Modelling of Near-Wellbore Formation Damage for Open Hole Horizontal Wells in Anisotropic Media

Abstract

Abstract It is well known that formation damage due to drilling fluid has a huge impact on well productivities, especially for open hole completed horizontal wells. To evaluate this impact to keep maximum oil productivity, a numerical model has been developed to simulate formation invasion and damage during the drilling phase of a well. This model can be used to study the impact of various parameters related to drilling fluid properties or drilling conditions on well performances. In previous papers1,2, a methodology combining both experimental and numerical approaches has been proposed to model the near-wellbore formation damage for both water-based mud (WBD) and oil-based mud (OBM) and filter cakes removal by natural clean up procedures. However, anisotropic permeability was not considered, although heterogeneities were taken into account. The main difficulty of near-well modelling of anisotropic media is that the generally recognised elliptic flow behaviour in the well vicinity is not valid as the well cannot be considered as a point (or line) source. Flow behaviour is radial at the wellbore boundary and becomes gradually elliptic when going away from the well. To accurately model near-wellbore formation damage, a new grid system, which is obtained based on elliptic coordinate transformation, is proposed in this paper, and a new numerical scheme is given for the modelling of near-well flow in anisotropic media. The proposed model can be used to study non-uniform formation damage and cleanup results around the well due both to permeability anisotropy and gravity effect. Examples are presented to study sensitivity of various parameters (fluid properties, drilling conditions, ...) on well performances. This model is developed for the modelling of horizontal wells as well as slanted wells. It can be used as a tool for well design and construction. Finally, a field example in a deep offshore reservoir is presented to show an application of the developed model. Introduction It is well recognised that near-wellbore flow properties are altered by mud and mud filtrate invasion during overbalanced drilling operations. The degree of alteration, generally called "Formation Damage", depends on a large number of parameters such as nature and characteristics of the drilling mud, formation properties and operating conditions. Formation damage can cause significant decreases in oil and gas productivity, especially for open hole completed horizontal wells3–10. Worldwide yearly lost production due to formation damage is equivalent to billions of dollars in lost revenue. Identification and reduction of formation damage problems can lead to immediate returns in the form of increased production rates. Therefore, evaluation and prevention of formation damage is an important issue to optimise well productivity. Using experimental data from laboratory, near-well formation damage can be predicted through a near-wellbore flow simulator by doing some simplified assumptions.