Abstract

The world's ever increasing demand for energy has forced operators to go after more difficult reservoirs. These challenging reservoirs place a greater demand on the cement sheath's ability to maintain a long term seal. To form an initially successful annular cement seal, effective mud removal is required. If effective mud removal can be achieved, the resultant cement sheath should be elastic and resilient enough to survive future downhole thermal stresses. Cyclic steam injection presents some of the harshest conditions under which a cement sheath's resilience and elasticity can be put to the test.

Derived from the industry's latest innovative science in sheath durability, a third generation cement design has been developed for a California heavy oil field. The paper will discuss the evolution from the initial wells to the improvements made for the second generation and the extensive testing and qualification process that went into the third generation slurry design. Best practices and lessons learned are covered.

Background

As in-place oil viscosity increases, its capability to flow to the well bore and up the production string decreases. The most common method used to change the flow properties of heavy oil is to introduce steam directly into the formation. As the oil absorbs heat from the steam, the viscosity decreases sufficiently to allow flow. In addition, the steam can increase or maintain formation pressure further increasing productivity. Pumping super-heated steam down the casing places extreme stress loads on the cement sheath. At the beginning of an injection phase the well is rapidly heated up, which causes the casing to expand. During periods when the steam injection is halted the well can cool, which causes the pipe to contract. This contraction places additional stresses on the cement sheath. When injection begins again, the stress cycle is repeated. If the cement sheath is not sufficiently elastic and resilient, the annular seal may fail during one of these stress cycles.

The reservoir produces -oil from depths of about 600 ft to 3,000 ft, depending on structural position. The reservoir is a biogenic silicious deposit consisting of the shells of diatoms with varying amounts of detrital material (principally clay and sand) so that individual depositional cycles are identifiable. In the upper reservoir intervals, ranges from 45–70% and permeability is 0.1–4 mdis approximately 1,000 ft thick. With increasing depth and temperature, the dominant mineralogic phase changes to porcelaniteciated reduction in porosity (<A A waterflood has been in operation since the late 1980s to mitigate reservoir compaction and improve hydrocarbon recovery. Vertical wells that are hydraulically fracture stimulated in stages over the gross interval are used throughout most of the field.

History

Prior to the year 2000, wells were cemented with conventional cement in conjunction with a universal fluid (UF-Slag) mud system that was pumped ahead of the cement. This was an effort to pump a heavier density conventional cement to combat the various stresses encountered in the wells. The theory was, if cement to surface was not achieved, the universal fluid would harden and provide adequate zonal isolation. Foam-Nitrified cement was used from 2001 to present.

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