The Drilling Mud Dilemma Recent Examples (includes associated papers 14481 and 14527 )
Authors: Simpson Jay P. 
Published: December 2007

Distinguished Author Series articles are general, descriptive representations that summarize the state of the art in an area of technology by describing recent developments for readers who are not specialists in thetopics discussed. Written by individuals recognized as experts in the area, these articles provide key references to more definitive work and present specific details only to illustrate the technology. Purpose: to informthe general readership of recent advances in various areas of petroleum engineering.

Introduction

Those involved in drilling and completing oil and gas wells usually areaware of the importance of drilling mud. They know that mud can determine lowor high cost, and a productive or abandoned hole. All concerned want a goodmud, but the mud that the log and core analyst wants may not be what the drilling supervisor needs. In almost every drilling operation there aredrilling mud dilemmas to be faced and compromises to be made. The following are common examples.

Density

High mud weight may be neededto avoid influx of formationfluids that can cause mud contamination, corrosion, kicks, or blowouts andto support the walls of the hole for borehole stability.

Low mud weight canpermit faster drilling, avoid lost circulation, and combat differentialpressure sticking.

Rheology

High viscosity and gel strength may be desired to combat hightorque, bridging, drag and fill caused by poor hole cleaning and to providegood suspension of weight material. Low viscosity and gel strength, however, result in faster drilling and more efficient separation of drilled solids.

Filtration

High filtrate will minimize chip hold-down and provide forfaster drilling, but low filtrate may be desired to combat tight hole caused bythick titter cake, differential pressure sticking, and formation productivity damage.

Clay Hydration and Dispersion

Dispersion of clay and shale solids may bedesired in water-based muds for easier control of mud viscosity, gel strength, and filtration. Inhibition of shale swelling and dispersion is desired forborehole stability, low mud maintenance costs, and protection against formationproductivity damage.

Many recent developments in drilling mud technology make for difficult choices. Some examples are discussed and suggestions are presented forresolution of the dilemmas.

New Weight Materials

For more than 40 years barite has been the principal material for weightingmuds. There are several reasons for its popularity.The high specific gravity permits weighting the mud with as littleincrease in solids as practical.Its chemical inertness and virtual insolubility in water or oil minimizeeffects on mud properties and the environment.Barite's intermediate hardness is soft enough to prevent serious abrasionof metal but hard enough to prevent excessive attrition and creation offines.It is available in large quantities at a low cost to meet the industryrequirement of millions of tons per year.

Over the past few years two other weight materials have come into use. Bothmeet the requirements for chemical inertness and availability, but they differfrom barite in specific gravity and hardness. Properties of alternative weightmaterials as compared with API-grade barite are:

Principal Specific Hardness, Component Gravity Moh's Scale

Barite BaSO 4.20 2.5 to 3.5 Ilmenite FeO TiO 4.65 5 to 6 Itabirite Fe O 5.05 to 6 (Micaccous hematite)

The higher specific gravities of the alternative weight materials aredefinitely an advantage in high-density muds.

JPT

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