Sorry for the confusion. There are many different practices, and I'm not sure what exactly was done in MJ#1. But, yes the drilling fluid is removed during casing because of the cement process. Here are several practices that I'm aware of:
1. The open hole could be swabbed to remove the drilling fluid prior to casing to see if oil would flow as a test to see if it's worth spending the cash to case the well or plug it. But this is not likely done since MJ#1 had high pressure zones in the Jurassic. Well control might be lost if the drilling fluid was removed. Also the swabb may not seal against the rock and may chip rocks into the wellbore.
2. More likely is that the casing was set in place with drilling fluid in the wellbore, then the cement was injected down into the casing and pressed to the bottom with a plug. As the rig pushed the plug down, first drilling fluid, then cement would flow into the pit from the casing backside. After waiting a few days for the cement to dry, they would fill the casing again with drilling fluid and "drill out" the plug to continue drilling deeper to TD.
3. Once TD is reached and casing set and cemented, they would likely fill the casing with drilling fluid again before perfing. This would control the well in case high pressure was released from the formation after opening communication from the formation into the casing. Since they are not actually drilling at this point, I guess calling it drilling fluid is confusing. But, they probably used the drilling fluid since its on site. When they first swabb after perfing, they will watch the lifting load on the rig to make sure the rig is actually pulling up a load and not being excessively pushed from underneath by pressure from formation fluids (liquids and gas). Having things shoot out the hole is the most dangerous thing to avoid on a well site.
Hope this helps!
