Troublesome Shale Control Using Inhibitive Water-Base Muds

by
Vincent Francois Pernot, MSE

University of Texas at Austin, 1999
Supervisor: Martin E. Chenevert

Over the years wellbore stability in shales has been achieved using oil-base muds. Because of the expense and environmental problems associated with oil-base muds, industry has continued to search for equally effective water-base muds. It has long been recognized that the primary difference between oil-base and water-base muds, is that shale formations react with the water-base muds, produce swelling, and often collapse into the wellbore. Until recently it was not obvious which mechanisms were operative during shale/water-base mud interactions.

The research reported herein discusses the development of swelling pressures when water-base muds are in contact with the troublesome Speeton and Gumbo shales. A mechanistic theory is presented that focuses on increases in pore pressures that are reflected in swelling pressures. The results obtained show that different mud filtrates produce swelling or shrinking pressures that are related to the type and quantity of salt present in the mud. The effectiveness of various fluids for inhibiting swelling of certain shales is presented in term of membrane partitioning coefficient. Fluids tested include de-ionized water and solutions of CaCl₂, NaCl, potassium formate, glycerol, methylglucoside and NaCl/methylglucoside. It was found that while methylglucoside stops swelling for the gumbo shale, potassium formate as well as methylglucoside maintained the strength of both shales. It was also shown that the Speeton shale/fluid system behaves like a natural "leaky" membrane whereas the Gumbo shale does not.

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