The intermediate casing hole size can be calculated from empirical data in the Williston basin. Using this calculation reduces the cement excess required, helps achieve the top of cement objectives, and reduces the cementing costs. Many operators in the Williston basin are using an outdated assumption of average hole size for calculation of cement volumes on their intermediate casing section. This results in excessive cement volumes to sometimes detrimental effect; breakdown is occurring frequently at the Mission Canyon. This occasionally causes operators to fail to meet cementing job objectives. Very few open hole calipers are run in the intermediate section in this basin, making direct measurements very rare.
In an extensive review, cement evaluation logs from approximately 300 wells in a 15-month period from January 2014 through March 2015 were used to approximate top of cement and then derive an average effective hole size. The logs evaluated were from cement bond log (CBL) and ultrasonic imaging tools. The data are controlled in a statistical manner for cement job anomalies such as losses or severe channeling.
The findings prove that intermediate casing hole size in the Williston basin is much smaller today than in the recent past. The switch from water-base mud to oil-base mud and a marked improvement in drilling speed have limited the size of the intermediate section hole. One operator was able to substantially reduce cement volumes, keeping the top of tail slurry a minimal height above the weakest formation and thus avoiding losses. As a result, this operator is achieving top of cement objectives more frequently and has substantially reduced cementing cost.
This analysis contributes to the community valuable empirical data about intermediate section hole size in the Williston basin using a large dataset and statistical controls to ensure quality of the conclusions.