Efficiency of Steamflooding in Naturally Fractured Reservoirs
Authors: Babadagli T. 
Published: December 2007

Abstract
This study aims to identify the effective parameters on matrix heating and recovery, and the efficiencies of these processes while there is a continuous flow of steam in fracture. For this purpose, an experimental study was conducted on a single matrix-single fracture model and the results were used to verify the numerical solution of energy equation. A parametric analysis was performed using numerical output. The results show that as the steam injection rate is lowered, the contact time between matrix and steam flowing in fracture increases causing more conductive heat transfer to the matrix. On the other hand, more heat energy is introduced into the system with increasing steam injection rate and this results in more matrix heating desirably but it increases the cost of the process. Therefore, an optimization study is needed. It is observed that there is a critical injection rate optimizing the process and the critical injection rate for an efficient matrix heating is defined for different matrix sizes and matrix heat transfer coefficients.
In the second part of the study, similar analysis was performed to investigate the effect of injection rate on the matrix oil recovery. Efficiencies of steam injection processes in a laboratory scale single matrix-single fracture model and field scale case with different horizontal fracture configurations were investigated. The critical injection rate was defmed for laboratory scale simulation for different matrix properties. Finally, critical rate concept was evaluated for different number of horizontal fractures and steam qualities in field scale simulations. The approach and results can be used in further studies to analyze the efficiency of thermal applications and to obtain correlations for steam injection performances in naturally fractured reservoirs.

Introduction
In order to recover heavy matrix oil in naturally fractured reservoirs (NFR), steam injection has been recognized as an effective enhanced oil recovery technique. Many mechanisms are involved in the recovery process from NFR during steam injection. Due to numerous parameters that are effective on the mechanisms, accurate recovery estimation becomes highly complex.
Limited number of experimental investigations is available on the recovery mechanisms under temperature effect in NFR. Reis reviewed these mechanisms and the characteristic recovery times for them. Briggs et al. numerically and Briggs et al. both numerically and experimentally investigated the temperature effect on the matrix recovery. More recently, Babadagli experimentally studied the effect of temperature on the capillary imbibition performance from a matrix containing heavy-oil. Most of these studies are based on the matrix recovery at static conditions. Matrix recovery under dynamic conditions, i.e. during steam flow in fracture, was also investigated. Jensen and Sharma performed steam and hot water injection experiments on artificially fractured samples and evaluated the recovery performances for the fracture properties, the temperature of injected fluid and matrix lithology. Behavior of matrix recovery under different injection rates and injection patterns was examined both numerically and experimentally by Sumnu et al. with the emphasis on the effect of fracture relative permeabilities and capillary pressure. Pooladi-Darvish et al. studied fracture-matrix heat transfer analytically under static conditions for single matrix block. Babadagli investigated the effect of injection rate on the matrix heating under dynamic conditions for single fracture-single matrix model experimentally and numerically.
Field scale simulation studies were also conducted for different purposes. Nolan et al. numerically simulated steamflooding into a carbonate reservoir and investigated the effects of reservoir thickness, porosity, directional permeability and well spacing on the performance. A similar modeling study was done by Dreher et al. P. 665^