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

摘要

Removal 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 tubing-casing 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.