Recent Developments In Slim Tube Testing For Hydrocarbon-Miscible Flood (Hcmf) Solvent Design

Abstract

Abstract Testing of solvent effectiveness related to a given crude oil under simulated reservoir conditions began in the early 19508. 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 the hydrocarbon miscible flood (HCMF) process has increased, slim tube test equipment and procedures have become more sophisticated. The current state of the technology is discussed, including laboratory equipment, instrumentation, operating procedures and analysis of results. Procedures for analysis and interpretation of slim tube test results are discussed, leading to improved solvent design 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 parameters. Design engineers seek to find a solvent which will have a reasonable chance of attaining the desired increase in oil recovery in the reservoir. Enrichment with intermediates must exceed an economically acceptable safety margin above the minimum miscibility concentration (MMC), which defines the boundary between multi-contact miscible (MCM) and immiscible performance. This usually implies selecting the solvent analysis corresponding to one or more percentiles of slim tube oil recovery above the MMC recovery, 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 can then be used as the basis for solvent 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 coreflood 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 concurrentwith the fluid flow direction are included in the result, but not the tri-dimensional sweep efficiencies characteristic of a real reservoir element. Data enhancement procedures, verified by improved agreement between measured and calculated initial oil density and effluent reservoir volumes, improve comparisons between slim tube runs. Interpretion benchmarks can be established to clarifythe characterization of each run as MCM or immiscible. A set of four key performance correlations for each run show how solvent performance compares to benchmarks, and aid characterization of the dominant solvent process.