Drilling fluid density can be increased using drilled solids for the lowest initial weight-up cost. If drilled solids had no other effect, using them for a weighting agent would be cost-effective. However, using drilled solids could potentially increase drilling costs because slow drilling, poor cement jobs, stuck pipe, and lost circulation will frequently follow. Downtime on a rig or reduced penetration rates are far more expensive than the cost of using adequate weighting material.
Removal of drilled solids as they reach the surface usually costs less than any other method of mitigating their effect. Conversely, controlling drilled solids with dilution is usually the most expensive method. Although drilled solids concentration can be reduced by half with dilution by doubling the system volume with clean drilling fluid, this clean fluid is expensive.
Polymer drilling fluids, with X-C, PHPA, and so on, require relatively low drilled-solids concentrations. The polymer attaches to all solids in the system whether they are desirable or not. Failure to keep a low drilled-solids concentration will result in using excessive quantities of chemical (polymer). This can make the use of polymer drilling fluids cost-prohibitive. The use of polymer systems requires the utmost in planning solids control equipment for optimum performance.
A word of caution is appropriate here. Neophytes in drilling have a tendency to try to minimize the cost of each category of drilling expenses with the misconception that this will minimize the total cost of the well. It is important to realize that additional expenses can be incurred because of inadvisable decisions to cut costs in easily monitored expenses while drilling wells. When line-items are independent of each other, minimization of each line-item will result in the lowest possible cost. When line-items are interconnected, minimization of each line-item may be very expensive. Drilled solids concentrations and trouble costs (or the cost of unscheduled events) are very closely related.
For example, a common mistake is to allow an initial increase in mud weight to occur with drilled solids. Clearly, less money will be spent on the drilling fluid if no weighting agents are added. These savings are easily documented. Less apparent, however, will be the additional expenses incurred because of excessive drilled solids in the drilling fluid. These problems will obviously increase the well cost as described previously.
Another common mistake, usually made while drilling with weighted drilling fluid, is to relate the cost of the discarded weighting agent with the drilled solids discard. The cost of discarded weighting agents (barite or hematite) can be relatively small compared to the problems associated with drilled solids. This is particularly true in the expensive offshore environment. Even in less expensive land drilling, a comparison normally justifies discarding weighting agents to eliminate drilled solids.
Solids control equipment properly used, with the correct drilling fluid selection, will usually result in lower drilling costs. Decisions made for specific wells depend on the well depth and drilling fluid density. Shallow, large-diameter, low mud-weight wells can tolerate more drilled solids that more complicated wells. Each well must be evalu ated individually with careful consideration of the risk of the problems associated with drilled solids.
Failure to remove drilled solids with solids control equipment results in solids control with dilution. This creates excessive quantities of fluid that must be handled as a waste product. It can be very expensive if this excess fluid must be removed from the drilling location. Even if the fluid can be handled on-site, large quantities of fluid frequently increase costs.
Smaller quantities of waste products can significantly decrease the cost of a well. Decreasing the quantity of drilling fluid discarded with the drilled solids will decrease the cost of rig-site cleanup. Dilution techniques for controlling drilled solids concentrations greatly increase the quantity of waste products generated at a rig. This results in an additional expense that ultimately adds to the total cost of drilling.
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