RE: Days of well design20 Jan 2021 21:14
Well design is essentially a balancing act, maintaining enough hydrostatic head to keep the bore from collapsing and fluids from entering from the rock strata and fracturing the rock with to high a hydrostatic pressure with consequent loss of mud possibly at such a rate the pumps cant keep up, ending up with an empty hole.....we wont even go there!. In fact a little over pressure from the mud column is a good thing as it allows a cake to build up on the open hole wall. This is from the various additives added in the mudroom. I have unhappy memories of pouring crushed asbestos into the hoppers, touch wood no sign of asbestosis, as the mud fluid passes into the rock it leaves the solids behind, like a filter. An understanding of the pressures and porosity with depth dictates the casing points. Once a hole is cased it is generally considered secure, though poor cement jobs might dictate otherwise. Trying to control a long length of open hole with mud weight (adding heavy earth or barites) is problematic, hence casing.
This will all be based on previous drills in the area and industry standards, amended to actual conditions as required.