Development of an Offshore Gas Condensate Reservoir by Nitrogen Injection vs. Pressure Depletion (includes associated paper 18560 )
Authors: Hagoort J. , Brinkhorst J.W. , van der Kleyn Piet H. 
Published: December 2007

A conceptual study is presented of the development of an offshore gas-condensate reservoir by nitrogen injection vs. pressure depletion. Nitrogen injection could enhance condensate recovery, but requires additional capital. investments and is more costly to operate. The results of the study indicate that nitrogen injection is a realistic alternative for pressure depletion, provided that the reservoir is not too heterogeneous and the reservoir fluid is sufficiently rich in condensate. Under these conditions, nitrogen injection could be as profitable as conventional pressure depletion and in addition may double the condensate yield. The business risk for nitrogen injection is greater, however, and for this reason governments should consider granting special tax and/or royalty reliefs for enhanced condensate recovery schemes.

Introduction

Development of gas-condensate reservoirs by pressure depletion is attended by retrograde condensation: the dropout of liquid condensate within the reservoir at reservoir pressures below the dewpoint pressure of the reservoir gas. Because this dropped-out condensate is practically unrecoverable, pressure depletion of gas-condensate reservoirs results in a poor condensate recovery.

Historically, gas cycling has been applied to enhance the recovery of condensate from gas-condensate reservoirs. It involves the separation of the produced gas into condensate and dry gas, and the subsequent reinjection of the dry gas into the reservoir. Through this reinjection, the reservoir pressure can be maintained above the dewpoint pressure, while the wet gas is being displaced by the dry gas and driven toward the producers.

The drawback of gas cycling is that for a long period, a valuable commodity is to be injected that otherwise could have been sold. For this reason, gas cycling is in many cases economically unattractive, especially in offshore environments where initial capital investment is large and one cannot afford the postponement of gas sales.

A possible alternative for gas cycling is the injection of low -cost, inert nitrogen. Produced cryogenically from air with an established and proven technology, nitrogen can be made available for continuous, trouble-free injection in vast quantities at virtually any location. The disadvantage of nitrogen injection is that the produced hydrocarbons sooner or later become contaminated with nitrogen, which requires additional separation facilities. This does not pose a technical problem, but does lead to extra costs.

As far as petroleum engineering is concerned, nitrogen injection into subsurface reservoirs does not present any major problems. It is being practiced successfully in an increasing number of recovery projects. Also, the displacement of condensate gas by nitrogen in the reservoir is a relatively simple process that has proved to be very efficient. Hence the feasibility of nitrogen injection as an alternative for gas cycling is mainly an economic problem.

In this paper, we present a conceptual study of the development of a gas-condensate reservoir by nitrogen injection vs. pressure depletion. As a prototypical reservoir for this study, we have taken a hypothesized, offshore reservoir located in the Dutch sector of the North Sea. For this prototype, we have worked out a production development scenario complete with production projections for both pressure depletion and nitrogen injection. The economics of both scenarios have been evaluated with the discounted-cash-flow-analysis (DCFA) method.