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The well integrity modeling work will be presented at the upcoming Celle Drilling 2022 conference (September 13-14)

Posted July 6, 2022
Al Moghadam, TNO

Assessing the Impact of Temperature Cycles on well Cement in Low Enthalpy Geothermal Wells

Geothermal energy is an important candidate to aid the transition to a low-carbon economy. Lowenthalpy geothermal wells are particularly important to jurisdictions with a low geothermal gradient, such as the Netherlands. In order to be economically viable, low-enthalpy wells should be relatively cheap to drill and last for several decades, longer than typical oil and gas wells. The required longevity for these wells presents new challenges in terms of well integrity, particularly related to cement and casing damage.

Geothermal wells experience temperature cycles as they are suspended for various operational reasons. This leads to a change in the casing/cement temperature along the well, for hundreds or thousands of cycles during its lifetime. The temperature cycles lead to changes in cement stress, particularly due to the differential thermal expansion of the casing/cement/formation system. In order to ensure a long operational life, the cement formulation should be designed to be able to withstand the cyclical stresses induced by the temperature cycles. A damaged cement sheath can expose the casing to corrosive fluids and open a leakage pathway to shallow freshwater aquifers.

In this work, we have developed a 3D well integrity model that incorporates the cement hydration process. The model is previously verified using laboratory experiments. A case study is designed to represent a typical lowenthalpy geothermal well in the Netherlands, using well designs and inputs from publicly available data. The cement stresses are tracked over the life of the well, to understand the magnitude of the stress cycles and assess the potential long-term damage to the cement sheath.

The results show that the pore pressure drop due to cement hydration causes an increase in shear stress in the cement sheath. The pore pressure drop during hydration can debond the cement from the formation, barring the use of expansive agents in the cement recipe. During the temperature cycles, the tangential and axial stresses in cement change more than the radial stress. Axial stress in the cement sheath is particularly sensitive to temperature changes due to the axial movement of the casing. The shear stress in the cement sheath cycles between 16 and 18 MPa (2,300 and 2,600 psi) as the temperature fluctuates between 80 and 30 °C (176 and 86 °F), at a depth of 2000 m for the particular conditions in the present case study.

The modelling technique presented in this work provides a robust methodology to estimate the magnitude of cyclical stresses in the cement sheath of low-enthalpy geothermal wells. This is a critical input to design cement recipes that can withstand thousands of load cycles throughout the lifetime of a geothermal well.

Modelling stress evolution in well cement during hydration

The integrity of cement/casing bond in a plug or an annulus must be maintained to ensure zonal isolation in wells. The initial stress state of cement after curing is a critical parameter that sets the safe operating window for the subsequent pressure and temperature changes in a well. Recent experimental studies indicate that cement plugs experience a loss in pore pressure and radial stress during hydration. The drop in radial stress could result in cement debonding from the casing.

In this work, we have developed a methodology to model the stress evolution in cement plugs during hydration. The model begins with the slurry state of cement and calculates the water consumption and void creation over time as the hydration reactions progress. The void volume change due to chemical shrinkage is imported into a coupled mechanical model that calculates the pore pressure drop and the resulting change in radial stress. The results of the proposed modelling methodology are verified using lab experiments from the literature.

The results provide new insights in understanding cement behavior under lab and field conditions. Under most scenarios, the cement pore pressure drops to the water saturation pressure which leads to evaporation of the remaining pore water. This pore pressure drop controls the radial stress change, according to the theory of poro-elasticity. For a plug set under an initial pressure of 40 MPa, the radial stress drops to 20 MPa after 40 hours of curing. For an initial pressure of 20 MPa, radial stress decreases to 8 MPa after 25 hours. This stress drop can cause the cement to debond from the casing, if the fluid pressure below the plug exceeds the final radial stress. The bulk shrinkage of cement can also be estimated using the model. Bulk shrinkage is equal to chemical shrinkage until a solid skeleton is formed in cement. Afterwards, bulk shrinkage is driven by the pore pressure change and the physical boundary conditions.

The proposed methodology in this work demonstrates that the pore pressure and stress drop in cement can be effectively modelled. This allows us to determine the initial stress state of cement after curing to better understand the integrity of the annular cement and cement plugs, under field conditions. Furthermore, understanding the mechanisms responsible for the cement stress drop helps in designing better cement recipes that can maintain higher initial tress levels.

Current work on supercritical cement was presented at the Stanford Geothermal Workshop

Posted Nov. 1, 2021

This work is a part of a joint international effort to develop “sustainable geothermal well cement for challenging thermo-mechanical conditions” (Test-Cem) under the umbrella of Cofund GEOTHERMICA. The paper describes an exploratory study of cementitious composites for applications in geothermal wells under super-critical conditions. Preliminary screening of a wide range of blends was done on formulations mixed at room temperature, cured under hydrothermal conditions at 85oC (overnight) followed by overnight hydrothermal curing at 300oC and the final curing under the super-critical conditions at 400oC and pressure of 25.5 MPa (3,700 psi). The tested formulations included high-temperature Portland cement modified with silica (CaO-SiO2 system), P2O5-Na2O-CaO-Al2O3-SiO2 – based systems with varied Al/Si ratios, and P2O5-Na2O-CaO-Al2O3-SiO2-Fe2O3 phosphate systems, Na2O-CaO-Al2O3-SiO2-MgO-Fe2O3 and Na2O-CaO-Al2O3-SiO2-MgO Mg-containing systems, and Na2O-Al2O3-SiO2 cement system.

Mechanical properties of the blends and crystalline phase transitions were determined after the initial curing and exposure to the supercritical conditions to understand effect of phase transitions upon materials performance. Formulations with short-term compressive strength development of no less than 6.9 MPa (1,000 psi) and stable phase compositions were identified for further testing to fully evaluate their true potential.

Cement Formulations for Super-Critical Geothermal Wells