DESIGN AND DEVELOPMENT OF A HIGH PRESSURE-HIGH TEMPERATURE WELLBORE SIMULATOR FOR INVESTIGATION OF THE IMPACT OF CYCLIC STRESSES ON THE INTEGRITY OF WELLBORE SECTIONS WITH CEMENT/CASING INTERFACES
Creators: Lin, Zichao
Publisher: name: University of Alberta Library
Year: 2022

<p>An ideal cement job is expected to provide a perfect zonal isolation during the entire production life and even after the abandonment of the oil and gas wells. Variations of downhole stress conditions due to, for example, pressure cycles during the multistage hydraulic fracturing operations and temperature changes during the cyclic steam stimulation process, can affect the integrity of the cemented wellbore sections and, as a result, the zonal isolation may be lost. Investigation of the effects of downhole stress conditions and cement dehydration on the permeability of the cement matrix as well as the integrity of the cement/casing interface is, therefore, needed for a better understanding of why and how the zonal isolation may be lost under various downhole stress conditions, which would also be useful for the development of solutions to mitigate the associated formation fluid leakage problems.<br>
Various blends of small-scale cement samples were prepared by using industrial procedures. Porosity and permeability of the cement samples were then measured by using standard testing protocols. After conducting initial screening tests, porosity measurement by drying was identified as the most appropriate method. Core flooding tests were conducted to measure the cement matrix permeability. A steady-state nitrogen permeability measurement of the wet cement sample was identified as the most suitable technique in this case. <br>
A high pressure (43 MPa) and high temperature (120 °C) wellbore simulator was designed and constructed to investigate the integrity of the cemented wellbore sections under variable pressure and temperature conditions. The wellbore simulator is capable of measuring the permeability of the cemented casing sections (i.e. annular cement column between two casing sections) under cyclic temperature and pressure conditions. Potential leakage pathways due to micro-fractures along the cement matrix and/or debonding along the cement/casing interfaces were visually inspected after conducting the permeability tests. Strength of the shear bonding between the inner casing and cement column was also measured.<br>
Annular column of cement sections in the wellbore simulator was cured at 80 °C and 25 MPa for one week. Three groups of permeability measurements were conducted under constant pressure and temperature conditions. In the first and second groups of experiments, permeability measurements were conducted at constant pressures of 0, 10, 25 and 43 MPa while the temperature was kept constant at 80 °C and 50°C, respectively. The third group of tests was conducted to determine the effect of cement dehydration due to the time on the permeability of the cemented wellbore section under the same pressure and temperature conditions used in the first two groups of tests. <br>
Based on the results of these initial experiments, three factors were identified as having the most significant effect on the permeability of the cemented annular (casing/cement/ casing) wellbore sections: i-) material (cement and casing) shrinkage/expansion caused by the temperature, ii-) casing shrinkage/expansion caused by the inner casing pressure change, and iii-) cement dehydration due to the nitrogen injection during each permeability measurement. The final permeability of the cemented annular section was controlled by the combined effects of these three factors. <br>
Permeability of the cemented wellbore section was also measured under cyclic pressure (varying up and down between 3MPa and 43MPa) and constant temperature (50 °C) conditions. The permeability of the cemented wellbore section was measured after 12, 24, 36 and 50 cycles of pressure changes. Three groups of experiments were conducted under this category.</p>

<p>Comparison of results from the six groups of experiments conducted using the wellbore simulator revealed that once the debonding occurred at the cement/casing interface due to initial change in pressure and/or temperature, applying cyclic pressure did not significantly alter the permeability of the cemented wellbore section.<br><br>
After finishing each group of permeability tests, cement to casing shear bonding strength measurements were also made. Visual observation of the casing surfaces indicated that the surface roughness conditions of the casings changed after each cement permeability test was conducted. The shear bonding strength increased (while the final permeability of the cemented wellbore section decreased) significantly with the increasing casing surface roughness. It was also found that the cyclic pressure tests caused a significant reduction in the cement to casing shear bonding strength and a corresponding increase in the final permeability of cemented wellbore sections. </p>