Predesigned Bottomhole Pressure (BHP) in Aerated Drilling Matches in the Field and Improves Drilling Performance in Carbonate Reservoir
Authors: Shirkavand Farid , Hareland Geir , Behbahani Mohammad Haeri , Mostafavi Vahidreza 
Published: January 2008

Abstract
A field case analysis of under balanced drilling (UBD) in the "A" structure located in Southern Iran has been carried out in the present paper. It was initially reported that 51% of an average well's cost was Non-Productive Time (NPT). Typical recordable NPT categories and key performance indicators used include tight hole, tool failure, hole cleaning issues, well control and lost circulation. It was decided that aerated drilling could be applied with advantages such as higher penetration rates, less lost circulation and overall lower drilling cost. As part of designing these wells the bottom hole pressure (BHP) was minimized. This paper shows that in the planned UBD, pre-simulated BHP is in good agreement with the operational BHP. The well drilling design of mud and air rates and the corresponding pressures are in this paper have been plotted against the field recorded pressures for different mud and air rates. The result of the pre-simulations also revealed that there is an unfavorable range of mud flow rate that provides a low BHP of the aerated mud for different mud rate and air injection rates. By illustrating the BHP (dynamic and static), annulus back pressure and different mud rates, it has been shown that an optimum combination, of mud and air rates must be determined in order to maximize the penetration rate. The design process of BHP includes checking for required cuttings carrying capacity, which is determined by ensuring that kinematics energy per unit volume is enough for all planned rates.
The design method presented herein also suggests injecting air into mud during drilling of the lost circulation intervals as the best mud loss controlling method. The methodology and the calculation procedures used to pre-design the operation are presented herein with the field data against the pre-estimated. The results of this approach in the field have given reduction in NPT with some results presented herein.

Introduction
A well was drilled with an aerated drilling program to reach the "A" structure located immediately to the east of the central Iranian fault along with Dashtak and Kutah structures, in Fars North area. Close by, on the western side of the fault, other fractured gas and oil fields are located. During drilling of well "A-1", the larger challenge was controlling mud weight to avoiding lost circulation, tight hole and wellbore collapse (Figure 1). At the depth of 3850m, loss of circulation with rate of 90–170 bbl/hr was observed, which was controlled with LCM (Lost Circulation Material) after about 3.72 days. Drilling operation continued with 18–42 bbl/hr losses down to the depth of 3890 m and to 3928 m with 67.5 pcf mud and 8–21 bbl/hr losses. At 4242 m the mud weight was decreased to 65.5 pcf and well started to flow at 20 bbl/hr. At this point the mud weight was gradually increased to 67.5 pcf while continuous loss and salt water flow was occurring. It is believed that the well could only have been drilled with aerated drilling practices because of the specific challenges encountered.