Abstract

The ultimate objective for cementing a casing string is to achieve long-term integrity of the well. This can be achieved through long-term zonal isolation between the penetrated formations with appropriate mechanical properties, low permeability, and shear bond strength properties provided by the cement sheath placed in the corresponding annuli. Experiences in the Hassi-Messaoud field have shown that zonal isolation is a very complex problem for some formations such as the LD2 zone, which exhibits, in some cases, corrosive fluids and highly saline formations as well as lost circulation zones. Cementing operations are critical and the need for cementing systems capable of solving these problems is crucial.

This paper focuses on the factors that have contributed to the potential failure of the existing cement system. Stress modeling has been used to select the appropriate cement system to overcome the challenges faced by the cement sheath in this type of environment. Stress analysis calculations, combined with mechanical properties testing of the cement system, has suggested the use of a novel cementing technology. This new system has been able to solve these problems to provide flexibility, improve corrosion control, expansion properties of the set cement, reduce the chances of microannulus creation, and therefore avoid overpressurization of the annulus casing. To ensure correct cement placement in the lost circulation zones, this slurry system has been successfully combined with fibers specifically engineered for cement slurries.

This technology has been a cost-effective solution for cementing 12 1/4-in sections in the Hassi-Messaoud field and 8 1/2-in sections in the Berkaoui and Hassi-Guettar fields. Field results have proven that significant workover costs, up to USD 1 million on wells in production, can be saved with the application of this technology.

Introduction and Hassi-Messaoud Field Characteristics

Zonal isolation is one of the primary objectives of cementing a well and if not achieved, may result in fluids migration to surface, microannulus, annular pressure development and alteration of the integrity of the cement sheath. This problem, if encountered, can be time and money consuming for the operating company causing workover and remedial work extra costs.

Algeria is a major hydrocarbons producer in Africa, producing currently 1,300,000 bbl/day of crude oil and exporting about 70 billion m3 of gas per year. By far the largest oil field in Algeria is Hassi-Messaoud, located in the center of the country, which produces about 350,000–400,000 bbl/day of 46o API crude, down from 550,000 bbl/day in the 1970s, but up from 300,000 bbl/day in 1989. The Hassi-Messaoud area contains an estimated 6.4 billion barrels, just below 60% of the country's proven oil reserves. It is expected to double production from the field to 700,000–750,000 bbl/day within the next 3 years. In order to satisfy the increasing market demand, the Hassi-Messaoud field is currently under important development.

The Hassi-Messaoud field is characterized by a very thick sandstone reservoir covering an area of 2000 km2. The producing layers occur at an average depth of 3400 m and are located in the Cambrian Ra, R2 and R3. In the past few years, horizontal wells in Hassi-Messaoud field have started to be drilled in order to optimize the reservoir contact.1 In most cases a 7-in. liner is landed at a depth corresponding to the top of Cambrian Reservoir Ra, corresponding at a deviation of 45° and a 6-in. drain is drilled to target depth (TD) inside the reservoir Ra, R2 and R3 to a measured depth (MD) of an average of 4300 m, corresponding to a true vertical depth (TVD) of 3400m and an inclination of 89°. The 6-in. section across the Cambrian is then completed by production tubing in open hole (Fig. 1).

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