Rock/Fluid Interactions of Carbonated Brines in a Sandstone Reservoir: Pembina Cardium, Alberta, Canada

Abstract

Summary When CO2 is injected into petroleum reservoirs it forms carbonic acid in the brine phase and interacts with reservoir rock. Flow tests were performed by continuously circulating CO2-saturated brines through Cardium formation cores. All the cores initially showed a large drop in permeability, after which permeability rose steadily but did not regain its initial value. Microscopic examination of the cores indicated that fines had been released and had migrated toward pore throats, reducing permeability. In addition, mineral alterations occurred, including the dissolution of calcite and siderite, which may account for the gradual rise in permeabilities noted in the experiments. Introduction When CO2 is injected into petroleum reservoirs, it partitions between the oil, brine, and gas phases, thereby forming carbonic acid in the brine phase. As a result, the brine has a lower pH and contains carbonate and bicarbonate ions and undissociated CO2. The work in this paper is concerned primarily with the effects of this mechanism on the properties of reservoir rocks, and consequently, on the EOR process. In this paper, we review previous relevant works, describe the experirnental procedure used to flood Pembina Cardium cores with carbonated brines, and present test results. Literature Review Sandstone reservoir rocks, in general, contain siliceous material, clays, and various carbonates. The latter are predominantly calcium, although iron and magnesium carbonates are also common. These minerals are present in various proportions in different rocks and react differently to the changing environment brought about by CO2 injection. Thus, the net effect of the CO2-injection process depends on the type of rock, the fluids being injected, the injection rates, and the reservoir conditions. Silica is inert to CO2 and to carbonated brines at normal reservoir temperatures because a very strong acid (e.g., hydrofluoric acid) is required to dissolve quartz-rich sandstone. Some siliceous minerals like iron chlorite, however, are unstable in acidic environments and become water soluble. On the other hand, alkaline materials at pH>9 react readily with silica.1 At higher temperatures, typical of steamflood conditions, silica reacts with water to form the soluble silicic acid H4SiO4, as discussed by Stone et al.2 Carbonate minerals like calcium and magnesium carbonate react readily with carbonated brines.3–7 The main reactions that occur during the dissolution of CO2 in water areEquations In the presence of CaCO3, MgCO3, or FeCO3, the following reactions lead to the formation of water-soluble bicarbonates:Equations These latter reactions can cause dissolution of the main rock matrix of carbonate reservoirs, leading to the formation of new flow paths and an increase in rock permeability. On the other hand, carbonate material may be the cementing agents for sand and clay particles in sandstone reservoirs. In such a case, the dissolution of the cement can cause these particles to be released, allowing them to move in the flow path, to accumulate at pore throats, and to reduce permeability. Conversely, an increase in permeability may be observed if the released particles are smaller than the size of the pore throats and can be flushed out. Ross et al.8 and Bathurst9 mentioned that an increae in (1) temperature at constant CO2 partial pressure, (2) pressure at constant CO2 concentration and temperature, or (3) the amount of CO2 dissolved in the brine (but only to a certain extent, after which the effect is reversed) causes an increase in the solubility of calcite. Thus a decrease in pressure can cause the precipitation of calcium in the form of CaCO3 according to the following reactions.4,9Equations These reactions lead to the formation of an insoluble scale of CaCO3, which can cause a reduction in matrix permeability. This can be a problem at wellbores because large pressure drops occur and scale is liable to form. Przybylinski7 studied the problem of scale formation in detail and concluded that its rate of formation is proportional to the seed surface area available and the concentration of carbonate and bicarbonate ions. Hall and Lansford10 proposed that sodium oxalate be added to the chase brine of CO2 slugs to precipitate out the dissolved calcium ions as calcium oxalate. Clays are a group of minerals whose behavior under carbonated-brine flooding is poorly understood. An excellent summary on clays and their characteristics is the monograph edited by Longstaffe.11 As Brindley12 describes, clays are layered silicates, containing sheets of tetrahedrally and octahedrally coordinated atoms. Various cations, water molecules, hydroxy and hydrated complexes, and organic liquids can be found between these layers. Almon and Davies13 subdivided clays into four different types on the basis of mineralogy and reactivity. 1.Kaolinite: a hydrated alumino-silicate that often occurs in the form of plates and strings and can migrate as fines. 2. Smectite: an alumino-silicate in which part of the aluminum has been replaced by magnesium, sodium, and calcium. These clays are prone to swell in the presence of water, to dislodge from pore walls, and to migrate. 3. Illite: a hydrated potassium alumino-silicate that has several morphologies and can sometimes be found as long fibrous crystals. These crystals can form bundles or break off and pile up against pore throats, resulting in a loss of rock permeability. 4. Chlorite: a hydrated alumino-silicate that contains large amounts of iron and magnesium and that reacts readily with many acids and oxygenated waters. Iron dissolved from the chlorite can be reprecipitated in the form of Fe(OH)3 if the acid is spent.