Gas Well Deliquification: Gas Lift - Part-I
Recently we discussed in detail, a novel gas well deliquification technology called Foam Assisted Lift. This blog is targeted towards introduction of the second technology, called Gas Lift in our blog series on Gas Well Deliquification. Gas lift has many forms and has been one of the first artificial lift techniques that was used to aid production from oil & gas wells. Through thick & thin, this technology has made its mark as one of the best solutions to production enhancement & maintenance from wells, becoming a million dollar industry with-in itself.
Ideally, a well cannot achieve its absolute flow potential due to hydrostatic head of liquid column in the wellbore and pressure drop due to frictional losses from sand face to the wellhead. These two factors exert added back pressure on formation resulting in well load-up in late life as the reservoir pressure depletes supplemented by increase in water cut or WGR. Pressure drop due to friction can be minimized by optimizing completion design. However, unfortunately, sooner or later, a water drive reservoir will start producing water which will load the well. As talked extensively in our blog series, this phenomenon is known as liquid loading. We have a dedicated blog series on Gas Well Deliquification techniques, and this installment serves to introduce & describe Gas lift as a deliquification technique.
Principle: High pressure gas is injected in production casing which enters into tubing from gas lift valves installed in Side Pocket Mandrels (SPMs). The entrant gas mixes with liquid and lightens the liquid column (reducing its density hence hydrostatic head or weight of the column), thereby assisting the flow from the well.
Gas Lift Types
Continuous Gas Lift is usually employed on wells with higher productivity index. Gas stream is injected uninterrupted into the casing annulus, enters into tubing through gas lift valve and lowers the overall density of the fluid column. Being high productivity well, the liquid column in the wellbore rises quickly and is lifted by continuous gas injection. Thus, for a given average reservoir pressure and productivity index, the well is able to flow at a higher rate.
Intermittent Gas Lift is designed for lower productivity wells. High pressure gas slugs are injected intermittently into casing annulus which lifts the accumulated liquid in the production tubing. Being low productivity well, the reservoir influx is low and sufficient time is required to restore the liquid level. As soon as the fluid is produced on surface, gas injection is interrupted, and the cycle of liquid accumulation to production through gas injection is repeated.
Gas lift systems have several advantages when compared to other gas well deliquification techniques such as plunger lifts & Foaming systems. However gas lift, is one of the most CAPEX demanding artificial lift system. As we are injecting gas against static column in well, we need high pressure gas for injection. High pressure gas at surface requires multi-stage compression, separation, scrubbing, et cetera. However once the investment in surface facilities is made, Gas lift is one of the most efficient and low maintenance artificial lift system currently used in the industry.
In a nutshell, few pros & cons of this timeless deliq technique:
Pros
Capability to handle solid production
No issues in high GLR wells (a limitation with pump assisted lift)
Can be used in deviated wells
Gas lift valves of different specification can be installed and retrieved through slickline.
Low foot print; choke and ESV are required
Can be used in high temperature and corrosive environments
Full-bore access, rigless jobs can be performed
Cons
Availability of gas for injection
Less efficient as compared to other lift system
Availability of compression requirement
In the next installment on Gas lift, we will discuss the mechanics behind modeling gas lift wells and a complete tutorial on gas lift design in industry standard nodal analysis software.
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