Conventional overbalance drilling in naturally fractured reservoirs causes the invasion of mud and fine rock fragments into the fractures, which result to decrease the productivity of hydrocarbon from these reservoirs. In some cases, the sever problem of mud invasion into the fractured prevents flowing of HC's into wellbore after initial well completion especially in the depleted fractured Iranian reservoirs.
Underbalanced drilling (UBD), with a reduced exerted pressure down in the wellbore avoids the build up small solids being deposited on the sides of the hole, it also prevents the invasion of mud and fine rock fragments into the fractured formation which, together, help to improve the productivity of hydrocarbon from the reservoir. UBD method avoids many of the problems associated with the use of conventional drilling such as stuck pipe. Also, the rate of penetration increase by 2 to 5 folds by using UBD method, which resulted in low cost of drilling.
The main objective of this study is to investigate the use of UBD method in the fractured reservoirs in southern Iranian oil field in terms of drilling equipment's, drilling operation, drilling fluid, formation characteristics, and economic evaluation as compared with conventional overbalance drilling method.
Underbalanced technology, while still on a sharp growth curve, is finally becoming accepted as a "normal" method for handling the drilling and completion of wells .
Underbalanced drilling occurs when the effective downhole circulating pressure of fluid system in contact with the rock formation is lower than the existing formation pressure. UBD has been utilized to minimize formation damage due to invasion of mud solids and fluid filtrate into permeable and fractures formations. However if the drilling fluid does not have the adequate invasion property, the severity of losses to the formation could increase in underbalanced conditions are not maintained at all times .
Special equipment and procedures are required to control formation fluid inflow during underbalanced drilling. Nevertheless, drilling underbalanced offers several significant benefits over conventional drilling techniques. These include [1, 5]:
Increased penetration rate and bit life,
Minimized lost circulation while drilling,
Improved formation evaluation,
Reduced probability of sticking the drillstring downhole,
Increases well productivity, and,
The requirement for primary stimulation treatments can be reduced or eliminated.
Also the best formations candidates for underbalanced drilling include [2, 3]:
Any formation subject to impairment: particularly naturally fractured reservoirs drilled with straight hole or horizontal laterals.
Lost Circulation: Major problems with lost circulation suggest using underbalanced drilling procedures. If the interval to be drilled is sub-normally pressured, it will be necessary to use a lightened fluid to minimize or avoid significant fluid loss.
Penetration Rates: If offset well data indicates low penetration rates for mud (for example, less than fifteen ft/hr) and the formation are hard, dry gas, mist or foam are potential alternative to mud.
UBD techniques include: Gas Drilling, Stable Foam and Stiff Foam Drilling, Gasified Drilling and Flowdrilling. Parsi field located in South of Iran was selected for this study.