As oil well drilling encountered more and more difficult conditions, hole problems finally became undeniably associated with excessive drilled solids. Many years ago, a controversy raged concerning the effect of drilled solids on the cost of a well. Many thought that drilled solids were beneficial as an inexpensive substitute for weighting agents. Frequently, production horizons near the surface were normally pressured and could be drilled with unweighted drilling fluids. Usually, these drilling conditions were relatively trouble free, and a poor-quality drilling fluid was used for drilling. Of course, drilling performances and well productivity could be enhanced with better-quality drilling fluids, but those effects were difficult to quantify. As these areas graduated from unweighted drilling fluids to weighted drilling fluids, better drillingfluid properties were required to prevent trouble. The primary problem was that large quantities of drilled solids were intolerable. The drilling trouble costs could easily be traced to failure to limit drilled-solids concentration. This provided the impetus for most drilling rigs to upgrade their surface systems handling drilling fluids. The benefits of a clean drilling fluid have been well stated in previous chapters and have been well validated.
Some rigs now process all drilling fluid sequentially in accordance with good practices, as discussed in Chapter 5 on tank arrangements. Drilling fluid type does not affect proper rig plumbing. Dispersed or nondispersed, fresh- or saltwater, clay-based or polymer-based, any drilling fluid must be treated sequentially to remove smaller and smaller drilled solids.
The cost of solids-control equipment was justified initially economically as an insurance policy to prevent catastrophes. Subsequently, more expensive drilling fluids required lower drilled-solids concentrations. Polymer additives that adhere to active solids require significantly lower concentrations of drilled solids to prevent loss of too much polymer. Environmental concerns also dictate minimization of waste fluid; this requires careful attention to mechanical removal of drilled solids.
DILUTING AS A MEANS FOR CONTROLLING
One way that drilled solids can be kept at a manageable level is to simply dump some of the drilling fluid containing the drilled solids and replace it with clean drilling fluid. One half of the drilled solids is eliminated if one half of the system is dumped and replaced with clean fluid. Generally this is too expensive, so mechanical equipment is used. Traditionally, large volumes of drilling fluid were dumped from the system by aggressively dumping sand traps. This makes room available for clean drilling fluid needed for dilution without calling it ‘‘dilution.’’