The flow properties of several asphaltenic crudes were studied at reservoir temperature in rocks of different morphology and mineralogy. The experiments performed showed a progressive reduction in permeability to oil during injection, varying in rate according to the system considered. The existence of organic deposits was verified by Rock-Evalpyrolysis measurements made on sections of samples taken at the end of flow at different distances from the entry face. This technique enables the profile of the deposits to be quantified. The interpretation of the permeability damage experiments and their simulation are treated by comparing the asphaltenes in oil to colloidal particles in suspension, capable of being deposited at the surface of the pores and thus reducing the permeability of the porous medium. The first simulations were carried out using the PARISIFP particle damage model, which has recently been extended to the case of multi-layer deposition. A satisfactory qualitative agreement is observed with the experimental results. Les propriétés d'écoulement de plusieurs bruts asphalténiques ont été étudiées à la température du réservoir d'origine dans des roches de morphologie et minéralogie différentes. Les expériences réalisées mettent en évidence une réduction progressive de la perméabilité à l'huile au cours de l'injection, plus ou moins rapide selon les cas. L'existence de dépôts organiques a été vérifiée par des mesures de pyrolyse Rock-Evaleffectuées sur des sections d'échantillons prélevées en fin d'écoulement à différentes distances de la face d'entrée. Cette technique permet de quantifier le profil des dépôts. L'interprétation des expériences de colmatage et leur simulation sont traitées en assimilant les asphaltènes dans l'huile à des particules colloïdales en suspension, susceptibles de se déposer à la surface des pores et ainsi de réduire la perméabilité du milieu poreux. Les premières simulations ont été réalisées en utilisant le modèle IFP d'endommagement particulaire PARIS , qui a été récemment généralisé au cas de dépôt en multicouches. On observe un accord qualitatif satisfaisant avec les résultats expérimentaux.

SummaryPermeability is one of the most important petrophysical parameters for reservoir characterization, but also one of the most difficult to obtain. Logs provide a good estimate of porosity and saturations, but the accuracy on permeability derived from nuclear magnetic resonance (NMR) is rather poor. So far, reliable values of permeabilities are obtained only from laboratory measurements on core samples for local measurements and well testing for a larger scale-averaged determination.We present an original method for measuring the permeability of drill cuttings without any specific laboratory conditioning (cleaning, coating, etc.). A volume of approximately 100 cm3 of cuttings is placed in a pressure vessel. The cell is then filled with a viscous oil. The process of oil invasion into the cuttings always traps a certain amount of gas. When a pulse of pressure is applied on the cell, the oil enters into the cuttings thanks to the gas compressibility. The permeability is then derived from the dynamic of the oil invasion by using a simple model.The method was tested by using various crushed-rock samples of known permeability. Excellent reproducibility and good agreement between cores and cuttings permeabilities were found for many decades of permeabilities. This method presents many advantages. The measurements can be performed in a few minutes, leading to the possibility of operating on site during drilling. The limitations of the method are related mainly to the size, the representativity of the drill cuttings, and the absence of the confining stress.In developing this method, our purpose is not to replace core analysis but, rather, to provide additional quick and inexpensive information on reservoir characterization.

Summary Prediction of formation damage that occurs in horizontal wells, often openhole completed, is a critical point for optimizing an oilfield development. The economic impact of near-wellbore induced drilling damage and cleanup efficiency has led to significant progress in both experimental and numerical studies designed to assess the wellbore flow properties during oil production. In a previous paper, a methodology combining both experimental and numerical approaches was presented to evaluate the natural cleanup of horizontal wells drilled with an oil-based mud (OBM). This paper presents an extension of the methodology for simulating both (a) near-wellbore invasion and permeability damage generated with a water-based mud (WBM), and (b) natural cleanup during oil backflow when the well is put into production. There is a fundamental difference between WBM and OBM invasions. In an oil-bearing formation, the displacement of the oil in place with an OBM filtrate is a miscible displacement process, while the displacement with a WBM filtrate is a two-phase flow process (imbibition), generating high wetting-phase saturation in the invaded zone. Then, during oil backflow, a portion of the wetting phase is trapped, leading to residual wetting-phase saturation greater than the initial one. Even in the absence of chemical interaction between filtrate and fluids in place, this induces an adverse water/oil relative permeability effect, which is an additional permeability impairment. This paper describes a numerical approach to model the formation damage with WBM and to predict well performance for natural cleanup when the well is subject to a pressure drawdown. The kinetics of fluid filtrate invasion, the filter-cake properties, and the filtrate/oil relative permeability curves in imbibition and drainage, together with damaged and return permeabilities, are obtained from specific drilling fluid damage laboratory tests. Using these data, the fluid filtrate invasion during the drilling phase is simulated, leading to a cone-type invasion depth along the horizontal well. This approach has allowed us to study the impact of various parameters related to fluids or cake properties, drilling conditions, and natural cleanup processes on the well performance.

