Simulation of Nitrogen Injection into Naturally Fractured Reservoirs
Authors: Vicencio O.A. , Sepehrnoori K. , Miller M.A. 
Published: December 2007

Abstract
This paper addresses the simulation of nitrogen injection in naturally fractured reservoirs through different scenarios by using a conceptual model with isotropic and anisotropic matrix blocks.
Since, in practice, nitrogen is injected under surface conditions, there is some concern about nitrogen injection breakthrough because higher density values under lower temperatures could accelerate the movement of nitrogen in the reservoir and, therefore, it could result in an early breakthrough in production wells.
By using the ECLIPSE commercial simulator, we constructed black oil and compositional simulation case studies under isothermal conditions in order to adequately represent the production conditions of the field and the monitoring field data before nitrogen injection. Simulation results indicated that injected nitrogen moves straight to the gas-oil contact because gravity forces destabilize the displacement.
Furthermore, we considered the injection of different gases (reservoir gas and methane) at the same reservoir conditions, our simulation results show that the gravity force effect under small variations of density between the injected gas and reservoir gas has a big impact on the distribution of injected gas in the gas cap and oil recovery from matrix blocks.
Subsequently, we constructed a thermal compositional case study for a dual porosity system injecting nitrogen at surface temperature conditions. Results indicated that the variation of temperature mainly occurs in the gas cap near the bottom of the gas injector wells. An analysis of nitrogen injection at higher temperatures was also performed in order to define the impact on the distribution of gas and the oil recovery from the matrix blocks.

Introduction
Naturally fractured reservoirs are complex and constitute a large amount of the world's oil reserves. Their complexity usually is represented in numerical models based on conceptual abstractions. When most recoverable oil is present in low-permeability rocks, the dual porosity model appears to be the most efficient approach for simulating such reservoirs.
The rhythms of exploitation in a fractured reservoir can cause a field pressure to decline drastically and be reflected in the reduction of oil production rates and the formation of a secondary gas cap. Therefore, field pressure maintenance with gas injection could result in an exploitation alternative from the technical and economical points of view1.
The success of this pressure maintenance project considers many variables, but the rock-fluid properties and distribution of the fracture system require special attention. The main mechanisms in our case studies are:the gravity force mechanism, which occurs in the gas cap zone and is mainly due to the difference between densities of injected gas and reservoir gas, andthe gravity drainage mechanism, which occurs mainly in the oil zone when the fractures are full or partially saturated with gas.
In fractured reservoirs, gravity drainage is an important recovery mechanism and plays a major role in the hydrocarbon recovery. Our simulation case studies consider the nitrogen injection for pressure maintenance. A similar practical project is receiving attention in the Cantarell field, the largest oil field in Mexico characterized as a dual porosity system with a black oil fluid.
The motivation of this study is to focus on the nitrogen distribution impact by injecting the gas at surface temperature conditions. Since, in practice, nitrogen is injected under surface conditions, there is some concern about nitrogen injection breakthrough because higher density values under lower temperatures could accelerate the movement of nitrogen in the reservoir and therefore, it could result in an early breakthrough in production wells.