SCAL : AN INDISPENSABLE STEP FOR FORMATION DAMAGE EVALUATION

摘要

The impact of wellbore formation damage on the economic viability of a field development is becoming increasingly widely accepted by the oil industry.Productivity losses due to permeability damage generated by drilling and completion operations are of main concern specially for long horizontal open hole completed wells in which the near wellbore damage is not by-passed by perforations.Formation damage evaluation from specific laboratory tests becomes a large part of the SCAL activities and many papers have focussed on this area of research.This paper is a contribution to i)understand mechanisms of drilling mud and mud filtrate invasion,ii)quantify drilling-induced permeability damage and iii)evaluate the performance of various cleanup procedures to restore the near wellbore flow properties.Specific laboratory equipment and procedures derived from techniques used in SCAL were developed to evaluate filtration properties of drilling muds and permeability damage generated by the wellbore invasion.The performance of 8 mud formulations(WaterBased Mud(WBM)and Invert Emulsion Mud(IEM))was evaluated on 40 cm long sandstone core samples.Then,a comparative study aimed at determining the efficiency of various cleanup treatments using specific”breakers”to remediate filter cakes has been performed on core samples damaged either with a polymeric Water-Based Mud or an Invert Emulsion Mud.Results shows that the IEM’s present better filtration properties and are less damaging than the WBM’s tested here.Specific WBM”breakers”(enzymes,oxidants)are efficient and may restore a large part of the initial damage.On the other hand,breakers for IEM(surfactants,emulsified acid,solvents)may create additional damage if the breaker is allowed to invade the formation.Finally,some recommendations are given to design laboratory tests for evaluating drilling-induced formation damage and for restoring near wellbore flow properties.