Mature reservoirs in a state of decline (brownfields) account for 60% of the world's oil production. Even with declining production, and in spite of deepwater discoveries, these important reservoirs remain the most significant source of hydrocarbons and are the best hope for meeting future energy demands. However, the challenge in brownfields is improving in their traditional 35% recovery factor.

Brownfields are enjoying resurgence as smaller and more aggressive operators replace the majors in exploiting these declining reservoirs. With the majority of the world's hydrocarbons coming from fields 30 years old and older, the industry is looking for methods to increase ultimate recoveries while enjoying attractive returns on investment. So why are these maturing fields only recovering 30% of their hydrocarbons in place? One of the main reasons is formation damage, particularly in depleted reservoirs. Stimulation techniques are not always the answer. Underbalanced drilling (UBD) technology can eliminate most forms of drilling-induced damage to leave the reservoir as close as possible to its virgin state. In these cases, UBD delivers superior net present value (NPV) and ultimate recovery.

Technologies that cut drilling costs or boost production are especially important in brownfields because the declining productivity of these reservoirs leaves little margin for error. Horizontal wells have played a large role in revitalizing depleting reservoirs and, like UBD, took many years to gain acceptance.

However, horizontal wells and UBD are a powerful combination and one that will prove hard to ignore.
While horizontal well technology has played a significant role in extracting more hydrocarbons from these fields, it has had more than 35 years to perfect its application. Research by McKinsey & Co., a leading consultancy, has shown that the oil industry, for all its innovation, is inexcusably slow in implementation, with an average of 22 years, compared to 11 for broadband technology and eight for medicine. Energy demands are not going to give UBD 35 years to prove itself. The time is now.

Now well-documented, UBD properly applied improves the profit picture by cost-effectively:

• Reducing the cost of top-hole drilling to reach the pay zone;
• Reducing the cost of drilling within the pay zone itself;
• Increasing reservoir characterization and discovering bypassed pay;
• Increasing productivity by reducing reservoir skin damage; and
• Reducing the reservoir's ultimate abandonment pressure.

UBD for performance

UBD's classic role is also enjoying a renewal as the technology's performance features are called upon to decrease the cost of accessing the reservoir. Increasing rates of penetration (ROPs), extending casing points, preventing lost circulation and handling large water flows are familiar challenges and well-known to drilling optimization engineers. Non-reservoir drilling could be considered non-productive time (NPT), and reducing it, particularly in brownfields, is the main objective of UBD. In UBD, gas or multiphase fluid systems allow for low hydrostatic pressures that promote higher ROPs because unburdened rock shatters more readily than rock under pressure from heavy drilling mud. The lack of hydrostatic pressure on the formation also prevents lost circulation and differential sticking problems caused by fractures and thief zones.

Permian Basin, United States. In the once-prolific Permian Basin of West Texas, lost circulation is a significant and common problem. One example of its cost impact is in conventional wells, which typically lose thousands of barrels of drilling mud. This loss, combined with saltwater influxes of 2 bpm contaminating the drilling mud and causing shale swelling, was making the wells uneconomical. Switching to a UBD system using stable foam solved the problem: Saltwater influx no longer affected drilling performance, and no lost circulation occurred. One operator set a local drilling record of 8,000 ft (2,440 m) in 68 rotating hours, compared to more than 200 hours of drilling with contaminated mud.
United Kingdom. Perhaps one of the more striking examples of performance drilling has been a major operator's experience in the Brent field. When the operator decided to allow the field to depressurize, the depleting reservoir lowered the fracture gradient, causing significant lost circulation problems on new drills. UBD techniques were employed in the sandstone reservoir to drill 700 psi underbalanced relative to the pore pressure of interbedded shales. By eliminating the lost circulation while maintaining wellbore stability, dramatic savings were achieved. NPT was reduced from US $1.8 million to less than 1% of that. The techniques saved the program, extending the estimated life of this brownfield by 5 to 10 years.

