Abstract

Testing of solvent effectiveness related to a given crude oil under simulated reservoir conditions began in the early 1950's. The initial apparatus used a small-diameter stainless steel tube. capable of sustaining high internal pressures to contain a porous medium similar to reservoir rock. This "slim tube" was mounted within an oven maintained at reservoir temperature. The desired test result was the minimum pressure required to achieve solvent/oil miscibility, characterized by high oil recovery from the slim tube.

As interest in enhanced oil recovery by hydrocarbon sol vent flood has increased, slim tube equipment, operating procedures and analysis of results have become more sophisticated. The current state of the technology is discussed, including laooratory equipnent J instrumentation and operating procedures.

Procedures for analysis and interpretation of slim tube test results are discussed, leading to improved sol vent desi gil concepts. Examples are provided to demonstrate the impact of solvent design on the hydrocarbon miscible flood process.

Introduction

The object of solvent design is to find the combination of available NGL and mixing gas streams to give a hydrocarbon mixture which demonstrates interaction with the reservoir oil as good as or exceeding some reference parameter. Design engineers seek to find a sol vent which will have a reasonable chance of attaining the desired increase in reservoir oil recovery, but with enrichment limited to an economically acceptable safety margin above the minimum miscibility concentration (MMC). This usually implies selecting the solvent analysis corresponding to one or more percentiles of slim tube oil recovery above "just first contact miscible"., or a critical temperature one or more degrees higher than for MMC. This definition requires results from successful slim tube tests with solvents characterized as both immiscible and multi -contact miscible. The plot of recovery against solvent mole average critical temperature (1) can then be used as the basis for sol vent selection.

Slim tube tests are performed to observe actual solvent-oil interaction in a physical simulation of reservoir pore space. If the slim tube is packed with angular grains, the variability of pore and pore throat sizes and shapes simulates reservoir rock but allows each test to be run at a fraction of core flood cost. The slim tube can then be considered to represent a single chain of connected reservoir pores exhibiting realistic solvent displacement efficiency. Diffusion and convective dispersion effects concurrent with the fluid flow direction are included in the result, but not the tri-dimensional sweep efficiencies characteristic of a real reservoir element. Although the fluid flow rate in a conventional slim tube run is about 10 times reservoir flow rates in any given direction, longer runs at 1/10 to 1/30 at laboratory rates do not indicate severe changes in performance.

Data enhancement procedures, verified by improved agreement between measured and calculated initial oil density and effluent reservoir volumes, improve comparisons between slim tube runs. Interpretation benchmarks can be established to clarify the characterization of each run as MCM or immiscible.

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