An Analytical Approach to Removing Mud Solids
Authors: Field L.J. , Anderson D.B. 
Published: December 2007

Here are methods for determining the particle-size distribution in a drilling fluid and for estimating the amount of solids being introduced into the circulating fluid. The results are used to evaluate the efficiency of the solids removal equipment in circulating systems.

Introduction
One of the prime functions of a rig circulation system is to remove the drilled solids from the mud before recirculation, since all solids adversely affect drilling rate. A major problem in reducing drilling costs has been the inability to remove these solids effectively. Since very fine solids, such as clays, are exceedingly detrimental to drilling rate, they are kept at a minimum by extending them. This is done by using additives that make a smaller amount function like a larger amount. Low-solids muds contain bentonite (which is added to obtain desired mud properties) and the drilled solids that are produced while properties) and the drilled solids that are produced while making hole; these solids may also contain bentonitic material.
Removing solids by dumping the mud into the reserve pit, or sump, is always costly and inefficient. The solids removal problem must be approached systematically. First, the given flow stream is analyzed to determine particle size, then the coarse particles are separated by screening; the next-size particles, in the medium or intermediate range, are removed by mechanical means; and the fine particles can be moved by flocculated settling. particles can be moved by flocculated settling. In this way, process efficiency is assured.

Classification of Solids
The present API designation of particle size is of limited use. In terms of volume percent, it defines as sand the particles larger than 74 microns, and as silt those smaller than 74 microns. This has become known as an abrasive test because it is based on the assumption that all solids larger than 74 microns are abrasive sand. However, this assumption is not necessarily valid. Some of the particles larger than 74 microns are nonabrasive, and some of those in the silt category are abrasive. Another breakdown for classifying solids is needed, since solids in a mud system can be from less than I micron to more than 3,000 microns. In addition, the solids content should be expressed as a weight percent rather than a volume percent. As noted above, the percent size classification percent. As noted above, the percent size classification does not indicate the nature of the particles definitively enough; however, it is still the only method of classification being used.
Table 1 is a suggested particle-size classification. In a weighted system, the same particle-size classification can be used by considering two specific gravities: 4.2 for barite, and 2.65 for the remainder of the solids. The suggested classification deals with size rather than type of solids since this is the most important criterion. Table 2 compares various common particle sizes to illustrate how small the ultrafine particle sizes to illustrate how small the ultrafine and colloidal mud solids are in relation to other common particles.

Field Method for Determining Particle Size
After deciding how to classify solids according to size, the next step is to develop a procedure to measure particle sizes in the field. Although several laboratory methods are available they are complicated, not necessarily accurate, and require too much time to obtain results.
JPT
P. 663