One word of caution is appropriate here. Neophytes in drilling have a tendency to try to minimize the cost of each category of expense on the basis of the misconception that this will minimize the cost of the well. Minimizing individual items will only minimize a total if there is no dependence of variables on other costs. For example, increasing mud weight with drilled solids is cheaper than using barite. The cost savings from not purchasing barite is easy to calculate. The cost of all of the problems that ensue is much more difficult to predict. This is the insidious nature of drilled solids.
Decreasing individual costs to decrease the total cost is somewhat analogous to the accountant with appendicitis who decides to save money by renting a room at a cheap motel and calling a doctor friend rather than going to a hospital for an appendectomy. Room and board might be cheaper, but the net cost of improper care will probably make the decision very costly. Extra costs 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 costs of unscheduled events) are very closely intertwined.
One common mistake, usually made with the misconception that the well will be less expensive, is to allow the 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 to it. These savings are easily documented. Less revealing, however, will be the additional expenses because of the excessive drilled solids in the drilling fluid. Many of these problems will increase the well cost and have been discussed in the preceding sections.
Another common mistake, usually made while drilling with weighted drilling fluid, is to relate the cost of the weighting agent discarded with the drilled-solids discard. The cost of discarded weighted agents (barite or hematite) can be relatively small compared with the tragedies associated with drilled solids. This is particularly true in the expensive offshore environment. Even in cheaper land drilling, a comparison normally tilts in favor of discarding weighting agents.
Solids-control equipment, properly used, with the correct drilling-fluid selection, will usually result in lower drilling costs. Decisions made for various wells are very dependent on the well depth and drilling-fluid density. Shallow, large-diameter, low-mud-weight wells can tolerate more drilled solids than can deeper, more complicated wells. Each well must be evaluated individually with careful consideration of the risk of problems associated with drilled solids. As a general practice, however, since rigs drill a variety of wells during the course of a year, investing in a proper mud tank arrangement with adequate equipment is wise and frugal.
Yet another common mistake is to believe that different types of drilling-fluid systems will require different mud tank arrangements for solids removal. This is FALSE. Following the guidelines presented in this book will result in a system that will properly remove drilled solids from water-based, oil-based, synthetic-based drilling fluids. The waterbased drilling fluids could be dispersed or nondispersed, with or without polymers, or of low or high density or mud weight.
