High-carbonate rock

High carbonate rock is a pure form of limestone, which contains more than 95 % (m/m) of calcium and magnesium carbonates. It may be may be high calcium, magnesian, or dolomitic. 

Carbonate rocks are more frequently fractured than sandstones. In many cases open fractures in carbonate reservoirs provide high porosity / high permeability path ways for hydrocarbon production. The fractures will be continuously re-charged from the tight (low permeable) rock matrix. During field development, wells need to be planned to intersect as many natural fractures as possible, e.g. by drilling horizontal wells. 

A reservoir is not a subterranean lake of pure oil or a cavity filled with gas. It is a porous and possibly fractured rock matrix whose pores contain oil, gas, and some water, or else, more rarely, it is a highly fractured rock, whose fractures contain the fluids. Such a resewoir is usually located in sandstone or carbonate rock. The rock matrix of an exploitable reservoir must be porous or fractured sufficiently to provide room for the hydrocarbons and water, and the pores and fractures must be connected to permit fluids to flow... 

When the temperature of a carbonate reservoir that is saturated with high-viscosity oil and water increases to 200° C or more, chemical reactions occur in the formation, resulting in the formation of considerable amounts of CO2. The generation of CO2 during thermal stimulation of a carbonate reservoir results from the dealkylation of aromatic hydrocarbons in the presence of water vapor, catalytic conversion of hydrocarbons by water vapor, and oxidation of organic materials. Clay material and metals of variable valence (e.g., nickel, cobalt, iron) in the carbonate rock can serve as the catalyst. An optimal amount of CO2 exists for which maximal oil recovery is achieved [1538]. The performance of a steamflooding process can be improved by the addition of CO2 or methane [1216]. 

So-called "wormholes" can be formed when the injected acid primarily enters the largest diameter flow channels in carbonate rock further widening them (107). Acid only invades the small flow channels a short distance greatly reducing treatment effectiveness. High fluid loss rates, low injection rates, and reduced rates of acid-rock reactions decrease the wormhole length. 

Extractions of three of the coals were also carried out in a rocking autoclave (method B) mainly to allow analysis of the gases to be carried out. The results of these extractions are shown in Table II. The high carbon dioxide yields obtained were indicative... 

Let us now consider the problem from the standpoint of calcite precipitation kinetics. At saturation states encountered in most natural waters, the calcite reaction rate is controlled by surface reaction kinetics, not diffusion. In a relatively chemically pure system the rate of precipitation can be approximated by a third order reaction with respect to disequilibrium [( 2-l)3, see Chapter 2]. This high order means that the change in reaction rate is not simply proportional to the extent of disequilibrium. For example, if a water is initially in equilibrium with aragonite ( 2c=1.5) when it enters a rock body, and is close to equilibrium with respect to calcite ( 2C = 1.01), when it exits, the difference in precipitation rates between the two points will be over a factor of 100,000 The extent of cement or porosity formation across the length of the carbonate rock body will directly reflect these... 

It can be seen that in the region of Devonshire Marsh, located near the thickest part of the freshwater lens (Figure 7.27), that the waters are subsaturated with respect to calcite and aragonite and have high CO2 pressures, apparently derived from organic matter oxidation in the marsh area. The waters have low salinities, low Sr2+ concentrations, and little Mg2+ and Ca2+ derived from dissolution of carbonate rock minerals. Toward the south shore of Bermuda and eastward from Devonshire Marsh, the salinity of the waters increases, and the saturation state approaches near-equilibrium with calcite, and supersaturation with respect to aragonite. Lower Pc02 values characterize the waters farther away from Devonshire Marsh. 

The timing of the dolomitization of carbonate rock bodies and emplacement of dolomite cements has been one of the more controversial aspects of the "dolomite problem." Most of the basic factors controlling dolomite formation, where were discussed in Chapters 6 and 7, also apply to dolomite formation during the later stages of diagenesis. However, the extended periods of time, the solution compositions likely to be encountered, and the elevated temperature and pressure that occur during deep burial provide highly favorable conditions for dolomite formation. 

Halley R.B. and Schmoker J.W. (1983) High porosity Cenozoic carbonate rocks of south Florida Progressive loss of porosity with depth. AAPG Bull. 67, 191-200. 

P. Simell, J. Leppalahti and E. Kurkela, Tar decomposing activity of carbonate rocks under high C02 partial pressure. To be published in Fuel in 1994. 

High HC03 and pH and low Cl below the carbonate rock section, along with high Ca and Mg (samples 1-36 and 1-41 in Table 10.2). 

Defluorinated Phosphate Rock. There is substantial production of defluorinated phosphate rock for fertilizer use in Japan (about 100,000 mt/year). Ground, high-grade rock is mixed with small proportions of sodium carbonate or sulfate and wet-process acid. The mixture is calcined at a temperature of 1350°C in an oil-fired rotary kiln 45.0 m in length and 2.7 m in diameter. The product contains 38-42 percent P205 of which more than 90 percent is soluble in neutral ammonium nitrate solution and is an effective fertilizer on acid soils. During the production of defluorinated phosphate rock, substantially all fluorine is driven off. Sodium bifluoride (NaHF2) is recovered as a byproduct. A similar product is made in the United States, but it is mainly used for animal feed supplement. 

Most natural water systems in contact with calcite (oceans, rivers, lakes, carbonate rock aquifiers) are, however, near equilibrium, and PCO2 dependence cannot be ignored. According to our model, the rate of backward reaction is a significant function of surface pH, and surface pH is determined by calcite equilibrium at the surface PCO2. At the relatively high pH, low PCO2 conditions of most natural waters, the surface pH is least well defined and may depend, in part, on the flux of CO2 between the surface and bulk fluid. 

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