UBD in the reservoir

When formation pressure is not enough to overcome the choke effect caused by skin damage, the well can no longer be produced economically. Thus, while reservoir drilling involveds many of the same issues as performance drilling (lost circulation, differential sticking and higher ROP) that allow longer horizontal sections to be drilled, the most critical factors for the reservoir are avoiding skin damage and lowering the reservoir's ultimate abandonment pressure.

The one factor that characterizes virtually all brownfields is reduced reservoir pressure; however, if an aquifer is maintaining pressure, then these fields are very likely encountering severe water management problems and their negative effects on reservoir performance. Horizontal wells have done a tremendous job in reducing these effects; unfortunately, the majority of them are still being drilled with conventional overbalanced systems. UBD is particularly suited to improving the productivity of horizontal wells because of the amount of time spent in the reservoir drilling the lateral. In low-permeability reservoir rock, permanent damage can occur with overbalanced mud systems.

UBD reduces skin damage to a bare minimum. Despite the empirical math, true "zero skin" does not exist. However, UBD holds the key to getting as close as possible and achieving the true potential of the payzone. Because the reservoir if flowing while it is being drilled, no invasion is occurring that could cause a reduction in permeability. As an additional benefit, the operator gains valuable early information because, in effect, he or she is performing a well test while drilling.

United States. The Hugoton field is one of the most prolific fields in the United States and is still being exploited despite a reservoir pressure of 600 psi. The severe depletion has created lost circulation problems with resultant formation damage and permanent productivity reduction. It is of special interest to note that, after 50 years of applying fracturing technologies in this brownfield, there remains no consensus as to which type is the most effective.

UBD, to the contrary, is allowing operators to drill extended lateral sections that were previously undrillable. The technology also has been applied in some reservoirs where fracturing would connect water zones. Currently UBD is used in the majority of all new wells drilled in the Hugoton, which are enjoying incremental production rates that are 300% to 400% greater than those of their conventionally drilled and completed cousins.

Eastern Europe. The brownfields of Peitu Siupariai, Diegliai and Pociai in the Gargdzai region of western Lithuania were exploited with vertical overbalanced wells over the past 40 years until Minos Nafta took over as operator and began studying and experimenting with new technologies, including fracturing and UBD. As a form of testing while drilling, UBD sometimes finds oil in unexpected places. For example, the uppermost zone in one field was not expected to be productive and was drilled through conventionally without detection of hydrocarbons. But while the field was being drilled underbalanced, this same zone suddenly started producing 4,000 bo/d - a 30-fold increase compared to a nearby offset well produced conventionally from the known pay zone. The oil influx from the new zone was so prolific that drilling was suspended, and the well was completed after 16 ft (5 m) of penetration. Field production soared from 2,000 b/d of oil to more than 8,400 b/d of oil.

Conclusion

While increasing energy demands provide some motivation for producers to apply new techniques, contractors are obliged to look beyond and reduce the barriers to their acceptance. Although no longer considered an emerging technology in North America, UBD is still a niche technology for the majority of the world. In recent years it has encountered and overcome numerous technical and management challenges, allowing its practice to spread quickly.

For the foreseeable future, brownfields will remain the greatest potential source of hydrocarbons, and UBD technology is crucial for recovering these trapped resources. While UBD technology has passed the safety hurdle and clearly demonstrated that it can provide superior economic returns, its application remains expensive compared to conventional drilling techniques. Looking to the future, new systems will be brought to the field that will reduce the costs of UBD applications and improve the chances of commercial success. Today, however, the tools and expertise exist to provide better risk management in candidate selection and project implementation that supports the decision to apply UBD. Asset managers should be looking to UBD and begin asking the same question they ask of horizontal well technology: "Why should I not drill this well underbalanced?"

Editor's Note: This paper was originally prepared as SPE/IADC 91725 and has been reprinted with permission.

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