocarbons, so large inflows of hydrocarbons can destabilize them. –Temperature limits of current foams, about 180°F, restrict the use of foam to depths less than 12,000 ft.