Abstract

The theory of hydraulic optimization of drilling with conventional (incompressible) drilling fluids is well known and has been widely practiced in the industry.

Classical theory of hydraulics optimization for maximum drilling rate calls for either the use of empirical correlations or the use of optimization theory to maximize some arbitrary objective functions such as maximum bit hydraulic horsepower or jet impact force.

Concept of hydraulic optimization for maximum drilling rate when drilling with foam, however, is not well investigated. Compressible nature of the foam makes the use of conventional optimization theory difficult.

A transient-mechanistic model of cuttings transport with foam has been developed and numerically solved recently. In this study, the new model has been used to re-visit classical theory of hydraulic optimization (i.e. maximum bit hydraulic horsepower/ jet impact force criteria).

A new methodology has been suggested to determine optimum gas/liquid injection rates for maximizing drilling rate when drilling with foam while keeping the bottom hole pressure minimum.

The new method can be easily used in the field to determine best combination of gas/liquid injection rates and total bit flow area (i.e. jet nozzle sizes) such that maximum drilling rate is achieved.

Introduction

Foam is often used as a circulating fluid in underbalanced drilling operations because of its high viscosity and variable density. Applications of foam as a drilling fluid in many drilling operations, and the results from various field cases are well documented1–6.

Drilling hydraulics has long been recognized as one of the most important considerations for maximizing drilling efficiency and minimizing drilling cost.7–10 The concept of hydraulics optimization for drilling with incompressible fluids have been well investigated.11–17

Optimization of drilling hydraulics requires calculation of frictional pressure losses in the system and of the minimum fluid velocity to carry the cuttings in the annulus. Finding satisfactory answers for both of these problems has been a challenge for engineers even for incompressible drilling fluids.

The hydraulic optimization problem becomes more complicated when drilling with foam due to the compressible nature of the foam. Determining the optimum back pressure and gas/liquid injection rates for effective cuttings transport while achieving maximum drilling rate are some of the major questions need to be answered.

In this paper, a new methodology is presented for determining optimum combination of back pressure, gas/liquid ratio and total flow area in order to maximize the drilling rate when drilling with foam.

Drilling Hydraulics Optimization

Classical theory of hydraulic optimization for maximum drilling rate calls for either the use of empirical correlations (such as Fullerton18 charts, or Amoco curves19) or the use of optimization theory to maximize some arbitrary objective functions such as maximum bit hydraulic horsepower and jet impact force.11–17

As stated by Swanson et al.15 "drilling hydraulics optimization, similar to many other engineering optimization problems, involves the manipulation of several independent variables to obtain a maximum (or minimum) in one or more dependent variables within boundaries imposed by cost, safety and the physical properties of the system under analysis".

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