Evaluation and Optimization of Low-Density Cement: Laboratory Studies and Field Applications

摘要

Cementing a string in one stage is a challenging task, especially in the presence of weak formations. Cement slurry losses during placement is highly possible if the equivalent circulating density (ECD) exceeds 82 pcf during placement. A conventional method to overcome this challenge is to use multi-stage cementing by setting the stage tool above the loss circulation zone. However, field data indicate that the tool can fail, thus causing serious delay and economic loss. In addition, stage tools are considered weak point and not good for long term seal. A second method for zonal isolation is to use low density cement. In this study, we considered cementing the intermediate and production casings in S-1 (sandstone) and S-2 (carbonate) shallow formations and BJD (dolomite) deep formation in a single stage using lower density cement based on hollow microspheres at 70 pcf. The shallow conditions simulated in the lab tests were 150 oF curing temperature, 2,400 psi conditioning pressure and 1,800 psi confining pressure for 3 months. The deep conditions were 260 oF curing temperature, 5,000 psi conditioning pressure and 3,000 psi confining pressure;also for 3 months. Hollow microspheres cement was used in oil and gas wells without encountering any operational problems. However, the high cost of microspheres cement was a limitation for potential field applications. In this study, we present extensive lab work to optimize hollow microspheres low density cement (LDC) by elimination of micro fine cement from the blend. Experimental studies (shrinkage, compressive strength, porosity, gas and brine permeability, and chemical analysis cement) were conducted to determine the effect of this optimization on the properties of cement. The testing was performed in Saudi Aramco facilities to reduce the cost of hollow microspheres systems. Data generated during three months supported the use of the optimized system to cement casings at the shallow conditions tested. The removal of micro fine cement did not reduce the compressive strength with an average of 2,013psi;neither shrinkage nor liquid permeability was observed during the three months of testing. The use of the optimized system will help to bring cement to surface in one stage. The optimized blend price is 40% less than conventional hollow microspheres low density cement. This paper will discuss case histories that include job design, execution, and evaluation of the LDC. Field treatments were conducted without encountering any operational problems. The treatment was successful and maintained isolation for more than three years.