P262 A FAST AND DIRECT METHOD OF K-PHI MEASUREMENTS ON DRILL CUTTINGS 1 P. EGERMANN 1 R. LENORMAND 1 D. LONGERON 1 and C. ZARCONE 2 1 Institut Français du Pétrole (IFP) 1 4 Avneue de Bois Préau 92500 Rueil Malmaison France 2 Institut de Mécanique des Fluides de Toulouse (IMFT) Permeability and porosity are necessary for reservoir characterization. An early evaluation of these parameters is commonly obtained through Log measurements. The most difficult parameter to obtain is the permeability as the accuracy on permeability derived from NMR is rather poor. So far reliable values of permeabilities are only obtained

Near wellbore flow properties are affected by mud and mud filtrate invasion during overbalanced drilling operations. The degree of alteration depends on a large number of parameters such as nature and characteristics of the drill-in fluid, formation properties and operating conditions. Laboratory mud filtration experiments have been conducted for many years to determine the extent and the degree of formation damage due to drillin fluid invasion. This paper proposes a new methodology to interpret formation damage tests performed with water-based muds. Numerical simulations are performed to quantify independently, on the one hand the impact of mud solids invasion and on the other hand the effect of multiphase flow process on the global permeability damage. Effective oil return permeability profiles are first determined through local pressure measurements along the core at different backflow rates. Then, corresponding relative permeabilities are determined from matching of the pressure differences and cumulative production evolution as a function of time. Results show that a good fit between experiment and simulation is obtained with a unique set of relative permeability curves for a given formation permeability. Their shapes are very similar to what is obtained using standard water/oil displacement experiments. The efficiency of the restoration of effective permeability is highly dependent on the oil backflow rate. The higher the imposed oil flow rate, the better the restoration of initial permeability. The main advantage of the proposed procedure is to provide, from a single laboratory test, a consistent interpretation of both permeability damage mechanisms (mud solids deposits and filtrate invasion ). This leads to a better diagnosis of the origin of the damage. Finally some recommendations are given to improve the design of laboratory formation damage experiments and to interpret natural cleanup of open hole completed wells. Guidelines are also provided to select the least damaging drill-in fluid formulation for a given permeability formation.

Formation damage in horizontal wells, often open hole completed, is a critical point for oil fields developed in deep off-shore, where acceptable development costs are based upon a limited number of highly productive wells. This paper describes a simplified numerical approach to model filter cake removal by natural cleanup when the well is put under a drawdown pressure difference. A flow simulator taking into account the near well permeability variation has been developed using a cylindrical grid with very small gridblocks around the well. Laboratory data obtained on sandstones damaged with an oil-based mud have been used as input data to model filter cake removal by natural cleanup. The purpose of this modelling is to optimize the applied drawdown pressure, to study the variation of well productivities as a function of the pressure drawdown, and to get the best possible well performance preventing sand production problems in poorly consolidated formations.

