Hydrocarbon Solvent Design Using Miscible Equivalence
Authors: Batycky J.P. , Vittoratos E.S. , Wu R.S. 
Published: March 2010

The operation of a miscible gas displacement project involves the blending of one or more dry gas and LPG feed streams to obtain a solvent for injection. The solvent scream composition must, according to the desired design criterion (e.g. first-contact miscibility), be rich enough to achieve miscibility within the reservoir oil without being richer than required.

Most project designs have been based on miscibility assessments using simple (two- or three-component) solvents and reservoir oils in VLE experiments, slim tube tests, and/or equation-of-state predictions. Because each stream making up the solvent is invariably multi component with both dry gas and solvent components present in each, this simplistic approach can neither adequately nor easily incorporate the impacts of changing feed stream compositions.

This report presents a new technique which simply requires hand calculations to rigorously accommodate alterations in feed stream compositions and reservoir pressures in arriving at a suitable solvent design. As a first step it is necessary to quantify the contribution of each component, termed miscible equivalence, ME, toward achieving solvent/oil miscibility. This can be accomplished using for example, a "tuned" equation-of-state. Application of the method to specify solvent compositions in the field involves computing the amounts of dry gas and LPG streams until the summed miscible equivalence value is equal to the solvent design requirement.

The accuracy of this procedure is illustrated for a number of pools by comparing the ME procedure with equation-of-state results. In addition, the comparison of ME values for individual components in different fields has merit in scoping LPG distribution options.

Introduction

The development of universal guidelines to predict the attainment of miscibility between an injected solvent and the reservoir oil has been pursued for more than 25 years with mixed success. Evaluations of miscibility have been conducted experimentally using vapour-liquid equilibria (VLE) measurements and/or slim tube displacement tests. As pointed out by Benham et al.(1), a suitable correlation must depend on reservoir fluid composition (including saturation pressure), reservoir temperature, operating or design pressure, and solvent composition.

It has been recognized that each component in a solvent stream can contribute positively or negatively to solvent/ oil miscibility with varying levels of effectiveness.

Benham et al.(1) suggested using the molecular weight or the hydrocarbons making up the intermediates in the solvent as an indicator of solvent/oil miscibility. Rutherford(2) went one step further and showed explicitly with ethane and propane as intermediates, that miscibility could be achieved when the pseudocritical temperature of the solvent exceeded a certain value which presumably would be reservoir and fluid dependent. Jacobson (3) quantified the impact of acid gases such as CO2 by correcting their critical temperatures for purposes of miscibility assessment. However, based on the time-consuming nature of the experiments, the appropriateness of pseudocritical temperature was not rigorously examined.

The use of molecular weight or pseudocritical temperature to characterize a solvent would each produce similar results since the miscibility contribution of higher molecular weight components would be valued above those with lower molecular weights.