Overcome the Impasse with Casinghead Gas
December 4, 2014

Process Description

Casinghead gas is natural gas produced with oil in oil wells and is usually the flash gas from the oil reservoir. Casinghead gas collects in the annular space between the well tubing and casing of an oil well. The weight of the casinghead gas contributes to reducing the bottom hole pressure and lowering well production – specifically by holding a back-pressure against the formation.  Reducing the gas pressure on the well casing (annulus) can increase oil production.  The goal is to maintain casinghead pressure as close to zero as possible. In areas or the country where it is allowed, these systems are often configured to pull a vacuum.  

 VRU_casinghead_gas_compressor.jpg

Historically, much of the casinghead gas is flared or vented to the atmosphere as a method of pressure reduction. Typically casinghead gas is flared or vented when the wellhead pressure drops below gas sales line pressure.

A casinghead gas compressor is often used to lower back pressure in the well annulus (casinghead gas pressure).  This allows gas and fluids to flow more easily into the well bore and increase oil production. This casinghead gas can be sent to a gas gathering or sales pipeline.

Recovery of casinghead gas volumes of less than 25,000 SCFD can be economic. Volumes greater than this are easily justified.

 

Measurement   

Direct measurement of the casinghead gas flowrate is the best method for the purpose of sizing a casinghead gas compressor and to determine the recovery of gas possible. Methods used to measure flowrate include. 

  • Turbine meter 
  • Thermal mass flow meter
  • Orifice plate meter

Always collect a sample of the casinghead gas to determine the molecular weight of the gas and to obtain the VOC and methane content of gas. 

Emissions Composition

Vented casinghead gas will emit methane and VOCs with the majority of the gas made of methane.  Normally, casinghead gas is relatively wet with a specific gravity of 0.85 (air = 1) and containing 16 gallons of liquids per thousand standard cubic feet of gas. 

If flared, emissions will be mostly CO2 and NOx, CO and some VOCs. Flared sour gas (containing H2S) will emit SO2.

Recovery Methods

VRU compressors are effective recovery systems for casinghead gas.  The compressor type used will depend on casinghead gas chemical composition, flow rate and compressor discharge pressure needed.

Casinghead compressors are often used with electric submersible pumps and rod pumps where formation gas is required to be separated downhole and then transported through the annulus. Often the compressor discharge is routed to either a booster or flash gas compressor or to a low-pressure gathering system. Similar to vapor recovery compressors, casinghead compressors operate with low suction pressures, high compression ratios, and low gas throughput rates. Most systems operate on pressure demand, allowing the well bore to remain clear and stable, resulting in a steady flow of gas and fluids. 

Casinghead gas recovery projects are commonly used in wells with water or carbon dioxide floods, typically used in enhanced oil recovery (EOR) where the produced gas can be re-injected into the reservoir.

Multiple wells that are located in close proximity can be routed to a single compressor package. The key component is distance. If the wells are located too far from each other, then pressure drop can become an issue. For such a system, all wells need to at or be near the same surface pressure, so the compressor can pull equally on each well. 

Consider using a backup casinghead gas compressor for single well and multi-well applications to reduce downtime and maximize gas recovery. 

Future

Flaring and venting of all gas streams including casinghead gas is getting more and more public and regulatory attention every day. Expect that all sources of natural gas will be judged on how much is sent to sales or used for a useful purpose. More regulation can be expected. 

Let HY-BON help your company with vent gas management. We can assess your wells for casinghead gas opportunities to recover the gas, increase profits and reduce your environmental liabilities.

Vent Gas Management – HANDLED!