Abstract

Due to an increased understanding of the effects of formation damage, it has become commonplace to test the damage potential of drilling fluids with reservoir core, prior to drilling. This preventive approach has shown to be more effective than neglecting pre-drilling laboratory testing. However, the current approaches are lacking in many areas.

Through research, as part of an E.U. funded project, well productivity 2002, a standard procedure for testing the formation damage of drilling fluids has been developed. The research provides a number of explanations as to why previous "round robin" studies display large variations in results between laboratories. False ranking of drilling fluids and formations were subsequently produced due to the varied procedures currently used. Explanations and solutions are specified in order to allow future testing to be repeatable in any laboratory and most importantly, representative of the wellbore situation.

The result of the research is a cost-effective procedure, which can be scaled to the reservoir after testing. Although the laboratory equipment used is not greatly different from that used in many laboratories, there are procedural changes. These changes allow accurate testing and ranking of formation damage potential. The results detail representative fluid application, basic fluid ranking procedures and a more advanced investigative procedure, which determines type and extent of damage expected from the formation and fluids. Additions to these procedures allow for the testing of solid and chemical completions, to provide a strong tool for well planning.

The research was part of Work Package 1 of the E.U. project "Well Productivity 2002" and the recommended methods are already being used in a number of reservoir studies. It is intended to present future work using these methods, in parallel with well data.

Introduction

The existence of formation damage is the existence of lost revenue. To reduce or prevent formation damage is to control that loss of revenue. One approach that has come to the forefront of formation damage prediction and mitigation is to utilise core analysis. Recent studies have shown that preventative formation damage testing, using representative core and reservoir conditions of temperature and pressure, have proved successful (Watson et al1). Previous recommended practices have attempted and failed to allow an acceptable level of repeatability (Marshal et al2). It is our suggestion that the relevance of the conditions in the wellbore should first be determined. In our research we examined the ability of the formation damage test to represent the conditions present in the wellbore in terms of:

  1. Filtrate loss control properties

  2. Cake development

  3. Damaging mechanisms

  4. Return permeability

The intention is for a further publication to examine well data in comparison to laboratory work, utilising different fluid compositions and rock lithologies.

Methodology
Core Material

Blaxters sandstone was selected as representative of a medium permeability North Sea reservoir.

Blaxters is medium grained sandstone with framework grains of predominantly quartz and also mica. An open macro-pore network was present. Abundant kaolinite clay cement partially fills intergranular pores, which increases the relative proportion of microporosity. The occurrence of abundant crystal faces of quartz suggests that extensive precipitation of quartz cement has taken place at the surface of quartz grains (overgrowths). Long needles of illite were occasionally observed.

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