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To alleviate excess gas production in the Prudhoe Bay field, a non-rig gas shutoff Prudhoe Bay field, a non-rig gas shutoff technique using sodium silicate has been developed. An activated sodium silicate mixture is squeezed through coiled tubing into the gas producing intervals where it gels into a gas producing intervals where it gels into a gas impermeable barrier. In addition, a temporary bridging agent has been successfully used to isolate oil producing intervals during a squeeze. This technique utilizes coiled tubing and can be accomplished for approximately 25% of the cost of a standard rig workover at Prudhoe Bay.
A non-rig gas shutoff technique using sodium silicate has been developed for the highly permeable matrix of the Prudhoe Bay field. The permeable matrix of the Prudhoe Bay field. The technique utilizes coiled tubing to squeeze an activated sodium silicate mixture into gas producing intervals separated by a competent producing intervals separated by a competent shale from the oil producing intervals. Once squeezed into the intervals, the mixture gels into an impermeable barrier according to a time/temperature dependent reaction. A temporary bridging agent can be used to isolate and protect oil producing perforations during the squeeze from the sodium silicate squeeze material.
Upon discovery, the Prudhoe Bay Sadlerochit formation contained estimated recoverable reserves of 9.6 billion barrels of oil and 29 trillion cubic feet of natural gas. It is bounded above by a gas cap and below by a water aquifer. The State mandated offtake rate maximum is 1.5 million BOPD. The associated gas production is currently 2 BSCFPD. In addition production is currently 2 BSCFPD. In addition to solution gas, gas may be produced via tonguing and/or coning mechanisms through high permeability sands ranging in permeability from permeability sands ranging in permeability from 100 to 4000 md. It is estimated that by the late 1980's, the reservoir will be unable to maintain the allowable offtake rate due to natural reservoir depletion has tened by an inability to handle the gas production.
Presently, two of the six separation facilities are processing gas at near capacity levels. Techniques used to date in an effort to lower gas production have included rate restriction of wells and costly gas shutoff rig workovers averaging $1 million. The relatively inexpensive gas shut off technique using sodium silicate and coiled tubing costs approximately $250 thousand, providing an economically attractive alternative to rig workovers.
This paper outlines the design and field implementation of a non-rig gas shutoff technique in the Prudhoe Bay field. This will be accomplished by first reviewing the material selection process which included field and laboratory testing of sodium silicate and temporary bridging agents. Next, the case histories of two gas shutoff squeezes in the Prudhoe Bay field will be described followed by a Prudhoe Bay field will be described followed by a generalized procedure based on the field experience gained to date. Finally, the conclusions of the study will be summarized.
Several materials were evaluated that would provide an impermeable barrier to gas influx. provide an impermeable barrier to gas influx. Also investigated were temporary bridging agents necessary to isolate and protect oil producing perforations while squeezing the gas producing perforations while squeezing the gas producing perforations. An important characteristic perforations. An important characteristic required of both shutoff and bridging materials was pumpability through coiled tubing, thus allowing selective downhole placement without a workover rig. In addition, the close proximity of the gas zone to be shutoff and the oil zone to be protected demanded an understanding of the compatibility of the squeeze and bridging agent materials.
Gas shutoff materials reviewed were expoxy resins, cement and an activated sodium silicate gel used successfully in the industry as a water shutoff material.
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