Acid Gases And Their Contribution to Miscibility
Authors: Jacobson H.A. 
Published: March 2010

Laboratory slim-tube displacement tests have provide new insight into the relative effectiveness of sour gas components in achieving miscibility with reservoir crude. Test results using subsurface fluid samples from the Comet "T" Pool of northwestern Alberta indicate that, as a miscible agent, hydrogen sulphide is slightly more effective than ethane. Carbon dioxide, however, had to be injected undiluted to obtain miscibility. Benham's correlation for predicting miscibility has been evaluated and found to be quite adequate for "sweet" gases, although conservative by at least 4 mol per cent in the required C2-C4 composition. For sour-gas systems, we find that individual components contribute to miscibility in proportion to the relative magnitude of their equilibrium constants. Pseudocritical temperatures are also indicative of miscibility, as suggested by Rutherford, and the possibility of extending this guideline to apply to sour-gas systems is indicated.

Introduction

IN RECENT YEARS, a fairly large number of reservoirs have been found in western Canada where the acid gases - hydrogen sulphide and carbon dioxide - are major components in the solution gas produced. Although there are attractive prospects for miscibly flooding some of these reservoirs, there is still little understanding of just how the presence of acid gas components influences solvent-crude miscibility.

The displacement of oil from a reservoir is miscible when there is no phase boundary or interface between displaced and displacing fluids. Displacement of oil by water is immiscible; displacement of oil by gasoline is miscible. Under certain conditions, fluids such as propane, mixtures of methane with propane or similar combinations of hydrocarbons will give a miscible - and therefore highly efficient - displacement of oil.

A. L. Benham(1) devised a relatively simple method for calculating the approximate conditions for a miscibility. This information should provide a better These gases included hydrocarbon components from methane through butane, but excluded hydrogen sulphide and carbon dioxide.

Through a series of displacement tests. W. M. Rutherford(2) found that miscibility between reservoir oil and displacing gas is a function of the pseudocritical temperature of the injected fluid. Again, this relationship applied only to light-paraffin hydrocarbons; the miscibility behaviour of other natural-gas components such as carbon dioxide and hydrogen sulphide was not investigated.

The main objective of the present work was to evaluate the effectiveness (in comparison with ethane, propane, etc.) of H2S and CO2 in contributing to miscibility. This information should provide a better basis for designing miscible displacement projects where acid gases occur as constituents of the available source injection fluids. With a sour injection gas it should be known, for example, whether a sweetening operation is justified or whether hydrogen sulphide should be left as a component of the solvent bank. In evaluating the economics of such alternatives it is necessary to know whether the add gas contributes tomiscibility in the same way as an equivalent concentration of butane, for example, or serves rather as a noncontributing diluents.

Experimental Procedure

In all, fourteen displacement tests were run using a 10-foot by 0.25-inch-I.D. "slim-tube", packed with fine (200-mesh) glass beads.