Laboratory Factors Influencing Slim Tube Test Results
Authors: Randall T.E. , Bennion D.B. 
Published: March 2010

The equipment and operating procedure for slim tube testing of hydrocarbon solvent-reservoir oil interactions within a porous medium has now been developed to the point where comprehensive checking of results is practical. Such tests have revealed laboratory factors which can lead to difficulties in interpretation of a solvent test program result. The discussion addresses the impact on test results of reservoir fluid sample quality, processing rate and the slim tube packing material.

If these factors are not considered, either apparent inconsistencies in laboratory results or misinterpretation of displacement process effects can result. Suggestions for avoidance and/or adjustment procedures to delete the extraneous effects will be included.

Introduction

The primary object in solvent design is to identify an injection fluid which effectively removes available oil from the pore space; maintains its integrity as a moving fluid zone in the reservoir; and is cost-effective. The petroleum industry has been working since the early 19505 to find test procedures to identify competent, practical solvents and injection processes for effective, commercial miscible floods.

Reservoir Criteria

Relative permeability effects dictate that any injected solvent must be single phase at reservoir conditions to prevent the formation of high gas saturation zones. Such zones, in turn, could preferentially divert solvent from contacting the remaining oil saturation, reducing volumetric sweep efficiency. Alternate injection of water with the solvent can suppress such problems, but can offset only minor free gas content in the pore space. The design objective is, therefore, to find an injection fluid in which the light and intermediate hydrocarbons will progress through the reservoir at the same rate. Extraneous phases which have low interfacial tension with the reservoir oil do not pose a problem, since they should move as a weak emulsion in the porespace.

Standard Testing

Development of the slim tube test has allowed solvent/oil interactions to be observed and compared in a standardized, controlled experimental environment. The results of several tests can then be compared on a Rutherford plot(1) to define the "minimum miscible concentration" (MMC) and the minimum concentration required for the attainment of effective first contact miscibility (JFCM). The design solvent analysis can then be selected in relation to these limits, balancing the risk associated with too lean a solvent composition against extra cost for unnecessary richness. The concept of design "safety" is thus implicit in the selection process.

The above description implies the need to test interactions between solvent and reservoir oil in a porous medium which resembles the reservoir as closely as possible with respect to temperature, pressure, hydrocarbon fluids in place and pore space configuration. The equipment and procedures often cannot be so precise, leading to the need for adjustment and interpretation methods to ensure valid conclusions from the comparison of results for several tests.

Practical Operation

The inherent instability in natural gas liquids (NGL) supplies has led to the development of solvent design correlations, which permit the injection plant operators to react on an hour-by-hour basis if required.