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首页> 美国卫生研究院文献>PLoS Clinical Trials >Thermodynamic Simulation of Carbonate Cements-Water-Carbon Dioxide Equilibrium in Sandstone for Prediction of Precipitation/Dissolution of Carbonate Cements
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Thermodynamic Simulation of Carbonate Cements-Water-Carbon Dioxide Equilibrium in Sandstone for Prediction of Precipitation/Dissolution of Carbonate Cements

机译：碳酸盐水泥-砂岩中水-二氧化碳平衡的热力学模拟，预测碳酸盐水泥的沉淀/溶解

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Carbonate cements, such as calcite, dolomite, ferrocalcite and ankerite, play important roles in the formation of pores in sandstones: precipitation of carbonate cements modifies pores and inhibits compaction, while dissolution creates secondary pores. This work proposed a precipitation-dissolution model for carbonate cements-CO2-H2O system by means of ion equilibrium concentration ([M2+], M = Ca, Mg, Fe or Mn) with different factors, such as temperature, depth, pH, PCO2, variable rock composition and overpressure. Precipitation-dissolution reaction routes were also analyzed by minimization of the total Gibbs free energy (ΔG). Δ[M2+], the variation of [Ca2+], [Fe2+], [Mg2+] or [Mn2+] for every 100 m of burial depths, is used to predict precipitation or dissolution. The calculation results indicate that the increasing temperature results in decrease of equilibrium constant of reactions, while the increasing pressure results in a relatively smaller increase of equilibrium constant; As a result, with increasing burial depth, which brings about increase of both temperature and pressure, carbonate cements dissolve firstly and produces the maximal dissolved amounts, and then precipitation happens with further increasing depth; For example, calcite is dissolving from 0.0 km to 3.0 km with a maximal value of [Ca2+] at depth of 0.8 km, and then precipitates with depth deeper than 3.0 km. Meanwhile, with an increasing CO2 mole fraction in the gaseous phase from 0.1% to 10.0% in carbonate systems, the aqueous concentration of metal ions increases, e.g., dissolved amount of CaFe0.7Mg0.3(CO3)2 increases and reaches maximum of 1.78 mmol·L-1 and 8.26 mmol·L-1 at burial depth of 0.7 km with CO2 mole fraction of 0.1% and 10.0%, respectively. For the influence of overpressure in the calcite system, with overpressure ranging from 36 MPa to 83 MPa, pH reaches a minimum of 6.8 at overpressure of 51 MPa; meanwhile, Δ[Ca2+] increases slightly from -2.24 mmol·L-1 to -2.17 mmol·L-1 and remains negative, indicating it is also a precipitation process at burial depth of 3.9 km where overpressure generated. The method used in this study can be applied in assessing burial precipitation-dissolution processes and predicting possible pores in reservoirs with carbonate cement-water-carbon dioxide.
机译：方解石，白云石，铁方解石和铁矾石等碳酸盐水泥在砂岩孔隙形成中起着重要作用：碳酸盐水泥的沉淀改变孔隙并抑制压实，而溶解则形成次生孔隙。这项工作通过不同因素的离子平衡浓度（[M 2 + ]，M = Ca，Mg，Fe或Mn）提出了碳酸盐水泥-CO2-H2O系统的沉淀-溶解模型，例如温度，深度，pH，<数学xmlns：mml =“ http://www.w3.org/1998/Math/MathML” id =“ M1”溢出=“ scroll”> P CO 2 ，可变的岩石成分和超压。还通过最小化总吉布斯自由能（ΔG）来分析沉淀-溶解反应路线。 Δ[M 2 + ]，[Ca 2 + ]，[Fe 2 + ]，[Mg 2+每100 m埋深]或[Mn 2 + ]用于预测降水或溶解。计算结果表明，温度升高导致反应平衡常数减小，而压力升高导致平衡常数增加相对较小。结果，随着埋藏深度的增加，温度和压力都增加，碳酸盐胶结物首先溶解并产生最大溶解量，然后随着深度的增加而发生沉淀。例如，方解石从0.0 km溶解到3.0 km，最大值为[Ca 2 + ]，深度为0.8 km，然后沉淀深度超过3.0 km。同时，随着碳酸盐体系中气相的CO2摩尔分数从0.1％增加到10.0％，金属离子的水浓度增加，例如CaFe0.7Mg0.3（CO3）2的溶解量增加并达到1.78的最大值埋深0.7 km时，mmol·L -1 和8.26mmol·L -1 ，CO2摩尔分数分别为0.1％和10.0％。受方解石系统中超压的影响（在36 MPa至83 MPa的超压范围内），在51 MPa的超压下，pH值最低为6.8；同时，Δ[Ca 2 + ]从-2.24mmol·L -1 略微增加到-2.17mmol·L -1 并保持负值，表明这也是在3.9 km埋藏深度发生超压的降水过程。本研究中使用的方法可用于评估埋藏沉淀的溶解过程，并用碳酸盐水泥-水-二氧化碳预测储层中可能存在的孔隙。

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