Avoiding Sustained Casing Pressure in Gas wells using Self Healing Cement

Abstract

Abstract Khuff gas reservoir is one of the most challenging gas fields in the Middle East; in the past several wells suffered from sustained casing pressure due to the downhole stresses and other contributing factors. The severity of the leaking annulus can range from the most hazardous blowouts, to less severe cases of residual gas pressure at the wellhead; recent zonal isolation solution consisted in using Flexible and Expanding cement system in the 12 ¼" open hole to case off the 9 ???? Liner, this system is good when the well operating envelope is well known, outside this envelope the Flexible and Expanding cement do not perform as well, this leads the cement to fail (A.S. Al-Suwaidi et al 2008). To further ensure a robust cement design and long term well integrity; Self Healing Cement is introduced in the 9 5/8" tie back section as an added assurance to provide a secondary barrier for the 12 ¼" open hole. Self healing cement is based on a responsive material with intrinsic self-healing properties automatically activated upon hydrocarbon exposure to rapidly seal by healing the damage; within hours the downhole well integrity is restored, reducing the health, safety and environmental risks and the extra costs associated to remedy to these problems including loss of production. This paper will cover the qualification and field implementation of the self-healing cement. It also includes case history covering Khuff gas wells. Introduction Gas producer wells drilled to the Khuff formation [Fig.1] have always been a challenge when it comes to achieving zonal isolation. Loss zones, high temperatures, high pressured gas formations and shales are just some of the obstacles that are encountered in this type of wells. Khuff gas producer wells typically surpass the 15,000ft depth and and reach downhole temperatures of almost 149 °C [300 °F]. The main area of concern for these wells is 12 ¼" section. After a 13 ??? liner is typically set and cemented at 2530m [8,300 ft] in order to isolate the loss zones in Mishrif formation and the high pressured gas zones at Thamama, the 12 ¼" hole is drilled to 4114.8m [13,500 ft], 15.24m [50 ft] above the Khuff formations. This section is usually cased of using a combination of 9 ???? liner and 9 ???? tieback to surface. The main risk in this section is the potential for gas migration (90% methane) from some of the overpressured Arab formations. The last section is an 8 ½" open hole where a 7" liner will be cemented. The 9 ???? casing is considered a production string and is tested to 41368.54kPa [6,000 psi]. The slurry placed in this section is Flexible and Expandable cement that can withstand such pressures as well as the pressures experienced during the production during the life of the well without failing either in traction or compression. However, in the event that the downhole stresses are greater than predicted; the Flexible and Expanding cement can no longer ensure zonal isolation; Self Healing Cement (SHC) was introduced and used as a secondary barrier in order to restore zonal isolation in case of gas leaks resulting from the failure of the Flexible and Expanding cement.