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 for- mation, temperature, environmental regula- tions, logistics, and economics (see “Critical Decisions,” next page). To meet these design factors, drilling flu- ids 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 (see “Basic Mud Properties and Ingredients,” page 36). 3 For any type of drilling fluid, all five prop- erties may, to some extent, be manipulated using additives. However, the resulting chemical properties of a fluid depend largely 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 envi- ronmental circumstances of the well. 33 April 1994 Ben Bloys ARCO Exploration and Production Technology Plano, Texas, USA Neal Davis Chevron Petroleum Technology Company Houston, Texas, USA Brad Smolen BP Exploration Inc. Houston, Texas, USA Louise Bailey Otto Houwen Paul Reid John Sherwood Cambridge, England Lindsay Fraser Houston, Texas, USA Mike Hodder Montrouge, France In this article MSM (Mud Solids Monitor) and FMP (Fluid Monitoring Package) are marks of Schlumberger. Fann 35 is a mark of Baroid Corporation. For help in preparation of this article, thanks to John Astleford, Schlumberger Dowell, Bottesford, England; Thom Geehan, Schlumberger Dowell, Houston, Texas, USA, Alan McKee and Doug Oakley, Schlumberger Dowell, St. Austell, England; Eric Puskar, Schlumberger Dowell, Clamart, France. 1. For a comprehensive review of the role of drilling fluids: Darley HCH and Gray GR: Composition and Proper- ties of Drilling and Completion Fluids , 5th ed. Hous- ton, Texas, USA: Gulf Publishing Co.,1988. 2. Geehan T, Helland B, Thorbjørnsen K, Maddin C, McIntire B, Shepherd B and Page W: “Reducing the Oilfield’s Environmental Footprint,” Oilfield Review 2, no. 4 (October 1990): 53-63. Minton RC, McKelvie DS, Caudle DD, Ayres RC Jr., Smith JP, Cline JT, Duff A, Blanchard JR and Read AD: “The Physical and Biological Impact of Processed Oil Drill Cuttings: E&P Forum Joint Study,” paper SPE 26750, presented at the Offshore Europe Conference, Aberdeen, Scotland, September 7-10, 1993. 3. For a full description of these properties and their measurement: Geehan T and McKee A: “Drilling Mud: Monitoring and Managing It,” Oilfield Review 1, no. 2 (July 1989): 41-52. 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 those limitations. Designing and Managing Drilling Fluid There are good reasons to improve drilling fluid performance and management, not least of which is economics. Mud may rep- resents 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. 1 High-angle wells, high tem- peratures and long, horizontal sections through pay zones make even more rigor- ous demands on drilling fluids. Furthermore, increasing environmental concerns have limited the use of some of the most effective drilling fluids and addi- tives. 2 At the same time, as part of the indus- try’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.