Abstract

Shale inhibition is considered to be the most important factor to prevent hole problems when drilling with water based drilling fluid (WBM). Some components are believed to be vital for well bore inhibition like KCl, glycols and designed polymers. The effect of the products is largely based on the cuttings quality seen in the return flow at the rig. When new systems are introduced they are often tested in relatively simple wells and performance are evaluated based on results form that well.

Some incidental observations where systems not regarded as inhibitive outperformed the state of the art water based drilling fluid systems. This observation trigged of a larger survey on drilling efficiency with respect to water based drilling fluid system. Water based drilling fluid systems used in Statoil's operations over the last 15 years in the North Sea were compared with respect to hole quality. Data from the survey shows that today's highly inhibitive systems do not perform any better than systems used for 15 years ago. The survey raises a question around the way drilling fluid systems are designed today. Are we using the right methods for designing the drilling fluids?

The paper describes the result from the survey. In the paper a method using different "indicators" to measure the hole quality is used. Such indicators include: stuck pipe frequency, tripping time, drilling progress, caliper data and cuttings quality. As a result of the first part of the survey, the drilling fluid chemical composition was altered both when drilling through non-reactive and reactive formations in a series of wells. An increased drilling efficiency was observed. The paper describes how the drilling fluid composition could be optimized dependent on the smectite content of the formation.

Introduction

Shales consist of a very complex compsition of different minerals. This composition varies with depth and age. The interaction between the drilling fluid and the complex mineralogy is very often the cause of hole problems while drilling a well. Finding the optimum drilling fluid to safely drill in such changing environment is challenging. Drilling fluid suppliers are constantly working to improve existing and to develop new and better drilling fluids. When new products or drilling fluid systems are tested it may be difficult to measure the improvement due to the influence of many other well design parameters. Especially the interaction between shale and drilling fluid and the resulting effect on hole stability and condition is difficult to evaluate. Presently, there is no direct measurement of hole stability and several hole condition indicators must be used instead. Using indirect measurement such as stuck pipe frequency, tripping time, drilling progress, caliper data and cuttings quality is only valuable if the data is sufficient to establish a trend. As a result any new drilling fluids or modifications must be tested and monitored over more than one well or section to verify a positive effect. We often fail to do such a long-term evaluation. More commonly drilling fluid systems and products are evaluated based on data from one or two wells. Another weakness, the lack of a historical database of hole quality indicators, it is making it difficult to benschmark drilling fluid modifications. The lack of historical data and the short test period of products or new fluid systems result in an evaluation with many unsertanties and are easily leading to incorrect conclusions.

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