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?

Underbalanced drilling is a non-conventional drilling operation where the hydrostatic pressure of the drilling fluid is maintained below the formation pore pressure. Many advantages can be provided by this specific drilling approach: higher ROP, minimised losses through formation, minimised formation damage in low permeability reservoir or depleted wells. Among the existing low-density fluids used in underbalanced drilling, foam is very beneficial due to its good cuttings carrying capacity. However, the knowledge of foam properties and especially of flowing properties is still incomplete. Foam can't be considered as a classical fluid and must be seen as a dispersion of gas bubbles in a continuous liquid matrix. Due to its own nature, foam is a thermodynamically unstable system, contrary to drilling fluids. The lack of knowledge on foam properties mustn't prevent engineers from considering this option in their technico-economical studies. The purpose of this work is to provide innovative procedures for foam rheology characterisation in order to better understand its behavior. The apparent viscosity of a foam is not only a function of its quality (ratio between gas volume and total foam volume) but also of its texture (bubbles size distribution), and of the presence of polymers in the aqueous matrix. A careful experimental procedure was elaborated, which uses a parallel plates geometry. Attention has been paid to foam stability, texture and evolution properties in order to have correct and reproducible measurements. In particular, wall slip phenomena and strain localisation were investigated and controlled. Shear flow properties were thus determined for different foams formulations with a very satisfying repeatability. The presence of a yield stress has been highlighted. Small amplitude oscillatory shearing experiments demonstrate that foam behaves like an elastic solid for small deformations. Use of a vane rheometer and observation of start-up in the creep mode confirm the elastic nature of the material. Excellent coherence was found between the different measurements. As a conclusion, a reproducible methodology for characterising foam rheological properties was elaborated. Those results constitute the first step on our way to accurately predict down-hole pressures while drilling with foam.

TX 75083-3836, U.S.A., fax 01-972-952-9435. AbstractThis paper provides a comprehensive analysis of hole cleaning with drilling foam and conventional drilling fluids in inclined wells using a fluid mechanics approach without inertial effect. A simple experimental set-up was designed to study hole cleaning properties of various drilling fluids. Foams with controlled quality were generated and accurately characterized from a rheometrical point of view. These foams behave like shear thinning yield stress fluids and can be described by a Herschel-Bulkley law. Since rheological parameters depend on foam quality, a master rheological curve was obtained by using a volume equalization procedure and the specific volume expansion ratio ε (ratio of the specific volume of the foam to the specific volume of the liquid). Foams and conventional drilling fluids with various rheological characteristics were compared and their behavior regarding hole cleaning was highlighted. The effect of dimensionless parameters such as the Herschel-Bulkley number H b (ratio of the yield stress to the viscous stress), the specific volume expansion ratio ε and the flow characteristic time T * (ratio of the flow time to the residence time of a fluid particle) were investigated. The influence of hole angle, particles size and concentrations were also evaluated. On the one hand, for shear thinning yield stress fluids with the same yield stress and the same flow characteristic time, the higher the Herschel Buckley number, the better the efficiency of cuttings removal in laminar flow. Similarly, increasing the flow characteristic time T * enables better hole cleaning. On the other hand, for higher H b, foam hole cleaning ability is improved at high inclination and is better than with conventional drilling fluids. At these inclinations, "particles dead zones'' are observed and can explain the poor hole cleaning when using conventional drilling fluids. Efficient hole cleaning with foam was found to be strongly dependent on the specific volume expansion ratio ε. Specific inclinations at which the worst hole cleaning is obtained (i.e. 40° to 60°) appears to be a function of particle size and particle concentration initially present in the hole. In conclusion, Hb and characteristic time are good indicators of cuttings removal ability. Extrapolating those numbers to an ERD well would eventually provide an evaluation of the minimum circulating time necessary to remove cuttings.

TX 75083-3836, U.S.A., fax 01-972-952-9435. AbstractAmong the existing non-conventional drilling operations, underbalanced drilling offers several benefits such as prevention of lost circulation and formation damage, increase of rate of penetration and reduction of differential sticking occurrence. In underbalanced drilling, the pressure of the drilling fluid is intentionally maintained below the formation pore pressure. Specific fluids are used in order to achieve underbalanced conditions : gas, mist, aerated mud or foam. Drilling with foam is very valuable due to its very low density coupled to its excellent cuttings carrying ability, but characterization of foam properties under drilling conditions is still incomplete and this could be an obstacle to the use of this technique by operators. This work is a part of a research program on drilling foams characterization conducted in collaboration between IFP and Totalfinaelf and focuses on the properties of aqueous foams when in contact with solids. The first part of the study deals with the incorporation of solids such as clays into the foaming liquid formulation; experimental measurements show that addition of swelling clay leads to very good stability properties without the necessity to add polymeric additive. Furthermore, clay addition provides good filtration properties with the formation of a filter cake, and this is important in case of accidental overbalanced condition during drilling operation. Lubricity properties of foam formulations were also characterized. The second part of this work is a characterization of the solid transport properties of aqueous foams. An experimental setup based on the weighting machine principle was elaborated. It allows the measurement of solids removal under various drilling conditions (circulating time and inclination angle, cuttings size and concentration). A new method for describing solids removal is proposed. Dimensionless numbers involving all the fluid parameters permit to describe transport properties and to bring out critical conditions. Extrapolating these numbers to the well could provide an evaluation of the critical parameters necessary to clean inclined and horizontal wells.