This paper was prepared for presentation at the 1999 SPE European Formation Damage Conference held in The Hague, The Netherlands, 31 May–1 June 1999.
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?
This paper describes the results of a three-year programme achieved in a Joint Industry Project aiming at evaluating formation damage arising from overbalanced drilling and completion operations. Eight typical drilling fluid formulations, including water-based and invert synthetic oil-based muds have been used to perform static and dynamic filtration tests on outcrop sandstone core samples. The performance of each mud formulation has been quantified from continuous monitoring of fluid losses during mud invasion and from return permeability measurements after mud exposure. The impact of various parameters on permeability impairment, such as initial core permeability, fluid saturations (oil or gas), temperature and shear rate has been evaluated. Results show that fluid losses and permeability impairments obtained with water-based muds are significantly greater than those observed with invert oil- based muds. Return permeabilities measured on six sections of long core samples. at increasing distances from the inlet face, showed a dramatic near-wellbore permeability reduction due to the invasion of filtrate generated from water-based muds. On the contrary, a stimulating effect, i.e. oil return permeability greater than initial oil permeability, was observed after invasion of filtrate from invert synthetic oil-based muds. This was attributed to a favorable relative permeability effect due to a reduction of connate water saturation during mud filtrate invasion. The use of a heavy brine as completion fluid has generated a significant additional permeability impairment due to the trapping of the wetting phase. inducing an adverse relative permeability effect. P. 237
TX 75083-3836, U.S.A., fax 01-972-952-9435. AbstractWells in the Girassol field, offshore Angola, are situated in very deep water and are being completed in unconsolidated sandy turbiditic reservoirs. These conditions have brought about significant challenges for well completions in this field: while the expectations in terms of production rate are high, the completions must be reliable and cost effective. Today, 28 wells have been completed and connected to production facilities. This field has been producing since December 2001.The choice of sand control techniques was a question much debated at the early stage of the project due to the different points of view on the subject among the partners. The completion strategies employed to date have included mainly sand alone screens in open hole and cased hole frac-packs. This paper will provide an overview of the Girassol field, and a report on the current situation after two years of high rate production. It presents an assessment of the different sand control techniques based upon well tests and production results along with PLT's and reservoir interpretations.The paper will also describe the design for both injectors and producers, along with the criteria in place today related to the sand characteristics, and off-shore implementation of the completions.
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