Experimental Study of Critical Flow Rates in the Tubing-Casing Annulus of Natural Gas Wells

Abstract

fax 01-972-952-9435. AbstractRemoval of water and hydrocarbon liquids from gas wells is increasingly recognized as an important topic for mature gas reservoirs. Accumulation of these liquids in the bottom of a gas well is often referred to as liquid loading. Liquid loading limits current productivity of 90% of the natural gas wells in the USA. Liquid loading first appears in the casing below the end of tubing (EOT). One way to reduce loading below the EOT is to install dead-end production tubing to the bottom of the perforations and force the gas to flow from the perforations through the tubing-casing annulus up to a cross-over connection near or above the top of the perforated interval. We conducted tests in a flow loop to evaluate such flow.The primary objective of our tests was to determine the critical flow rates for two phase flow through tubingcasing annulus using two different tubing sizes (2.88-inch-OD and 3.50-inch-OD) in a 4.00-inch-ID casing. Secondary objectives were to develop a method to predict critical flow rate, to identify the flow regimes that exist at the critical flow rate, and to evaluate the mode of liquid transport.For gas-water flow in vertical tubing, the Turner-Hubbard-Dukler (1969) prediction for critical flow rate (without the 20% correction) is very close to what we observe in our flow loop. However, the critical rates for flow in the tubing-casing annulus were found to be 20 to 50% less than predicted by multiplying the Turner-Hubbard-Dukler (THD) critical velocity and the annular cross-sectional area. It was observed from the tests that two types of flow regimes could occur at the critical flow rate: annular flow regime and transitional annular flow regime.In flow through an annulus, the film thickness on the casing wall is larger than the film thickness on the tubing wall. Theoretical analysis for one-phase flow shows that the maximum velocity in tubing-casing flow is closer to the tubing. We believe that this observation also applies to two-phase flow and that the higher velocity near the tubing pushes liquid toward the casing, which results in the observation of thicker liquid films on the casing wall.

Design of a High-Pressure Research Flow Loop for the Experimental Investigation of Liquid Loading in Gas Wells

“…The nitrogen and kerosene are stored at a pressure up to a maximum of 914 psi in a gas-liquid separator with a volume of approximately 396 ft 3 that is linked to a nominally 6 in. diameter flow loop.…”

“…For the compressor and water pump option, both equipment (1, 2) are delivering, separately or commingled (3,4,5,6), the required flow rates and pressures at the injection manifold. This option may demand a larger compressor and pump, since the multiphase pump will no longer provide the boosting pressure as depicted in the first option.…”

“…The system is then pressurized (by means of a water pump) until reaching the required operating value. Both phases leave the vessel at the same pressure and they can be either mixed (3,4) or distributed separately (5, 6) prior to entering the tubing (7). From an economic standpoint, the tank option is the least expensive since a water pump downstream from the process will not be necessary as the pressure of the liquid will be provided by the air phase at the tank.…”

“…3 Removal of water and hydrocarbon liquids from gas wells is increasingly recognized as an important topic for mature gas reservoirs where the continuous decline in reservoir pressure not only triggers the alteration of the flow regime, switching the annular mist flow to churn flow, but also dramatically decreases the liquid lifting capacity of the flowing gas.…”

Design of a High-Pressure Research Flow Loop for the Experimental Investigation of Liquid Loading in Gas Wells

“…Liquid loading limits current productivity of 90% of the natural gas wells in the USA. 3 Removal of water and hydrocarbon liquids from gas wells is increasingly recognized as an important topic for mature gas reservoirs where the continuous decline in reservoir pressure not only triggers the alteration of the flow regime, switching the annular mist flow to churn flow, but also dramatically decreases the liquid lifting capacity of the flowing gas.…”