Deliquification in a Mature Gas Field: Comparison of Predicted and Observed Rates after Well Workover to Install Velocity Strings

Liquid loading is a significant issue within gas and gas condensate wells, resulting in production decline and rate instability. The Southern Gas Field serves as a case study, to investigate a field experiencing liquid loadup. The field is located in the Southern North Sea, and is being produced via natural depletion with no pressure support from the surrounding aquifer. In Yr. 9 production well A ceased to flow as a result of liquid build up, and by Yr. 11 well B also showed signs of liquid loading. The load fluid consisted of liquid condensate, and this was confirmed by wireline investigations. A decision was made by the operator to deploy two 2 3/8" velocity strings, in an attempt to extend the field lifespan, by restarting well A and returning rate stability to well B. Velocity strings are a relatively low cost deliquification method, that aim to increase the gas flow rate above the critical flow rate, enabling the well to continuously unload liquids. The design of velocity strings is crucial, as if too small a tubing is used, the velocity string can act as a choke, suffocating the well. The velocity string run in well B proved a success, however the workover in well A failed to restart the well. Batch foamer treatments were later tried in well A but proved ineffective. The effectiveness of foam to deliquifying gas wells, depends upon the composition of the load liquid, as both water and liquid hydrocarbons react differently to surfactants. Typically liquid hydrocarbons such as condensate do not foam well, and must be agitated to maintain foaming. This study aimed to make use of Petroleum Experts Integrated Production Modelling (IPM) software to model the effect of the velocity string deployment, with regards to returning rate stability. Prosper models were created and Systems Analysis was performed and confirmed that 2 3/8" velocity strings will lift both well A and well B out of the Turner region. Prosper was also use to model the effect of using foam as an artificial lift method, with the results identifying that the water gas ratio (WGR) of the field is too low for effective deliquification using foams. A GAP model was also created for the Southern Gas Field to determine the incremental reserve gains to be had by deploying velocity strings. The simulations identified that well workovers can provide an additional 4.99 Bscf to the recoverable reserves