Development of an integrated assessment procedure for the integrity of wellbore cement in CO2 storage wells

[摘要] Overview of storage well integrityWells are the key technology for both storing CO2 and monitoring its reservoir migration. However new and existing wells could also represent a risk to storage assurance by potentially providing leakage pathways. The construction of wells involves the use of cement for two main functions; to cement well casing in place and to plug wells that are abandoned. CO2 could leak from abandoned wells by flowing through a degraded, damaged or incomplete plug or outside the casing, between the casing and the formation. While one outcome is that CO2 could migrate directly to surface via the well there is also the potential for CO2 to migrate to other formations which may not have appropriate geology to prevent subsequent leakage. Given the long time frames involved with CO2 storage even small leakage rates could lead to significant cumulative loss.Cement degradation is the result of a series of reactions that take place when acidic formation water, resulting from CO2 dissolution, comes into contact with the well. Depending on the solution chemistry this could lead to an increase in the porosity and permeability of the cement. There is also the potential for degradation to lead to a decrease in permeability as precipitates form in the cement porosity. Cement degradation essentially occurs in two stages; first, cement carbonation occurs as the carbonic acid present in water with dissolved CO2 reacts with the various constituents of the cement. These carbonation reactions form calcium carbonate, a solid. Loss of cement structure and strength can occur if this calcium carbonate is then dissolved. However, two conditions are required for calcium carbonate dissolution; a solubility deficit (i.e formation waters that are under-saturated in calcium and carbonate ions) and there has to be water flow to transport these solutes away and bring fresh water into contact with the cement. Since the solid structure of many formations can be largely composed of calcium carbonate, formation waters are often naturally high or saturated in calcium and carbonate ions and thus have little potential to dissolve calcium carbonate from reacted cement. Even though the solubility of calcium carbonate increases significantly with the concentration of CO2, many formations have so much of this mineral that formation waters are likely to remain saturated. However, under-saturated calcium carbonate conditions could still occur, especially in sandstone formations with silicate cements where there is little or even no solid calcium carbonate. In practice, the potential for cement degradation could be largely determined by the mineralogy of the target formation and its ability to keep formation waters saturated in calcium carbonate as the solubility increases with CO2 dissolution during CO2 storage.Experimental characterisation of cement degradationA difficulty in previous published experiments of cement degradation has been measurement of the degradation process as the permeability of intact cement is very low and thus water flow and degradation rates slow, leading to time consuming experiments. A different approach was used in this project; a composite cement-sandstone core plug was used in the core flooding experiments. While this reflects the cement-formation interface within a reservoir it also allows sufficient flow for regular fluid sampling due to the relatively high permeability of the sandstone. The fluid flow through the sandstone means that fresh fluid is brought into contact with the cement and reaction rates are maximised. The contrast between inflow and outflow chemical compositions of the water samples were used to estimate the mass balance of key ions and thus the reaction rates with time. After each core flood the mineralogy of the reacted cement was analysed using x-ray diffraction (XRD). In the plan of work for this project it was proposed that sets of experiments would be conducted to investigate degradation for t

[发布日期] 2014-08-19 [发布机构] CSIRO

[学科分类] 地球科学(综合)