The gypsum-salt formations are complex in drilling fluid. The gypsum-salt formations are characterized by salting-in property, plastic creep and anisotropy. Meantime, the rheological properties of drilling fluids become worse when contaminated by high pressured brine in the gypsum-salt formations. In the process of drilling operation in the long-section gypsum-salt formation, reasonable mud weight and good rheological properties of drilling fluids should be used to ensure the safety drilling in this complex formation. The main matters needing attention is as follows:
A field method for measuring solids sizes on the job. It is simple and easy to use and should improve the efficiency of solids control. There will be some sacrifice in accuracy; however, only the relative weight percentage distribution of the particles is needed to allow constructive steps to remove these particles.
The application of polymer muds has been hindered by two criticisms. First, they are difficult to run because mud-engineering maintenance guidelines are very different from conventional bentonite mud systems. Second, they are less solids-tolerant than dispersed or lime-based systems. Consequently, these muds often have proved uneconomical for drilling geologically young and highly dispersive shales or for drilling with high mud weights (greater than 14 lbm/gal [greater than 1700 kg/m3]).
Weighted muds are the ones that contain weighting materials. These mud systems are usually used for drilling at deeper depths because of increasing formation pressures. The typical composition of weighted clay/water mud is active clay, and inert solids like barite are used for enhancing the mud weight. As mentioned earlier, the mud arriving at the surface from the bottom while drilling is in progress contains active and inactive drilled solids. Hence, the low gravity solids must be removed first, using a screen, because their disintegration reduces the particle size to less than that of barite or in the similar range. This may cause a loss of costly barite if the mud is subjected to solids control. Once the larger particles are removed the mud must be passed through mud cleaner, where the hydrocyclones are used in series with screens. This system works best for muds with density less than 15.0 lbm/gal.