SPE Members
The objective of unloading valves in a gas lift string is to transfer gas to the lift point. Normally, the valve test rack opening (TRO) for nitrogen charged dome pressure valves is based upon the estimated flowing temperature profile. This is appropriate for most situations. However, the valve settings may not unload consistently when a significant difference exists between the shut in, unloading, and producing temperature gradients. Such temperature differences are very pronounced in either high rate wells or in wells with a large upper well bore heat sink, such as when penetrating the Arctic permafrost, and in deep sea completions.
A procedure has been developed to determine an appropriate TRO for each valve to accommodate the range of temperature conditions expected during kickoff operations. It reduces the likelihood of unloading valves operating out of sequence and improves efficient transfer of lift gas to the design lift valve under a variety of well bore temperature gradients.
The Prudhoe Bay field is located on the edge of Alaska's Arctic Ocean. The primary producing interval, the Sadlerochit, is found at approximately 8850 ft. SS(2698 m) with significant portions of the oil field undergoing water flood Seawater from the Arctic Ocean is injected in addition to produced water. All produced gas not consumed as fuel is injected into the gas cap for pressure maintenance. Well productivity index (PI) varies across the field from slightly below 1.0 to above 30 BFPD per psi drawdown (23 to 692 (m /d)(MPa). Water cuts currently average 60 percent. A range of tubing sizes are found in the producing wells, with 4-1/2 and 5-1/2 in (114 and 140 cm) diameter prevalent. Gas lift is the predominant artificial lift mechanism. Over 150 of the 300 ARCO operated producing wells in the Eastern Operating Area of Prudhoe Bay are normally on gas lift utilizing approximately 250 MMSCFD (81935 L/s) lift gas. Gas lift valves are normally 1-1/2 in (38 cm) O. D. The manifold operating pressure for the gas lift supply system is 1900 to 2000 psi (13.10 to 13.79 MPa). Due to the kick-off pressures and temperatures, valve dome pressures above 2000 psi (13.79 MPa) are common.
The top gas lift mandrels in Prudhoe Bay wells are normally located between 2900 and 3400 ft TVD (883.9 to 1036.3 m), well below the permafrost. Permafrost is essentially high water content, frozen soil id extends from just below the 18 in (457 cm) of active soil to 2500 ft TVD (762.0 m). This temperature sink creates a large differential temperature in the well bore between shut-in and flowing conditions, especially for high rate wells.
Prudhoe Bay wells with moderate to low PIs have been found to begin flowing while only lifting on upper valves and at very cool temperatures as they slowly warm up to flowing temperature. Designs based only on full flowing temperature profiles lifted from the orifice may incorrectly estimate the actual top valve operating temperature by as much as 15F (−9C).
Bellows operated gas lift valves are temperature sensitive and it's important to understand their characteristics over the range of well bore temperature gradient conditions. The difference between the unloading temperature gradient and the flowing temperature gradient has been well documented. The use of flowing temperature gradients in gas lift design techniques is common. However, the flowing gradient does not always result in an optimal gas lift design. Jones and Brown found that setting gas lift valve dome pressures based on a linear temperature change from top to bottom was inadequate. Likewise, Laing pointed out that linear temperature gradients are an oversimplification in deep water offshore production areas and could result in unloading problems.
Other methods have been proposed. One valve manufacturer's gas lift manual recommends using the geothermal gradient to provide some "safety factor".
P. 367^
