Dewatering Unconventional Gas Resources With Sucker-Rod-Pumping Equipment
Authors: Denney Dennis 
Published: February 2015

This article, written by Technology Editor Dennis Denney, contains highlights of paper SPE 104547, "New Methods and Equipment for Dewatering Unconventional Gas Resources Using Sucker-Rod- Pumping Equipment," by Mark W. Mahoney, SPE, and Randy DeWerff, SPE, Harbison-Fischer, prepared for the 2006 SPE Eastern Regional Meeting, Canton, Ohio, 11–13 October.

Dewatering coalbed-methane and low-pressure gas wells is a challenge. Compared to electrical-submersible and progressing-cavity pumps, sucker-rod pumps are very forgiving, and wells can be pumped at very low pump-intake pressure without damaging the pumping equipment. New designs in sucker-rod-pumping equipment and rod-pumping systems for high gas/liquid ratios (GLRs), coal fines, and solids have changed the approach to dewatering unconventional gas resources.

Introduction

Understanding gas wells and how they affect the sucker-rod pump is needed to apply sucker-rod pumps under mild-, medium-, and high-GLR conditions. Gas lock, gas interference, compression, and decompression often are misunderstood and lead to use of the wrong equipment.

Gas Separation

Downhole gas separation in front of pump intake can solve gas problems with sucker-rod pumps. The term gas lock often is used to describe any gas problem in a sucker-rod pump. Surface-valve checks and dynamometer-card analysis cannot distinguish between various gas problems. True gas lock exists when the hydrostatic pressure above the traveling-valve ball is greater than the pressure in the pump chamber at the bottom of the downstroke. On the upstroke, the gas expands in the pump chamber but the pressure in the chamber remains greater than the pump-intake pressure; therefore, the standing valve does not open. With true gas lock, the valves stay closed and the same column of fluid is raised and lowered with each cycle, with no fluid being pumped. Compression and decompression must be avoided. A better description of gas lock is "the pump has pumped down to its pump-chamber decompression limit."

The ideal-gas law gives the relationship between volume, pressure, and temperature. In simple terms, if the temperature is near constant, then pressure and volume are inversely proportional. On the upstroke, the pump chamber starts with an uncompressible volume according to the design constraints of the traveling and standing valves. As the plunger moves upward, the volume increases and the pressure decreases. When the pump-chamber pressure drops below the pump-intake pressure, the standing valve opens.

Pumps contain a certain volume that does not contribute to the pumped volume. This nonpumping volume includes the standing-valve section with the valve ball, ball-guide portion, and the upper-fluid-port portion. Another nonpumping volume is the traveling valve, with its seat, seat retainer, and cage, that functions inside the pump barrel. Pumps are assembled such that the traveling valve does not touch the standing valve when the valve rod bushing is seated in the clutch of the valve rod guide. Space out, when the well is put on pump, can be too far off bot-tom, which increases the nonpumping volume.