Foamed Cement Solution for Deepwater Tophole Challenges Offshore West Africa

Abstract One of the biggest challenges with cementing surface casings offshore in a deepwater environment is the requirement to place a cement column in unconsolidated formations with relatively low fracture gradients. This requires precise control of the equivalent circulating density (ECD) during placement so that it remains below the fracture gradient to help prevent losses. Rapid gelation and compressive strength development should occur quickly to help prevent leakoff of the cement into the unconsolidated formations. A major factor that affects ECDs, particularly in surface casings, is slurry density. A reduction in the density of the lead slurry is an effective method to control ECDs. However, densities below a certain limit prevent conventional water-extended cements from achieving the necessary gelation and compressive strength required to act as a barrier and fulfill the isolation objective. Low seabed temperature further complicates this. To achieve lower cement densities while meeting compressive strength requirements at low temperature, lightweight blended microsphere solutions (LBMS) or foamed cement are potential options. Foamed cement is a fine dispersion of gas in a cement slurry that includes a foaming agent. This three-phase system consists of cement, additives, foamer, nitrogen, and water. The foaming agent enters the already mixed base slurry through a suction created by the high-pressure downhole pump of the cement unit. This mixture then contacts energized nitrogen gas in the foam generator downstream of the pump before arrival at the wellhead. The result is a paste-like mixture with reduced density that, in turn, reduces hydrostatic pressure and overall ECDs. The foamed system does not maintain the same compressive strength of the base slurry, but the achieved compressive strength is adequate for isolation. The final foamed cement density depends on the foam quality, which relies on the base slurry density, gas quantity in the foamed cement, and nitrogen density. Foamed cement inherently possesses properties such as high viscosity, compressibility, low fluid loss, and stability. Once set, foamed cement achieves a high strength-to-density ratio, low permeability, and strong elasticity, which enhances bonding. For the tophole sections in the wells in this case history, foamed cement was chosen as an alternative to LBMS after the evaluation of factors such as rig bulk cement capacity limitations, logistical constraints related to field management of the two cement types, large LBMS volume requirements for successive cement operations in a four-well tophole campaign, uncertainty in slurry volumes required to meet seabed placement objectives, and the relative cost-effectiveness of a foamed cement operation compared to LBMS. This operation marked the first use of foamed cement for the operator in the area. Extensive discussions and detailed plans preceded execution. Rig surveys identified locations for the additional foam equipment on the rig and determined pipe work requirements. Plans included primary and backup nitrogen pumping units to address potential single-point failures. The liquid additive system currently onsite injected the foaming agent. Laboratory tests were performed on both the base and foamed slurries under field and well parameters and adhered to approved work methods for testing foamed slurries. Operations were successfully completed on four top holes in rapid succession, without deviation, per the approved programs. Cement returns reached the seabed for all operations, and simulations of top-of-cement analysis using the final circulation pressure confirmed the required height of tail cement to establish a solid shoe for the next section’s total depth. The successful application of foamed cement to address this challenge is outlined in this case study.