ABSTRACT

Once a drilling system is established, there are only few parameters that can be altered or changed due to the system constraints and limitations. However; with the variation in encountered formation characteristics the optimum value of drilling parameters changes and during drilling only few operational parameters can be changed to their optimum values. The real time data is used to re-evaluate the optimum values of such parameters (RPM, WOB etc) by using simple analysis technique. With the help of this simple program the operator can change the characteristics to their respective optimum values and get maximum rate of penetration within allowable constraints. Thus, an increase in rate of penetration will ultimately reduce the drilling cost. These result in an economic, cost effective and efficient drilling operation.

The paper elaborates the above discussion, shows how simple program can analyze the on going drilling process and suggests recommendations to acquire better results.

INTRODUCTION

One of the major benefits to be obtained from the use of a minimum cost drilling program is the determination of optimum weights and speeds during the course of drilling. This eliminates much of the need for reliance on offset bit records, as the formation drillability and bit life constants are obtained from drilling data (i.e., weight, speed, penetration rate). The only other parameter needed is bit footage and this must be estimated from an evaluation of the best available data. Using time-share computers and a teletype terminal, the updated information can be developed within 2 or 3 minutes i.e. a very small time lag.

This paper presents the algorithm, calculations and optimization procedure, which does not require any sophistication and can be applied to an ordinary drilling rig. The reported work in this regard and the methodology acquired for the optimization process require sophisticated instruments, sensors, smart rigs etc, which in turn increases the drilling costs. It is therefore, recommended in this paper that a simple optimization process with no additional costs will increase the efficiency without adding any real cost at large.

CONCEPT OF OPTIMIZATION

The drilling variables can be classified as alterable or unalterable, as shown in Table 1.

The classification is not strict, as some of the unalterable ones may be altered by a change in the alterable ones. For example, a change in mud type may allow for a change in bit type, resulting in a faster penetration rate through a particular formation. The compressive and tensile strength of the rock drilled remains constant, but the rock's drilling properties have been altered by changing the drilling fluid and bit type. Of course, there is considerable interdependence among the alterable variables. For instance, mud viscosity and fluid loss are considerably influenced by the type and amount of solids. The weight-rpm combination is interrelated; an increase in one may necessitate a reduction in the other for smooth economical operation. In considering which variables to choose for mathematical optimization, experien-ce and research suggest six: four alterable ones and two unalterable ones. These variables are listed in Table 2.

The basic interactive effects between these variables were determined by factorial design experiments. Variable interaction exists when the simultaneous increase of two or more variables does not produce an additive effect as compared with the individual effects. The meaning of variable interaction is illustrated in Fig. 1. This shows the related responses in drilling rate when the variables are increased from one level to another; first individually, second simultaneously.

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