Phase III - Department of Mines and Petroleum

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There is a tendency to treat geochemical and geomechanical effects as independent of each other. One concern is that the number of geochemical reactions predicted by the modelling (although predicted porosity changes are small), may geomechanically weaken the seal. If feasible, “chunks” of the seal should undergo geochemical autoclave experiments and the reaction products undergo geomechanical testing. Integrity of Existing Wells Long-term well integrity issues for the geological disposal of CO 2 are currently not well understood and are an area of active research by many groups internationally. The GJV have recognized this situation in their documents ‘Technical Evaluation and Basis for Development Concept’ and ‘Uncertainty Management Plan’, and have incorporated operational and abandonment practices to accommodate these uncertainties. The strategy for abandonment of the existing 27 (pre-Data Well) wells penetrating the Dupuy Formation will be reactive. If the monitoring and surveillance plan and plume migration forecasts indicate that CO 2 leakage from an existing well is imminent, plans will be developed which may include plug and abandonment operations. These operations may incorporate multiple levels of redundancy for long-term sealing. The generic cased-hole and open-hole remediation operations developed by the GJV are consistent with and generally surpass current international well abandonment approaches. For the cased-hole plug and abandon generic programme, the GJV has gone beyond operational concerns and recognised the importance of eliminating potential areas of leakage due to long term corrosion of the casing. The GJV have, however, identified three existing wells (the Data Well, P18J and U22J) that may potentially have the greatest exposure to the CO 2 plume and, it is recommended that a more comprehensive abandonment plan for these three 'at risk' wells should be adopted. The acquisition of downhole materials (casing steel and cement), during abandonement operations of these wells, and the assessment of their ageing characteristics could assist the GJV with managing the abandonment requirements of the remaining well assets on Barrow Island. This might also assist in formulating the most appropriate abandonment options for the injection and observation wells. Injectivity Injectivity estimates depend on the reservoir model and the data that was used to develop it. The GJV have undertaken a number of studies addressing injectivity and further issues are expected to be addressed as part of their Phase IV technical work programme. Injectivity Data Sources Injectivity performance predictions can be made using data from routine core analysis, mini-permeameter measurements, core-flood experiments, well testing and numerical reservoir simulations. The GJV analysed 1593 core samples and collected 4518 mini-permeameter readings over a 503 m interval from the Data Well. The GJV workflow makes significant use of numerical simulation, as is accepted practice in petroleum field development, and this is considered the most appropriate tool for analyzing injectivity in heterogeneous reservoirs. Well Tests An injection test was conducted in the Data Well using brine as the injection fluid as opposed to CO 2 . This allowed the horizontal permeability of the formation to be determined with greater certainty than if CO 2 had been used as an injection fluid. The GJV assessed the impact of multiphase flow on injectivity via a combination of an extensive review of published data and numerical reservoir simulation models. The GJV faced some operational difficulties during their well testing programme (e.g., cement debris and formation material plugging pore throats). We believe that the GJV responded to these problems with appropriate modifications to their well testing programme. The results of the three drill stem tests (DSTs) xxix
are consistent and show horizontal permeabilities of 21.7 to 28.8 mD (compared with 30-105 mD determined from routine core analysis for the same intervals). Vertical permeability was also assessed during one of these DSTs and during vertical interference tests conducted as part of the MDT programme. Several possible interpretations of these data which imply the vertical to horizontal permeability ratio (k v /k h ) might range from 0.03 to 0.26 were presented. The very high skin factors required in the DST interpretations were unexpected and subsequently attributed to pore throat plugging. Although the GJV has some contingency plans to remediate skin damage, it is unclear what skin factors would be encountered in future injection wells drilled in the field and this will need to be included as an uncertainty in subsequent injectivity analyses. Injectivity Indices Typically injectivity is controlled by four key parameters: • pressure, both the maximum achievable bottom hole pressure in the injection wells (to avoid fracturing the formation) and the fluid pressure in the formation; • the absolute permeability of the formation; • the relative permeability between the different fluids in the formation (i.e., irreducible water saturation); • skin factor in the injection wells. The injectivity index, which relates the injection rate to the pressure difference between the injection well and the reservoir, is best determined in heterogeneous reservoirs like the Dupuy by numerical simulation. The injectivity index is clearly affected by the maximum permissible well bore pressure in the injection well that avoids hydraulically fracturing the formation near the well. Creating a hydraulic fracture could enhance injectivity, although it is viewed as undesirable since it may create a fracture that propagates vertically upward and breaches the top seal. Thermal stresses also affect the potential to fracture the near well formation since cooling an annular region around the injection well can reduce the fracture gradient of the reservoir. The GJV have initiated studies to understand these effects on the fracture gradient, and have demonstrated that thermal effects are potentially significant. Geomechanical factors such as the in-situ stresses, and the elastic modulus and Poisson's ratio of the reservoir formation are also shown to be important in controlling the pressure at which these rocks will fracture. While geochemical issues, such as scale formation, carbonate dissolution and fines migration may also impact injectivity, they are considered to be secondary to the above geomechanical factors. The role of relative permeability in injectivity was assessed by the GJV using a Monte Carlo approach and indicates that relative permeability is one of the dominant uncertainties in injection predictions. While skin factor is not as significant an uncertainty, the GJV injectivity report recommends that laboratory experiments be performed to assess possible damage mechanisms. Phase Behaviour The main effects of CO 2 -H 2 O phase behaviour in reservoir simulation are captured via relative permeability curves. However, evaporation of pore-water into the injected CO 2 near the injection well, as discussed above, could dehydrate a region close to the injection zone and the significance of these effects should be calibrated by core-flood experiments. It is our opinion that relative permeability curves are the most important control on injectivity related to the interaction of CO 2 and brine, and should be calibrated by experiment. Geochemical issues, such as fines release from dissolution of carbonates, should definitely not be ignored and could also be assessed through experiments; however the shape of the total mobility curve is more likely to control injectivity on time scales of ~20 years. xxx
Page 1 and 2: Executive Summary Overview The Grea
Page 3 and 4: considered to be a thorough and sou
Page 5 and 6: Figure 2 Geological structure of th
Page 7 and 8: the CO 2 injectors in locations tha
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Page 15: Geochemical Modelling Comprehensive
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Page 21 and 22: Barrow Island the focus is on the e
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Page 25 and 26: y additional tensile strength testi

Phase III - Department of Mines and Petroleum

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