Reservoir Model Studies For Miscible Flood Projects
Authors: Flores Jorge , Siu Alan , Li Yau-Kun 
Published: March 2010

Predictions/rom a compositional numerical model/or first contact horizontal miscible flooding (5-spot pattern) by solvent injection show recoveries ranging from 81.5% to 52.2% DOIP depending on the type of miscible flood project, permeability heterogeneities and solvent-oil mobility ratio. Incremental recoveries over waterflooding range from 40.9% to 13% GOIP in a reservoir where a water zone was present. Model results show that ultimate recoveries with continuous solvent injection are insensitive to the time of implementation of a miscible flood project during waterflooding. Predictions also show that water-alternating solvent injection is effective only in reservoirs with high permeability contrast. Infill drilling (9-spot pattern) resulted in accelerated production with negligible incremental recoveries.

Introduction

The model predictions reported in this study form part of a joint project designed to develop an integrated engineering-economic model which could be used to examine secondary or tertiary recovery potential under miscible flooding. Selection of the reservoir that has considerable enhanced oil recovery (EOR) potential was accomplished by detailed survey of several reservoirs in Canada(1). Ultimate recoveries and production/injection rate forecasts for horizontal miscible flooding were obtained using a miscible flood reservoir simulator(2). The sensitivity analysis developed in this study will be used as first step guide lines for the development of an engineering economic model which will determine the economic feasibility of EOR projects.

Modelling Development

The development of the model was based on the options currently available in CMG's miscible flood reservoir simulator (MISIM3). The model assumptions are summarized below:First contact miscibility is taking place at all times.A physical dispersion of 0.2 m2/d is present in the horizontal direction and 0.002 m2/d in the vertical direction.Mixing by diffusion is insignificant compared to physical or convective dispersion. (Only one set of dispersion coefficients was applied in this study for the two types of reservoir description.)Hysteresis effect and viscous fingering are not incorporated in the model.The reservoir fluid is characterized by three components: oil, solvent and chase gas. Initially the reservoir fluid composition is 1, 0, 0, i.e. only the oil component.During solvent injection the calculated relative permeability of the hydrocarbon phase (solvent plus oil) is modified as a function of solvent concentration. The higher the solvent concentration, the higher the solvent mobility.Model production/injection rates for the wells represent 1/8 of the full well rates for primary and 5-spot pattern cases. For the 9-spot pattern the model production rates for the infill well represent 1/4 of the full well and for the original wells the rates represent 1/8 of the full well.The model inherently assumes that each producer in the pattern will exhibit the same performance. Producers are completed in the top three layers and injectors in the top four layers.

The 3-D Reservoir Model

The 3-D reservoir model, shown in Figure 1, represents Ys of a 5-spot and 1/8 of a 9-spot pattern. The model shows areas of no vertical communication between layers in the reservoir (thick lines).