Approximately 90% of the discovered fields in Iran are in carbonate reservoirs making this country as one of the largest carbonate producers in the world. Iranian depleted zones are increasing, as most of the fields become mature. High pressure drop in most of these fields such as Parsi, Karanj, Aghajari, Gachsaran, Bibi Hakimeh, Lab Sefid, Dehloran, Danan and Mal Kouh caused several drilling and production problems. The current condition for new operation challenges in Iranian oilfields emphasizes the significance of utilizing new tools and methods.
Implementing underbalanced drilling (UBD) technology in Parsi oilfield, located in south western part of Iran, may offer a solution to the challenges of operations of the field. Parsi has undergone substantial pressure loss over its history of nearly 50 years, and it is characterized by the highly fractured, vuggy Asmari formation. That combination of a challenging reservoir and low pressure has led to increasing problems for traditional overbalanced approaches. Many papers have been published discussing potential UBD advantages for southern Iranian oilfields; however, few papers are available about the challenges of this technology. Despite the increasing experience in this field, the value of UBD has not been fully acknowledged. This paper investigates the main UBD challenges in Parsi oilfield and summarizes the solutions to deal with them.
Successful application of UBD requires a detailed understanding of the potential benefits as well as limitations. Drilling activities in old fields have a paramount importance, either in trying to maintain the production level, or in reaching untapped hydrocarbon pools. Workover activity is also generally high(1). The challenge of working in depleted areas increases when zones with normal or original pressures coexist with the depleted ones. Several problems can be faced during drilling, completion, or workover that were not present when the fields were first developed. These changes often require modified procedures and techniques if the fields are to be developed economically. In some situations, fields will have to be abandoned if this economic development cannot be realized. One of the techniques developed to combat the difficulties of operating in depleted zones is UBD (2).
UBD has evolved into a new facet of the oil and gas industry. Some wells have been drilled underbalanced in southern Iranian oilfields since 2005 to reduce the drilling problems and maximize the economics of production. Effective UBD depends upon a complete understanding of the reservoir and its geology, as well as appropriate design and computer modeling of a multi phase flow circulating system. Severe losses, stuck pipe and other problems happened during conventional overbalanced drilling, made NIDC use UBD in Parsi (3), Figs. 1 and 2.Today operators are combating different challenges of UBD implementation worldwide with new tools, fluids, and techniques to make the operation more profitable. This paper summarizes not only the most known challenges happening during the implementation of underbalanced drilling but also concentrates on the most probable challenges in Parsi oilfield, Table 1.