The Blowdown-Limit Model
Authors: Coleman Steve B. , Clay Hartley B. , McCurdy David G. , Norris Lee H. 
Published: December 2007

This paper introduces the technology defining the gas-well productive limitknown as the productive limit known as the blow-down limit. This limit is thestatic reservoir pressure at which a gas well becomes incapable of unloadingthe fluids that collected during load-up. The theoretical background ispresented, along with field data that presented, along with field data thatsupport the theory. This technology establishes a method for determining theabandonment pressure of a depletion-drive gas reservoir.

Introduction

The first two parts of this series discussed the theory for predicting whenand why a gas well experiences liquid loading problems and the methods forpredicting problems and the methods for predicting behavior during load-up. Once load-up occurs, corrective action is generally required to return a wellto production. Methods typically used include swabbing, gas lifting, andpumping. In each of these methods, energy pumping. In each of these methods, energy is added to the well/reservoir system to aid in load-fluid removal.

Because of the cost of these options, many operators choose an alternativemethod known as blowdown to extend the productive life of the well. In thismethod, the productive life of the well. In this method, the wellhead pressureis reduced enough to induce sufficient reservoir flow to lift the load fluidsfrom the well and to blow the well down. Additionally, the operator mightchoose to shut the well in for an extended period to allow for near-wellborereservoir-pressure buildup, thereby aiding in the blowdown. In this case, thewell/reservoir system energy is used to unload the well. No external energy isadded to the system.

As one might imagine, many variables determine the success of a blowdown. The amount of energy available to remove the load fluid, the amount of loadfluid present, the well depth, the gas flow rate, and the amount of liquidfallback play important roles. This paper presents the logic, theory, and fielddata to further the definition of these variables and their effects on thelimits of blowdown. This technology is presented in the form of a new term, thegas-well blowdown limit. Defined in its simplest terms, the blowdown limit isthe Static reservoirpressure at which a gas reservoir is no longer capable ofunloading a well's natural load-up fluids' without external energy (swabbing, gas lift, etc.). The blowdown limit is controlled by the buildup of reservoirpressure in the near-wellbore region, pressure in the near-wellbore region, wellbore hydraulics during blowdown, and chants in the flowing wellheadconditions.

As this paper shows, the blowdown limit can be key to determining theultimate recovery from a depletion drive gas reservoir. The decision to makeinvestments and their timing regarding depletion of reserves from low-pressuregas reservoirs can be influenced significantly by the blowdown limit, Theblowdown limit also enhances the understanding of well operations for loadingwells. This understanding alone can maintain well production and increaseultimate recovery.

Blowdown-Limit Model Theory

Examining the stages of blowdown is useful in mathematically modeling theblowdown limit. As Fig. 1 shows, the blowdown limit is, in many ways, thereverse of the load-up model discussed in Ref. 2. Once a well loads (t=0), theaverage reservoir pressure equalizes with the sandface pressure(PR" "Psf) and the wellhead pressure equalizes with the system pressure(pwf-ps). As time passes, (PR=Psf) and Pe will equalize. passes, (PR=Psf) and Pe will equalize. After a period of shut-in, Psf may become higher than theload-up-induced hydrostatics of the wellbore plus psf if the well is thenopened to the system, the reservoir will begin to flow (t=l). If the reservoircan deliver enough gas under these conditions to change the wellbore flowregime from bubbly to slug flow, then the column of load fluid will begin to belifted from the bottom of the well.

As the slug is lifted to the surface and removed from the well, psf isreduced, causing a continued increase in flow from the reservoir (t=2). Assuming that the well can maintain a rate above its critical rate for a periodlong enough to remove the remaining period long enough to remove the remainingload fluid (owing to liquid fallback of the slug), the well will unload itselfcompletely and return to steady-state production (t=3). This proposed blowdownmodel indicates that three criteria must be met for a well to unloaditself.Psf must be capable of increasing (or pwf decreasing) to a point greaterthan the pwf decreasing) to a point greater than the load-up-inducedhydrostatics to set up a differential pressure across the reservoir, which willinduce reservoir flow.The reservoir flow performance in this insteady-state condition must becapable of producing initial wellbore velocities high producing initialwellbore velocities high enough to move the wellbore flow from a bubble-flow toa slug-flow regime.

JPT

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