Experimental Approach to Characterize Drilling Mud Invasion, Formation Damage and Cleanup Efficiency in Horizontal Wells with Openhole Completions

Abstract

Abstract The increasing number of horizontal wells being drilled, together with the continuing development and use of open hole completions has resulted in increasing reliance on formation damage testing to select the appropriate drilling fluid and/or cleanup technique. A two-year laboratory study was conducted to evaluatenear wellbore invasion and related damage due to two typical Drill-In fluids (D.I.F.) andperformance of various cleanup procedures using specific "breakers". In the first part of the paper, values of Flow Initiation Pressure (F.I.P.) and return permeability measured on rock samples damaged with an Oil-Based Mud (OBM) and with a Water-Based Mud (WBM) are compared to evaluate the self cleaning properties of sandstone core samples having a large permeability contrast. In the second part, the performance of various "breakers" (mutual solvent, emulsified acid, surfactants for OBM, oxidizers and enzymes for WBM) is presented. Results show that the OBM present better filtration properties and is less damaging than the WBM. The general trend is that near wellbore return permeability (0–10 cm) and self cleaning properties are strongly related to the Jamming Ratio (Mean pore throat diameter/mean mud solids diameter). The use of OBM breakers may induce additional damage if the soaking time is not carefully controlled. On the other hand, WBM breakers may be efficient if they are used under optimum conditions. Finally, some recommendations are given for designing a low-damaging D.I.F. and to define, if necessary, the best cleanup procedure. Introduction Horizontal wells are being utilized throughout the world in an ever increasing fashion to attempt to increase production rates by targeting multiple zones, maximizing reservoir exposure, reducing drawdowns to avoid (or to minimize) premature water or gas coning problems. Formation damage in horizontal wells is a matter of great concern, specially for oil wells that have been open hole completed. In such a case, relatively shallow damage, which is not by-passed by perforations, can result in very large skins. This is a critical point for oilfields developed in deep water reservoirs where acceptable development costs are based upon a limited number of high productivity wells1. The economic impact of near wellbore formation damage in horizontal wells has pushed towards the development of number of theoretical and experimental studies2–6 to assess drilling induced formation damage and to evaluate the performance of various cleanup procedures. However, mechanisms of drilling fluid damage and filter cakes cleanup are not well understood and laboratory methods for determining the type and extent of formation damage potential are not standardised. Recently, a comparative study presented by Marshall et al.7 showed that formation damage test results should be treated with considerable caution since a good level of repeatability and/or re productivity has not been achieved. This paper is a contribution to understand physical processes which take place during mud invasion, filter cake removal by natural production and/or cleanup treatment. Our primary objective was to provide insights to answer the following questions:how may vary drilling mud damage and natural filter cake removal with the reservoir permeability and the nature of the mud (OBM vs WBM)?Is it always necessary to use a breaker to destructurate the filter cake and for increasing the productivity of a long horizontal open hole well?