Evaluation of Formation Damage Caused By Drilling And Completion Fluids In Horizontal Wells
Authors: Van J. , Jiang G. , Wu X. 
Published: March 2010

Abstract
Horizontal wells are generally much more susceptible to the issue of formation damage than vertical wells because the contactarea of reservoir rock immediately adjacent to the wellbore with drilling and completion fluids is substantially larger and thecontact time is also much longer in horizontal wells. A method for the quantitative evaluation of the formation damage caused by drilling and completion fluids in horizontal wells was established in this investigation. The skin factor, flow efficiency and production loss were used as relative indices of damage. The whole work is made up of four parts:Develop a model for the calculation of the invasion depth, dt, on the basis of numerous data of tests, which allowed, determination of the radius of damage zone, rs. The anisotropy ratio of permeability of the rocks, β, was determined by means of the measurements of both vertical and horizontal permeabilities.Determine the skin factor, S, under different conditions using rs and other parameters.Calculate the flow efficiencies and production losses of horizontal and vertical wells using β; S and other parameters.Evaluate the formation damage of two horizontal wells located in Dagang Oilfield, and then make a comparison between the two wells and the corresponding vertical wells with regard to the extent of the damage caused by drilling and completion fluids.
It was shown that the effect of formation damage on the flow efficiencies of horizontal wells was generally less than that of vertical wells when β &lt; 3.50, but this was not the case when the value of β was relatively large. Under the condition of the same skin factors, the formation damage could be less detrimental to horizontal wells if the horizontal well length was relatively long and the value of β was relatively small. It was also found from the calculation that the formation damage could cause more reduction in oil production in a horizontal well than that in a vertical well even though the flow efficiency of the horizontal well was greater than that of the vertical well.

Introduction
In vertical wells, the flow of oil and gas through porous media towards the wellbore is governed by a radial flow. The flow rate depends mainly on the producing pressure drop and the permeability of production zone in the horizontal plane. When the formation is damaged by drilling and completion fluids, the permeability and flow rate will be reduced. The reduction percentage of the radial permeability of damaged core samples is usually used to evaluate the extent of formation damage of a vertical well.
In horizontal wells, however, the flow of oil and gas from reservoir to wellbore is the 3-dimensional flow, not only consisting of a radial flow in the horizontal plane, but also the vertical flows in the directions of different angles with respect to the horizontal plane. Therefore, the flow is affected by the anisotropy of permeability at different directions, which is designated as anisotropy ratio, β. For most reservoirs, the mean value of horizontal permeability is nine to 10 times that of vertical permeability(1). When the formation for a horizontal well is damaged by a completion fluid, the filtrate of the fluid will enter the formation 3-dimensionally. Hence, the damage scales of permeability will be different at various directions.