A Comparative Investigation of Minimum Miscibility Pressure Correlations For Enhanced Oil Recovery

Abstract

Abstract Multiple contact miscible floods involving the injection of relatively inexpensive gases into oil reservoirs represent one of the most cost effective enhanced oil recovery processes currently available. The experimental displacement procedures available for determining the optimal flood pressure, referred to as the minimum miscibility pressure (MMP), are both costly and time consuming. Hence, the use of a correlation proven reliable over a large range of conditions would likely be considered acceptable for the purposes of preliminary screening studies. This paper presents an evaluation of 15 rich gas, lean gas, and nitrogen MMP correlations published in the literature. Each method was developed using experimental data, equation-of-state (EOS) predictions, or a combination of the two. The accuracy of each correlation was evaluated by comparing predicted versus measured MMP's using a data base consisting predominantly of experimental results published in the literature. The most reliable MMP correlations were all found to have a common EOS developmental basis. These results support the applicability of EOS-based methods for accurate MMP predictions. Introduction The injection gases most commonly used for enhanced oil recovery processes are generally not miscible upon first contact with the reservoir fluids that they are displacing. However, under suitable reservoir pressure and temperature conditions, miscibility may gradually be developed between some injection-gas/reservoir-oil combinations by a mass transfer of components between the gaseous and liquid phases. Miscibility generated in such a manner is commonly referred to as multiple contact (dynamic) miscibility, and has been well described in the literature(1). The vaporizing gas drive (VGD) and the condensing gas drive (CGD) are the mechanisms through which dynamic miscibility is generally explained. Miscibility develops at the flood front during VGD processes, which are commonly referred to as lean gas (LG) drives. Dynamic miscibility in nitrogen (N2) floods similarly occurs through the VGD mechanism. During CGD processes, generally referred to as rich gas (RG) drives, miscibility develops at the injection point. Recent research has indicated, however, that miscibility in some RG drives may actually develop through a liquid extraction drive (LED) process(2,3). Similar to VGD's, miscibility develops at the flood front during a LED. An optimum displacement pressure exists for a dynamic miscible flood, commonly referred to as the minimum miscibility pressure (MMP). The object of this study is to evaluate available RG, LG, and N2 MMP correlations using a data base consisting predominantly of experimental MMP results published in the literature. Measurement and Prediction of MMP's The MMP for gas/oil mixtures has traditionally been measured by performing slim tube displacement experiments(1). These experiments involve constant temperature displacement of a live oil from the slim tube by an injection gas. The MMP has typically been accepted as the pressure at which a practical maximum recovery efficiency is observed following a series of displacements. MMP values have also been measured for many gas/oil pairs using the Rising Bubble Apparatus (RBA)(4), where a small bubble of gas is injected at the base of a column of live oil.