Economic Evaluation of Cycling Gas-Condensate Reservoirs With Nitrogen
Authors: Donohoe Charles W. , Buchanan Robert D. 
Published: December 2007

Summary
This work presents the results of an investigation of the economic feasibility of using cryogenically produced nitrogen as a substitute for natural gas to maintain reservoir pressure during cycling operations in gas-condensate reservoirs. Those economic factors unique to nitrogen cycling are discussed. It has been concluded that gas reservoirs with a condensate content in excess of 100 bbl/MMcf should be considered as potential nitrogen cycling prospects.

Introduction
Historically, gas-condensate reservoirs which exhibit liquid loss through retrograde condensation with reduction in reservoir pressure have been depleted under a full or partial pressure maintenance program by injection of dry natural gas. The technology to evaluate and accomplish this is available and well proved. The increased value and the shortage of natural gas, however, has made it generally uneconomic to divert this gas from the sales line to injection for the purpose of maintaining pressure in gas-condensate reservoirs.
This paper is concerned with the economic feasibility. of using cryogenically produced nitrogen as a substitute for natural gas to maintain reservoir pressure during cycling operations in gas-condensate reservoirs. Many factors affect the economics of a gas cycling project. Generally, the three most important factors are prices, stock-tank liquid content of the reservoir gas, and the degree of reservoir heterogeneity.
For the purpose of this evaluation, a hypothetical reservoir has been defined. Projections of performance have been made assuming the reservoir contains three different reservoir fluids with stocktank liquid contents ranging from 76.1 to 220.7 bbl/MMcf. The performance of each of these reservoir fluid systems has been evaluated assuming three different degrees of reservoir heterogeneity.
Performance and economic projections have been made assuming three different depletion methods:pressure depletion with no injection,pressure maintenance by nitrogen injection, andpressure maintenance by return of residue gas and purchase of makeup gas.
Multiple projections have been made for the injection cases assuming blowdown of the reservoir is initiated at various times, starting with breakthrough of dry gas. Performance and economic results are presented for those cases which produce the maximum income discounted at a rate of 15070 per year. Economic evaluations were made assuming prices and costs escalate at a rate of 6070 per year.

Projections of Performance

Reservoir Description
To make an economic evaluation of cycling a gas-condensate reservoir with nitrogen, it was necessary to obtain projections of reservoir performance. It was our desire to evaluate the effect of different reservoir fluid compositions and degrees of reservoir heterogeneity upon the economic potential of this depletion mechanism. To isolate these effects, performance projections were made on a hypothetical reservoir whose properties are shown in Table 1.
The hypothetical field contained six contiguous five-spot patterns of 360 acres each. This resulted in a conforming area under injection operations of 2,160 acres or 75070 of the total field area. In all injection cases investigated there were six injection and 12 producing wells. Under pressure depletion it was assumed that there were 18 producing wells.

Reservoir Fluid Properties
The compositions and basic properties of the three reservoir fluids investigated are presented in Table 2.

Reservoir Description
To make an economic evaluation of cycling a gas-condensate reservoir with nitrogen, it was necessary to obtain projections of reservoir performance. It was our desire to evaluate the effect of different reservoir fluid compositions and degrees of reservoir heterogeneity upon the economic potential of this depletion mechanism. To isolate these effects, performance projections were made on a hypothetical reservoir whose properties are shown in Table 1.
The hypothetical field contained six contiguous five-spot patterns of 360 acres each. This resulted in a conforming area under injection operations of 2,160 acres or 75070 of the total field area. In all injection cases investigated there were six injection and 12 producing wells. Under pressure depletion it was assumed that there were 18 producing wells.

Reservoir Fluid Properties
The compositions and basic properties of the three reservoir fluids investigated are presented in Table 2.