Underbalanced Drilling Through Oil Production Zones With Stable Foam in Oman

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Abstract In July of 1992, Petroleum Development Oman embarked on a series of field trials to assess the operational feasibility and commercial benefits of drilling depleted oil production zones under balanced with "stable foam". These trials called for a review of standard well control philosophies and practices, and the incorporation of a flow control concept. Three trials have been conducted, the first in a vertical well and the others in horizontal wells. Two of the trials were performed successfully and productivity results are encouraging. The third foam trial had to be discontinued due to sloughing shales in the horizontal section. More trials are planned to enable the quantification of commercial' benefits. Introduction Within several of the existing fields in Oman, reservoir pressures have dropped substantially to levels below the hydrostatic pressure exerted by conventional water based drilling fluids, causing significant fluid filtrate invasion during drilling operations. It is known that fluid invasion into the reservoir can lead to formation damage and well productivity impairment. Furthermore, in some fields like Fahud (fractured carbonate reservoir), the large pressure overbalance during conventional drilling causes lost circulation. When this occurs, water is pumped down the annulus in addition to the water being pumped down the drill string so as to prevent gas influx while drilling. Typically, for an 800 m 6 1/8" horizontal section in the Fahud field with lost circulation at the start of the section, about 10–25,000m3 of water and 15–25 m3 of cuttings (depending on the extend of overgauge hole) are lost into the reservoir. Consequently, the wells take between 2–6 months to produce back the drilling water and reach peak oil production. It is also believed that this formation damage leads to reduced ultimate oil production. Fluid and solids invasion into the reservoir can be prevented by drilling at balanced or under balanced conditions. Following a study of air-assisted drilling fluid systems and the development of well control procedures, Petroleum Development Oman conducted three field trials with under balanced drilling through oil bearing reservoirs between July 1992 and September 1993.