Nitrogen Injection Experience to Development Gas and Gas Condensate Fields in Rocky Mountains
Authors: Wu Xingru , Ling Kegang , Liu Dexin 
Published: March 2015

Abstract
Nitrogen injection into gas condensate or volatile oil fields has been practiced as a method of pressure maintenance as well as enhanced hydrocarbon recovery in Rocky Mountains for more than two decades. Reservoir management has experienced miscible displacement in the gas cap, reservoir pressure maintenance, and reservoir blow down in different fields. In the implementation of nitrogen injection, a tremendous amount of experiences on injection and reservoir performance had been accumulated without appropriate documentation. Furthermore, even though nitrogen injection has been widely used in the world to enhance recovery by miscible displacement or maintaining reservoir pressure, literature survey shows that the experience with nitrogen injection is sporadic.
This paper reviews reservoir characteristics and summarizes the lessons learned from nitrogen injection all of the world; then focuses on Rocky Mountains reservoir management to further analyze its production and surveillance, reservoir development stages in the life of fields, and the relationship between the fields and processing facility. Compositional reservoir simulation was performed to study the enhanced hydrocarbon recovery by injecting nitrogen and use nitrogen breakthrough information across the field as a continuous tracer to study the well connectivity between the injector and producer pairs.
The main contributions of the paper are that it highlights the accumulated experience associated with nitrogen injection, and provides information on amenable reservoir features which can be used to select nitrogen as a viable alternative for enhanced oil recovery purpose.

Introduction
Nitrogen injection to hydrocarbon fields to maintain pressure and enhance hydrocarbon recovery has been used widely in different formation and fluid types, and significant amount of experiences with reservoir surveillance, nitrogen separation and hydrocarbon processing, and fluid phase behavior have been accumulated and documented (Sinanan and Budri 2012; Sanchez et al. 2005). In Rocky Mountains, the fields are of particular interest because of the fluid type and formation characteristics.
Nitrogen injection into gas condensate or volatile oil fields in Rockies has been practiced for more than three decades. A number of fields (Figure 1) such as Painter Reservoir Units (PRU), and Ryckman Creek in overthrust, Anschutz Ranch East (ARE), and Glasscock Hollow, used large amounts of nitrogen to increase hydrocarbon recovery as well as to maintain reservoir pressure. Some of the fields with reservoir characterizations had been presented in literature (Clancy and Gilchrist 1983). In these fields, gas condensate/volatile oil was initially produced driven by gas expansion, supplemented by solution-gas and some with aquifer. After initial production, dry gas-re-injection and nitrogen injection was implemented. The experience associated with nitrogen injection and characteristics of reservoir development are valuable information for future nitrogen injection projects.
These fields are situated on the hanging wall of the Utah-Wyoming thrust belt, and they are producing from Upper Triassic-Lower Jurassic Nugget formation as shown in Figure 2. Some key parameters about the fields are compared in Table 1. A general stratigraphic description about the formations is presented in Figure 2. Some major fields such as PRU and ARE have been produced from the nugget formation which comprises Aeolian dune and inter-dune/sand-sheet sandstones with a stratigraphic thickness of approximately 900ft (Ring et al. 1999). The reservoir fluids are gas with volatile oil, and gas condensate, respectively. A brief introduction about reservoir characteristics is as follows.