The Effect of Thinners on the Fabric Of Clay Muds and Gels
Authors: Borst R.L. , Shell F.J. 
Published: December 2007

Here is a new freeze-drying technique for studying the fabric of clay muds with a scanning electron microscope. The resulting photographs make it possible to figuratively walk through the fabric and example the effects o heat, NaOH, and thinners, and to postulate relationships between clay fabric and rheology.

Introduction
The gelation and thickening of drilling mud at high temperatures causes problems in drilling deep wells. Thickening increases the pump pressures required to circulate the mud. It causes swabbing and pressure surges when running pipe. It keeps gas from breaking out or cuttings from settling out of the mud. It slows down drilling and may, in severe cases, make circulation nearly impossible.
Normally, muds are treated with organic thinners to reduce gelation and thickening. Heavy metal lignosulfonates (HMLS) are the most common thinners, but they cease to work well at high temperature. This is not an exact temperature like a melting point, but a range including 350 degrees F. In some wells the range may start below 300 degrees F; yet water-base muds have been used above 400 degrees F with few problems. A new product, Thinner D, performs well at high temperatures. product, Thinner D, performs well at high temperatures. Thinner D is a mixture of sulfomethylated tannin (SMT) and a chromium compound. It checks gelation and thickening at temperatures where other thinners no longer work, yet it is effective at low temperatures, even at low concentrations. It is used in amounts as low as 1/8 lb/bbl in the upper part of the hole to as much as 4 lb/bbl in the deep, hot section. This is in contrast with HMLS concentrations, which range from 2 lb/bbl in the upper part of the hole to massive treatments exceeding 30 lb/bbl in deep holes.
Thinning involves an interaction between the thinner and solids in the mud. The purpose of this study was to provide information on the nature of this interaction. Drilling muds are very complex; therefore, to simplify the interpretation of data, a simple clay-in-water laboratory mud was studied.
When clay muds thicken or thin, their fabric must change. Fabric is the three-dimensional arrangement of clay particles and fluid-filled voids. A correct picture of clay fabric should therefore help to explain picture of clay fabric should therefore help to explain how various thinners work. Several authors have proposed models for the fabric of clay gels. These are proposed models for the fabric of clay gels. These are summarized in Table 1. However, these models leave the nature of both interparticle association and particle bonding in doubt. particle bonding in doubt. Attempts have been made to observe directly the particles of clay gels using freeze-drying and electron particles of clay gels using freeze-drying and electron microscopy. Most of these efforts were unsuccessful because of changes in the gels during the freeze-drying process; Kittrick, however, was apparently successful. process; Kittrick, however, was apparently successful. His freeze-drying technique was modified and used to prepare specimens that were then examined with the prepare specimens that were then examined with the scanning electron microscope (SEM). When combined with other data, the SEM micrographs permitted inferences to be made concerning the effect of thinners on drilling muds.
The word "gel" is used in many ways in the drilling industry. In this report we shall refer to "clay gel" as clay mud that has been aged at high temperatures. This definition will be used even if the aged mud is quite fluid.

Experiments Low-Solids Laboratory Muds Were Used
The muds used were prepared by standard API methods. Two base muds were prepared. Base Mud I was 20 lb/bbl clay in deionized water.
JPT
P. 1193