Miscible And Immiscible Displacement Studies On Carbonate Reservoir Cores

Abstract

Introduction The use of nitrogen injection to stabilize water encroachment and to improve gas recovery from carbonate reservoir cores has been investigated. A series of linear displacement experiments was conducted on full-diameter plugs representing selected facies of the reservoir. Miscible displacement of methane by nitrogen and immiscible displacement of methane by water were measured. The recovery of trapped methane from the water flooded cores by nitrogen injection was also obtained. The rate of methane recovery was found to depend on the initial water saturation, the displacement rate within the plug and the lithology of the individual rock samples. A composite core made up of three plugs showed that the individual, plug tests gave conservative results for methane recovery efficiency. In order to compare methane recovery in a homogeneous system with the heterogeneous carbonate system, a Berea sandstone plug was subjected to the same suite of tests. It was found that the role of mass transfer had different effects on the rate of recovery of methane in the two systems. Introduction The effect of an active bottom water drive on gas recovery from an Alberta carbonate reservoir led to the initiation of this laboratory study. Gas displacement with water was conducted on cores from the reservoir facies in order to quantify trapped gas saturations. The effectiveness of nitrogen injection to reduce this trapped gas content by miscible displacement of the reservoir gas was also determined. Previous studies of displacement(1,2), and pore geometry(3,4) in sandstone or carbonate cores have involved the use of small plugs which have only given the effects of the matrix geometry. Unfortunately, interpretation of displacement data obtained using reservoir cores which contain vugular porosity can be somewhat subjective(5,6) and the dimensions of the heterogeneities must be relatively less than the size of the core being examined. Thus, the dolomite cores employed were full diameter and as long as possible in order to better reflect the presence of vugs and other types of porosity and their influence on gas recovery. Three selected non-preserved, full-diameter cores, which represented typical facies within the reservoir, were subjected to gas-gas, water-gas and gas-water displacement. The three cores, labelled A, B and C, are described in Table 1. They are all characterized by well-developed moldic to vuggy porosity with varying degrees of fracture development and matrix permeability. Similar displacements were conducted on a composite core made up of the three cores, a homogeneous Berea sandstone and a fractured carbonate core with well-developed inter-crystalline porosity. Each carbonate core is pictured in Plate 1. A qualitative description of the porosity types represented in each core could be summarized as:vug – tight matrixvug – fracturevug – matrixcompositematrix. (Berea Sandstone)matrix -fracture All measurements were conducted at constant flow rate and 6900 kPa (1000 psig) core outlet pressure. The tests included:Downward displacement of methane by nitrogen at several rates in dry cores.