Incremental Recovery Using Dual- Completed Wells in Gas Reservoirs With Bottom Water Drive: A Feasibility Study
Authors: Armenta M. , Wojtanowicz A.K. 
Published: March 2010

Water drive reduces gas recovery through the mechanism of water coning, which results in liquid loading that kills the wells. Installation of a Downhole Gas Water Separator (DGWS) in the well may prevent liquid loading. This paper presents another solution to this problem: a dual-completed well with downhole water sink (DWS) drainage and injection installation. DWS wells are different from DGWS wells by the inclusion of a second bottom completion that controls water outside the well and prevents a commingled inflow of gas and water to the top completion. DWS wells have been successfully tested in oil reservoirs.

This study analyzes the DWS concept for gas wells by comparing simulated performances of a conventional well and a DWS well. The simulation runs were made over a broad range of the initial reservoir pressure and permeability values for a gas reservoir with a large associated aquifer. Also included is a comparison between DGWS and DWS wells at selected conditions.

The results demonstrate the considerable advantages of dual completion over conventional wells in low-pressure (subnormal), tight (1 md) reservoirs-a 2.6 fold recovery increase before the wells are killed by excessive water production. The advantage, however, reduces to 10% for a reservoir with normal pore pressure gradient and a permeability of 10 md, and almost disappears for a permeability of 100 md and abnormal reservoir pressure. When compared to DGWS wells, the final gas recovery of DWS wells is the same, but DWS produces the gas 35% sooner than DGWS. The study also identifies the best DWS completion design where top completion is used only for gas production, and bottom completion for inverse gas coning, gravity separation, and water injection.

Introduction

Water production reduces gas recovery by shortening the well's life. Water could invade the gas well and kill it when a lot of gas may remain in the reservoir. Various concepts and techniques have been used to solve the water-loading problems. Hutlas and Granberry(1) discussed the techniques used in Kansas, Northwestern Oklahoma, and the Texas Panhandle: pumping units, liquid diverters, gas lift, and a concentric dual-tubing string. They concluded that the pumping units are best suited for shallow gas fields with very low permeability, while a combination of liquid-diverter and gas-lift installation would perform best in deeper, higher-pressure fields.

Libson and Henry(2) discussed the methods used to control liquid loading in the tight, low-gas rate reservoirs in the Intermediate Shelf area of Southwest Texas: pumping units, plunger lift, a small concentric dual-tubing string, soap injection, and flow controllers. They found the plunger lift and the small concentric dual-tubing string to be effective in wells with low water rates, while pumping units performed best when the liquid rate exceeded 10 BWPD.

Stephenson, Rousen, and Rosenzweig(3) evaluated different methods for dewatering (unloading) gas wells in the Box Church Field-a high water-cut gas reservoir in Texas. The methods include rod pumps, soap sticks, swabbing, coiled tubing/nitrogen, venting, plunger lift, and gas lift.