Trapped Gas Saturations In Leduc-age Reservoirs
Authors: Batycky J. , Irwin D. , Fish R. 
Published: March 2010

Gas production from Alberta's Leduc-age reservoirs, which originally contained more than 400 × 109m3 OGIP, has and will continue to be significant. Estimating recoveries from these bottom- water drive pools has been difficult. Recoverable reserves are dependent upon both the trapped gas saturation and aquifer performance. Generalizing core displacement tests and applying limited field experience has been problematic.

With a number of projects reaching maturity it is now possible to integrate core displacement data and field performance to clarify the factors that control gas recovery in these pools. This study is able to demonstrate the dependence of trapped gas saturation on a number of factors including wettability, initial fluid saturation and permeability. A simple analytical model was developed to estimate and interpret blowdown recoveries from these pools. The actual performance of a suite of Leduc-age pools has been interpreted and applied to validate the model's applicability.

Introduction

Gas recovery from a bottom water drive pool is primarily dependent on the amount of gas remaining unrecovered behind an advancing gas/liquid contact. Since 1990, Imperial has been producing the Leduc D-3A rese rvoir gas cap. The process is described as gas cap blowdown. This paper assesses the gas recovery that can be expected from the Leduc D-3A reservoir and implications for other Leduc-age pools. Evaluation of the trapped gas saturations included:reviewing the available documentation of laboratory studies;accessing industry expertise; and,material balance calculations on selected Devonian Reef reservoirs.

Recovery Concepts and General Terminology

Trapped gas, Gt, is defined as the total amount of unrecovered gas in a liquid-displaced volume of reservoir. The trapped gas therefore includes both the residual gas in the displaced pore volume and any gas remaining in unswept portions of the invaded reservoir. The amount of trapped gas within the liquid-swept zone can be determined using material balance calculations in conjunction with measurements that locate the gas/liquid contact. At any time during production, the amount of remaining or trapped gas, Gt, in standard units, sm3, can be determined from material balance, given by:

Equation 1 (available in full paper)

In Figure 1, the gas remaining unrecovered in the displaced volume, Vd, behind an advancing gas/liquid contact is the sum of the residual gas, Gr, which remains undisplaced within the liquidswept region and that remaining bypassed or unswept, Gu, i.e.,

Equation 2 (available in full paper)

During production, the amount of gas that is trapped by the displacing liquid phase is affected by the pressure changes that occur over time within the displaced zone, Vd. The pressure in Vd depends on withdrawal history and the underlying aquifer response. In Leduc's case the aquifer response is influenced as well by other Leduc-trend reefs on the common Cooking Lake aquifer. If the pressure in Vd increases over time, the initial value of Gt remains fixed even though the reservoir volume (and hence the saturation) containing that gas becomes reduced.