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.

Maximizing Storage Rate and Capacity and Insuring the Environmental Integrity of Carbon Dioxide Sequestration in Geological Reservoirs

“…Other studies have observed reduced injectivity in some calcareous sandstone and carbonate formations (Henry, and Feather, 1981, Ross, et al 1982, and Sayegh et al, 1990. Also, the tendency of scaling of production water during CO 2 injection has led some researchers to investigate the inorganic geochemical aspects of CO 2 reservoir interactions (Bowker andShuler, 1991, Shuler et a.1991).…”

Anisotropy and spatial variation of relative permeability and lithologic character of Tensleep Sandstone reservoirs in the Bighorn and Wind River basins, Wyoming. Final technical report, September 15, 1993--October 31, 1996

“…A geostatistical study of small-scale heterogeneity is more difficult because of the more subtle differences in sedimentation types within eolian sandstones. Authors who have done similar work on eolian and other types of sediments (Grant et al, 1994 andProsser andWilliams, 1993) (Ross et al, 1981;Omole and Osoba, 1983;Sayegh et al, 1990). However, carbonate scale formation and corrosion, which accompany CO2 flooding, are costly.…”

“…Assuming that changes in permeability should be due to dissolution or precipitation of carbonate minerals at the pore throats, Omole and Osoba concluded that CO2 flooding into carbonate reservoirs would result in the dissolution of carbonate minerals near injection wells and precipitation of them at production wells. Sayegh et al (1990) carried out CO2 flooding experiments at 45°C and 1.38xl07Pa (138 bars) using sandstone cores from the Pembina Cardium oil field, Alberta, Canada. They observed that the permeability of cores rapidly decreased at the early stage of the experimental runs and then increased gradually throughout the remainder of the experiment.…”

Experimental Investigation of CO<sub>2</sub>-Water-Rock Interactions during CO<sub>2</sub> Flooding in Carbonate Reservoir

“…Injected CO 2 dissolved in formation water to produce carbonic acid which will react with the reservoir rock [10] [11] [12]. The porosity and permeability of the reservoir [13] [14] [15] [16] [17], the wettability of the rock surface and the characteristics of the crude oil will be changed by the reaction between carbonic acid and the rock [18] [19], which will affect the efficiency of CO 2 flooding and geological sequestration of CO 2 [20] [21] [22]. And the carbonate minerals in the reservoir can especially easily react with CO 2 and water to affect reservoir property.…”