As increased volumes of acid gases (containing carbon dioxide and hydrogen sulphide) are processed, the technique of downhole re-injection of the concentrated acid gas for disposal of these unwanted fluids continues to become more popular. In many cases bottomhole injection temperature and pressure conditions are such that the injected acid gas phase is a supercritical fluid that is miscible with the existing reservoir fluids and, thus, the potential for adverse relative permeability effects (due to the creation of in-situ immiscible liquid and vapor phases) is avoided. In other cases, however, combinations of lower reservoir temperatures and/or initially depleted disposal zone pressures, or blending of the acid gas with the in-situ lean gas, can result in the formation of both liquid and vapor acid gas phases in the formation in the nearwellbore region. This can often cause very significant relative permeability effects, which may result in large reductions in injectivity of the acid gas on either a permanent or transient basis. This paper provides actual examples of such systems, as well as reviewing the design protocol that must be used to evaluate if potential phase behavior problems can occur downhole during an acid gas disposal operation. This has proven to be a key parameter in the successful evaluation of an acid gas disposal operation.
In many acid gas injection operations, the bottom hole pressure condition is high enough that, over the entire life of the injection operation and at all possible blended compositions between the acid injection gas and the reservoir fluids, a monophasic condition is present (generally represented by operation in the 'liquid' or 'supercritical' regions of Figure 1). In some cases, when the initial reservoir pressure is low, initial injection commences in the 'gas' phase region, as illustrated in Figure 1, and as pressure gradually increases, the phase condition of the fluids around the injection wells enters the two-phase region where a 'liquid' acid gas phase is in thermodynamic equilibrium with an acid gas 'vapor' phase. This may also occur when the acid gas blends with reservoir gas which may elevate the dew point pressure line upwards above the current injection pressure level, once again resulting in a condition of two-phase flow. The creation of these two immiscible phases (which have an interface and measurable interfacial tension between them) can, particularly in lower permeability formations, create significant adverse relative permeability effects which may result in large transient or permanent reductions in injectivity, possibly compromising the economics of the acid gas injection operation. Understanding the phase behavior in such situations and how it interacts with the capillary pressure and relative permeability forces present within the matrix of the proposed injection zone is essential, and it is this subject on which the case study presented in this paper concentrates.
Table 1 summarizes the composition of the proposed acid gas feed for a Western Canadian field injection project.