Field Method of Evaluating Annular Performance of Drilling Fluids
Authors: Walker R.E. , Korry D.E. 
Published: December 2007

Three criteria are considered here: the transition from laminar to turbulent flow, the annular pressure loss due to circulation, and the average annular viscosity. Field methods of approximating the criteria are presented, as well as a discussion of the accuracy of the field calculations relative to computer-determined values.

Introduction
A method used successfully to plan drilling hydraulics for various types of mud systems is based on predictions of the ability of a fluid to clean the predictions of the ability of a fluid to clean the hole under conditions that cause neither liquid erosion nor high annular pressure loss. The prediction is based on a mathematical simulation of flow in prediction is based on a mathematical simulation of flow in the annulus between the rotating drillstring and the wall of the hole. The value of the prediction technique is directly related to the inclusion of the pertinent physical forces and laws in the mathematical pertinent physical forces and laws in the mathematical simulation. Savins showed the influence of rotating drillpipe and explained why the simulation must contain the calculation for a helical flow path rather than for simple axial flow. The methods of solving the helical flow equations, which require a computer, are documented, as are the experimental results. Values for annular performance criteria can be calculated using the computer program to determine the capabilities of different types of mud systems as a function of depth and flow rate. Combined with appropriate standards for optimum hydraulics, this method furnishes a sound basis for designing a circulating fluid. However, to extend the technique so that it may be used at the rig site to improve the analysis of hole problems requires methods to approximate the annular criteria calculations and devices to measure the mud's flow properties within the effective range of shear rates. Hole problems are frequently related to cuttings-carrying capacity, annular pressure drop, or liquid erosion. Since remedial action for these situations may be based on changes in mud viscosity of flow rate the expected results of these changes on each of the annular performance criteria should be evaluated. For example, the mud viscosity can be increased at all shear rates or only in some ranges of shear rate. Incorrect viscosity increase can lead to no improvement in hole-cleaning, to a greater loss in flowing pressure, or to a reduction in drilling rate. An pressure, or to a reduction in drilling rate. An increase in flow rate may develop turbulent flow, which increases both the slip velocity of the particles and the tendency towards fluid erosion. The purpose of this paper is to outline methods that can be used in the field to estimate the annular performance criteria and to describe an attachment to performance criteria and to describe an attachment to the Fann Model 34 VG Meter that permits the measurements necessary for the "duo-power" model used to simulate the liquid's stress/rate relation.

Estimation of Performance Criteria
The three criteria needed to evaluate annular performance are the laminar-turbulent transition, the performance are the laminar-turbulent transition, the annular pressure loss, and the average annular viscosity. A duo-power model is used to simulate the stress/rate relation of the liquid. Laminar helical flow calculations made by the computer were approximated by a combination of equations and graphs suitable for use in the field. Examples for 8 1/2 -in. and 12 1/4-in. holes are presented.
JPT
P. 167