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CINGSA applies for pressure increase Tells AOGCC that an unexpected gas pocket in the gas storage reservoir creates a need for higher gas pressure than anticipated Alan Bailey Petroleum News
Following an application to the Alaska Oil and Gas Conservation Commission for approval to increase the maximum gas pressure allowed in its Kenai Peninsula gas storage facility, Cook Inlet Natural Gas Storage Alaska Inc., or CINGSA, presented its case to the commission in a May 6 hearing. The company has asked the commission for an approved maximum pressure of 2,200 pounds per square inch, rather than the existing 1,700 pounds per square inch limit.
The CINGSA facility, which went into operation in 2012, provides gas storage services for Southcentral gas and power utilities, enabling the utilities to store gas during the summer for use during the winter, when gas demand is high. CINGSA says that without approval for a higher reservoir pressure it cannot fully meet its contractual commitments for gas storage.
Six wells Richard Gentges, a consultant for the company, told the commission that the storage facility has five wells penetrating five distinct sand units within what is referred to as the Sterling C reservoir. The Sterling C is a depleted reservoir of the Cannery Loop gas field, immediately south of the city of Kenai.
Wells two to five target the more southerly part of the reservoir structure, while the number one well targets a part of the structure more towards its northerly end, Gentges said. And, given that gas reservoirs in the Cook Inlet basin tend to be compartmentalized, typically consisting of multiple sand-filled ancient river channels, the plans for the drilling of the wells accommodated the possibility of encountering gas in an isolated channel, he said.
Gas pocket The number one well, the first well to be drilled by CINGSA, was completed on Jan. 29, 2012, with the well penetrating all five reservoir sands. A rise in the pressure within the well suggested that the well may have encountered an isolated gas pocket. And subsequent stability of the pressure, at a moderate level, suggested cross-flow of gas between that pocket and other reservoir sands. An analysis of the temperature log for the well bore appeared to support that interpretation: The well had probably penetrated an isolated sand channel, containing relatively high-pressure gas that had never been tapped as part of field production.
Using data for a Cannery Loop gas field well that accessed the Sterling C reservoir when the reservoir was in production, CINGSA has been able to construct a graph showing how the reservoir pressure varies with the volume of gas in place in the reservoir, Gentges explained. Although that data was obtained as the gas volume declined during field production, the characteristics of the reservoir should remain consistent, as gas is added or withdrawn during use of the reservoir for gas storage.
Pressure measurement Each spring and fall CINGSA temporarily closes the storage facility for pressure and mass balance testing, in part to assure the integrity of the reservoir. And each of those shut-ins provides a gas pressure measurement. Following the third shut-in, in November 2013, CINGSA had established three pressure measurements, with those three measurements providing points that lie in line on the pressure-volume graph. That line of points indicated a significantly higher gas pressure than would be expected, based on the known volume of gas that had been injected into the reservoir, Gentges said.
And a pressure reading from this year’s spring shut-in almost directly overlays the pressure reading from the spring of the previous year, he said.
The graph showed that the reservoir actually contains more gas than would be expected from CINGSA’s gas storage injection and withdrawal, with that additional gas likely to have originated from the isolated gas pocket encountered by the number one well. A subsequent analysis of the data indicated the presence of 14.5 billion cubic feet of gas, additional to what CINGSA had injected, Gentges said.
Insufficient space But, with 14.5 billion cubic feet more gas in the reservoir than had been assumed as part of the storage facility design, there is only sufficient space in the reservoir for 8.2 billion cubic feet of the 11 billion cubic feet of working gas that facility is committed to hold, unless the reservoir pressure is allowed to exceed the approved level of 1,700 pounds per square inch. The requested new maximum of 2,200 pounds per square inch corresponds approximately to the pressure in the reservoir when the original gas field was discovered. Pressure testing in wells penetrating the reservoir has indicated that the requested maximum pressure would be only 63 percent of the threshold pressure at which fracturing of the rock would take place, Gentges said.
The 2,200-pounds-per-square-inch pressure limit would allow the storage of around 18 billion cubic feet of working gas, Gentges said in response to a question from Commissioner Cathy Foerster.
Moira Smith, CINGSA vice president and general counsel, commented that, although the gas volume accommodated by a maximum pressure of 2,200 pounds per inch actually exceeds current storage requirements, mandating a maximum below that level could limit potential future expansion possibilities for the facility.
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