Risk Assessment of Exploration Well Designs in the Oman Ara Salt
Authors: Tallin A. G. , Paslay P. R. , Cernocky E. P. , Ratchinsky M. A. 
Published: December 2007

Petroleum Development Oman (PDO) proposed to eliminate the 7-inch intermediate casing set inside 9⅝-inch casing in deep, high-pressure, gas exploratory wells within the Ara salt of southeastern Oman. The presence of the 7-inch casing has been driven by the limited burst capacity of the 9⅝-inch casing under the historical design criteria of full evacuation to gas. The 7-inch casing could be eliminated and the BOP could be downsized if the burst design load could be limited to kicks of small volume. This design change had the potential to reduce dry hole costs of exploratory wells significantly by eliminating the sour 95 grade 7 in. casing string and its premium connections; reducing the rig time spent running that casing from the drilling phase; simplifying the wellhead; downsizing the BOP; and allowing a larger bit with higher ROP to be used to TD.

A quantitative risk assessment was performed in order to quantify the risk of limited kick design and justify elimination of the 7 in. intermediate casing and downsizing of the BOP stack based on burst design with limited kick volume rather than with full evacuation to gas. A final design strategy is proposed here based on the risk of a limited volume kick and the option to bullhead down the casing annulus. Operational recommendations are presented concerning the criteria for kick detection, the procedure for circulating the gas kick out of the well, and the determination of when it is necessary to bullhead the well. The risk assessment incorporated measured properties obtained from offset wells in the area of interest, mathematical models, and statistical methods. Extensive use was made of the well control literature, References 1–76.

The results of the risk-based design approach are: The estimated kick probably from the Carbonate Stringers in the Ara Salt is between 12% and 19% per well. The majority of these kicks will not involve an influx that is gas at the bottom of the hole.The probability of a kick influx being gas at reservoir conditions is very low. Based on an analysis of the available offset well data, it is less than 10−4 for typical stringer reservoirs in the Ara Salt.Even if there is a gas kick, the likelihood of a casing rupture is very low – about 10−3 for 20% worn casing because the shut-in kick volume is likely to be very small.Because of redundancies, the BOP system is expected to be very reliable. However, this observation depends on strong operational policies and good maintenance. PDO has a superior BOP maintenance program.While limited kicks are not expected to pressurize the BOP system near its rating, the greatest risk in moving to the new design is failing to maintain the BOP system reliability.The risk of catastrophic failures can be significantly reduced by a policy of bullheading large, intense kicks before they pressurize the casing.There is a significant probability that a circulated, limited kick will fracture the formation below the shoe. Even though this kick is safe to bring to the surface based on the casing loads, the salt below the shoe is on average significantly weaker than the casing. Based on several conservative assumptions, it is estimated that 30% of gas kicks will fracture the formation.This design change can and should be made: discard the 7 in. (drill) casing string inside the 9⅝ in. casing, and drill to TD out of the 9⅝ in. casing; also replace the 15,000 psi BOP stack with a 10,000 psi BOP stack; even consider the option of using a 5,000 psi BOP stack in conjunction with the bullhead criteria.Limit the 9⅝ in. shoe to 3700 m if up to 20% wear is assumed and to 4800 m if no wear is assumed (historical maximum observed wear was 12% at shallow depths). Then, there is 10−4 probability of not being able to bullhead and fracture a kick into the formation without rupturing the casing. The well design can be used to drill reliably from the 95/8 in. shoe all the way to 7,500 m providing the hole remains stable.