Effects of ROP on Selected Foam Rheological Properties in Vertical Wells

Abstract

Foam drilling is one of the different techniques of underbalanced drilling which is to intentionally design the proper compressible fluid as drilling mud. The main characteristics of the foam fluids are its wide range of low densities which are generally from 1 to 5 ppg; and its high effective viscosities which are superior to those of both constituents; liquid and gas. Literature reported case stories of foam drilling in most challenging formations with excessive and complete losses due to reservoir depletion, fractured strata, unconsolidated beds etc. For good cuttings removal and desired hole cleaning, the technique of foam drilling relies upon foam high effective viscosity and annular mixture velocity. To guarantee adequate hole cleaning in foam drilling, factors affecting these two parameters must be carefully monitored and controlled during the entire hole sections. As foam rheological properties are among the most affecting parameters for hole cleaning, their detailed studies are not considered exaggerated. Carrying out foam drilling in producing formations results in annular foam rheological properties much different than those in the string due to the intrusion of formation cuttings and fluids into the annular mud stream. Therefore, detailed analysis of different parameters affecting on the annular foam rheology is crucial for optimum foam drilling. Rate of penetration (ROP) is a determinant parameter for drilling economy, but might also be one of the main causes for the changes of the annular rheology of foam. In this paper, we developed a recent program using VISUAL BASIC for a hydraulic model of foam drilling in vertical wells to investigate the influence of ROP on selected foam rheological properties. For the particular conditions in the Appendix Table of this paper, foam annular velocity, quality, density, power index, Reynolds number and cuttings concentration all increase proportionally with the increase of ROP, whereas, annular surface pressure, effective viscosity, consistency index and friction factor all decrease inversely with the ROP increase.