Cementing slurry suitable for oil and gas wellbore drill for application in South Africa
Date
2022
Authors
Dliwayo, Charity
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Abstract
South Africa's biggest local energy supply is coal, which is used primarily for generation of electricity, followed by crude oil which is imported and refined into petroleum products such as
fuel for cars. In future South Africa may reduce its oil imports as there have been discoveries in oil and gas reserves in the Cape Town coastal region. Therefore, an oil rig may need to be built to extract the oil. The rig wellbore will need to be cemented using oil well cements (OWC) to seal annular space between metal casing and walls of a wellbore. OWC slurry may have reduced strength with time when subjected to environments of elevated temperatures and pressures. This study investigates behavior of two well cement slurry additives that can be used as extenders on OWC slurries. Durapozz (DZ) and Bentonite (BT) extender additives were dosed from 0-5 % by weight of cement (BWOC) at 23 and 38 degrees during compressive strength testing. The results indicated that Compressive strength at 8 h and 38 ˚C was more than twice the required value of 2.1MPa. Bentonite showed signs of losing strength over 28 days of curing at 38 degrees compared to 23 degrees, while Durapozz showed increase in compressive strength due to its good packing ability caused by the presence of a wide range of particle size distribution. However, this caused high thickening times for both additives; DZ was found to act as accelerator at low dosages and retarder at high dosages. From 0-5% Bentonite and Durapozz had approximately the same thickening times therefore BT can be substituted with DZ. The Factorial Design (FD) method was used to determine Free Fluid Content (FFC), results showed low FFC (<5.9%) as slurries are lightweight and have less than 0.44 water to cement ratio. The rheology results indicated that the Bingham plastic model best describes flow behavior of prepared slurries, and that all slurries were shear thinning with thixotropic behavior. Rheology results from drilling software indicated that flow index (n) and plastic viscosity were the same but fluid consistency index (K) and yield pressure had an acceptable error of 5-7%. Slurries prepared with BT and DZ were all lightweight slurries with densities between 1500 and 1700 kg/m3.These slurries would be applicable in low formation pressure and therefore can be cemented in conductor and surface casings less than 1800m. Significance of DZ additive density in lowering cost of slurry as well as carbon footprint was observed.
Description
A dissertation submitted in partial fulfilment of the requirements for the degree of Master of Science in Engineering to the Faculty of Engineering and Built Environment, School of Chemical and Metallurgical Engineering, University of the Witwatersrand, Johannesburg, 2022