Wednesday, May 2, 2018

Designing and Managing Drilling Fluid

Gone are the days when drilling fluid - or mud as it is commonly called - comprised only clay and water. Today, the drilling engineer designing a mud program chooses from a comprehensive catalog of ingredients. The aim is to select an environmentally acceptable fluid that suits the well and the formation being drilled, to understand the mud's limitations, and then to manage operations efficiently within thoose limitations.

There are good reasons to improve drilling fluid performance and management, not least of which is economics. Mud may represents 5% to 15% of drilling costs but may cause 100% of drilling problems. Drilling fluids play sophisticated roles in the drilling process: stabilizing the wellbore without damaging the formation, keeping formation fluids at bay, clearing cuttings from the bit face, and lubricating the bit and drillstring, to name a few. High-angle wells, high temperatures and log, horizontal sections through pay zones make even more rigorous demands on drilling fluids.

Furthermore, increasing enviromental concerns have limited the use of some of the most effective drilling fluids and additives. At the same time, as part of the industry's drive for improved cost-effectiveness, drilling fluid performance has come under ever closer scrutiny.

This article looks at the factors influencing fluid choice, detailing two new types of mud. Then it will discuss fluid management during drilling.

What influences the choice of fluid?

Among the many factors to consider when choosing a drilling fluid are the well's design, anticipated formation pressures and rock mechanics, formation chemistry, the need to limit damage to the producing formation, temperature, environmental regulations, logistics, and economics.

To meet these design factors, drilling fluids offer a complex array of interrelated properties. Five basic properties are usually defined by the well program and monitored during drilling: rheology, density, fluid loss, solids content and chemical properties.

For any type of drilling fluid, all five properties may, to some extent, be manipulated using additives. However, the resulting chemical properties of a fluid depend largerly on the type of mud chosen. And this choice rests on the type of well, the nature of the formations to be drilled and the environmental circumstances of the well.

Issue & Decision

  • Specific healt and environmental concerns on type of mud and disposal of cuttings -> Determines mud system cuttings treatment/disposal strategy.
  • Remote location well -> May prevent the use of systems that consume large quantities of chemicals.
  • Composition and arrangement of the minerals in the formation and the clay chemistry -> Determines mud chemistry/ composition.
  •  Well profile/ angle -> Indicates the rheology needed to optimize hole cleaning. High-angle wells may need enhanced lubricity.
  • Strength and stress states versus hole angles -> Potential wellbore stability issues may concern mud weight. 
  •  Length of exposed open hole -> Greater inhibition needed for longer sections.
  •  Pore pressure -> Determines minimum mud weight needed to prevent blowout.
  • Rock strength-fracture gradient -> Indicates maximum mud weight that will not fracture well.
  • High -temperature well -> More than 275-300 degree F may cause product degradation.
  • Formation being drilled is pay zone -> Requires nondamaging mud to limit invasion, wettability effects of mud, potential emulsion blockage of the formation, fines mobilization and invasion, scale formation.

 Shales are the most common rock types encountered while drilling for oil and gas and give rise to more problems per meter drilled than any other type of formation. Estimates of worldwide, nonproductive cost associated with shale problems are put at $500 to $600 million anually. Common drilling problems like stuck pipe arise from hole closure and collapse, erosion and poor mud condition. In addition, the inferior wellbore quality often encountered in shales may make logging and completion operations difficult or impossible.



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