Abstract

The productivity of wells with non-perforated completions can be impaired by both mud filtrate invasion and incomplete mud filter cake removal. Well inflow modelling illustrates that the percentage of the interval flowing and the distribution of the flowing intervals, are more important than the filtrate induced permeability reduction around the wellbore. Therefore adequate filter cake removal is essential for optimum well performance. This paper addresses both physical and chemical methods of mud filter cake removal.

Introduction

The use of high angle and horizontal wells is increasingly widespread, primarily because of the increased productivity they provide. Many of these wells employ non-perforated completions and both mud filtrate induced formation damage and the incomplete removal of mud filter cake along the entire open hole section can impair productivity. This paper uses the results of inflow performance modelling to demonstrate the relative importance of each of these causes of productivity impairment. Field evidence of impaired productivity was highlighted in an internal company review of the performance of recent BP operated horizontal wells in the North Sea. Several of these wells did not produce from the entire completed interval. Geological variation and partial clean-up of the drilling mud filter cake are believed to be the two main reasons for reduced production.

Specially designed filter cakes which only need low differential pressures to lift off, or which can be readily removed by washing fluids, can maximise productivity from horizontal or high angle wellbore sections. This paper presents the results of laboratory work which demonstrates that the differential pressure required to initiate flow through filter cakes is a function of both mud type and formulation. In addition formation characteristics can impact upon filter cake removal success. Laboratory results show that when chemical washing fluids are used, they must be tailored to the mud filter cake that is to be attacked.

WELL INFLOW MODELLING

Modelling of inflow performance has been used to assess the relative importance of damage from mud filtrate invasion and incomplete filter cake removal. Several methods are available for the modelling of high angle and horizontal wells. The method of Goode and Wilkinson has been used because it allows flow from discrete sections of a horizontal well to be modelled.

The impact of near-wellbore permeability reduction on the flow efficiency (see Appendix A) of a horizontal well is shown in Figure 1 (Well details are given in Appendix B). Figure 1 shows that a small reduction in the near-wellbore permeability will have only a small impact on the flow efficiency of a horizontal well. If, however, the permeability reduction is in excess of 70% then the productivity of the well falls significantly.

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