Abstract

Gas production from Alberta's Leduc-age reservoirs, which originally containedmore than 400 Gm3 OGIP, has and will continue to be significant.Estimating recoveries from these bottom-water driven pools has been difficult.. Recoverable reserves are dependent upon both the trapped gas saturation andaquifer performance. Generalizing core displacement tests and applying limitedfield experience has been subjective.

With a number of projects reaching maturity it is now possible to integratecore displacement data and field performance to clarify the factors thatcontrol gas recovery in these pools. This study is able to demonstrate thedependence of trapped gas saturation on a number of factors includingwettability, initial fluid saturation and permeability. A simple analyticalmodel was also developed to estimate and interpret blowdown recoveries fromthese ools. The actual performance of a suite of Leduc-age pools has beeninterpreted using he model and to validate the model's applicability.

I.Introduction

Gas recovery from a bottom water drive pool is primarily dependent on theamount of gas remaining unrecovered behind an advancing gas/liquid contact.Since 1990, Imperial has been producing the Leduc D-3A reservoir gas cap. Thisprocess will be referred to as lowdown. In support of this project anassessment has been conducted to determine he level of recovery that might beexpected from the Leduc reservoir and any implications or other Leduc-agepools. The locus is to estimate the magnitude or trapped gas saturations in Leduc-age reservoirs by:

  • Assessing the available literature, accessing industry expertise and

  • Performing material balance calculations on selected Devonian Reefreservoirs.

II. Recovery Concepts And General Terminology

Trapped gas is defined to be the total amount of unrecovered gas in a Iiquiddisplaced volume of reservoir. The trapped gas therefore includes both theresidual gas in the displaced pore volume and any gas remaining in unsweptportions of the invaded reservoir he amount of trapped gas within the Iiquid-swept zone can be determined using material balance calculations inconjunction with measurements locating the gas/liquid contact. At any timeduring production, the amount of remaining or trapped gas, Gt, instandard units, sm3, can be determined from material balance, givenby:

Gt =Gi - Gp - Gc (1)

In Figure 1, the gas remaining unrecovered in the displaced volume,V3, behind an advancing gas/liquid contact is the sum of theresidual gas, Gr, which remains undisplaced within liquid-sweptregions and that remaining in bypassed or unswept regions, Gu,i.e.,

Gt = Gr+Gu (2)

During production, the amount of gas that is trapped by the displacing liquidphase is affected by the pressure changes that occur over time within thedisplaced zone, V3. The pressure in V3 depends on how theunderlying aquifer responds to the withdrawal history of the pool. In Leduc'scase the aquifer response is influenced as well by other Leduc-trend reefs onthe common Cooking Lake aquifer. If the pressure in V3 increasesover time, the initial value of Gt remains fixed even though thevolume (and hence the saturation) occupied by that gas becomes reduced.

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