Inorganic scaling is the process of mineral scale deposition that may occur in the well tubing, and/or in the near well-bore area of both production or injection wells. Scale formation is usually the result of poor compatibility between the brines injected into the reservoir and the minerals in the formation. It may also be the consequence of abrupt pressure and temperature variations to which production fluids may be submitted between the reservoir and the surface. The precipitation of mineral deposits (carbonates, sulfates …) may create significant permeability impairments due to plugging of pore throats and thus induce large well productivity or injectivity losses. In most cases, precipitation cannot be avoided and preventive treatments are recommended. One of the most efficient treatment consists in squeezing a scale specific inhibitor into the formation. This paper presents an original integrated methodology aiming at defining the best chemical inhibitor formulation for a given application and at optimizing its implementation into the corresponding reservoir. Both experimental and numerical approaches are used to select the inhibitors, to evaluate their performance at both core and field levels and to define the best strategy for the squeeze process. Laboratory tests include the classical inhibitor selection methods based on jar tests and tube blocking tests but also on more sophisticated testing procedures recently developed. Numerical simulations of the treatment are performed using a in-house reservoir simulator to upscale the squeeze life time of the treatment from laboratory scale to well geometry.

This article, written by Technology Editor Dennis Denney, contains highlights of paper SPE 73733, "Modeling of Both Near-Wellbore Damage and Natural Cleanup of Horizontal Wells Drilled With a Water-Based Mud," by Y. Ding, SPE, D. Longeron, SPE, G. Renard, SPE, and A. Audibert, SPE, Inst. Francais du Petrole, originally presented at the 2002 SPE International Symposium and Exhibition on Formation Damage Control, Lafayette, Louisiana, 20-21 February.

Abstract Formation damage in horizontal wells, often open hole completed, is a critical point for oil fields developed in deep offshore where acceptable development costs are based upon a limited number of highly productive wells. This paper describes a simplified numerical approach to model filter cakes removal by natural cleanup when the well is put under a drawdown pressure difference. A flow simulator taking into account the near well permeability variation has been developed using cylindrical grid with very small gridblocks around the well. Laboratory data obtained on sandstones damaged with an oil-based mud have been used as input data to model filter cakes removal by natural cleanup. The purpose of this modelling is to optimize the applied drawdown pressure to study the variation of well productivities as a function of the pressure drawdown to get the best possible well performance preventing sand production problems in poorly consolidated formations.

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 evaluate i) near wellbore invasion and related damage due to two typical Drill-In fluids (D.I.F.) and ii) performance 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.

Summary Formation damage risks are well documented for standard mud formulations but they are poorly analyzed for new types of non-polluting muds. This paper describes extensive laboratory work aimed at evaluating the behavior of these new types of mud formulations. First, static and dynamic filtration experiments were conducted on paper filters and rock slices. Examination of mud cakes by means of cryo-SEM has permitted correlation of filtration behavior with the structural characteristics of both external and internal mud cakes. Then, the simulation of the full process of mud invasion in oil-bearing reservoirs was achieved by performing static and dynamic filtration experiments in a specially designed core-holder cell containing 40 cm long sandstone core samples. Cumulative filtrate losses and pressure drops across six sections of the core, while circulating the mud and back flushing the oil, were continuously monitored to evaluate the permeability damage. Damage arising from overbalanced conditions has been evaluated in terms of cake permeability, fluid loss characterization and reduction in oil permeability after mud exposure. For the three mud compositions tested here, the filtration process in high permeability sandstones is mainly controlled by external mud cakes. Damage is severe but an additional damage due to the trapped aqueous filtrate phase can strongly affect the oil relative permeabilities. This approach has given a complete behavior understanding of new water based drilling fluid formulations including evaluation of their performances, limits of use at high temperature and assessment of risks for different operational conditions.

Today there are a number of oil fields that have been developed with horizontal wells with open hole completions. The performance of potentially high productivity wells gives the greatest concern. The economy of the whole development project may be jeopardised by any failure in obtaining the targeted production. This is particularly true for turbiditic deep water reservoirs where the economy relies on a limited number of highly productive wells. In fact, what is the problem? Generally these wells are completed with barefoot screens and run with the mud and the mud cake still in place in the open hole section. The section may be 1000 to 2000 meters long or even more. To our knowledge, there is no way to perfectly clean the entire annulus nor remove the entire cake during the completion phase. The well is put on stream hoping that Nature will complete the clean-up. In fact, as the reservoir is heterogeneous along the drain, and that these highly productive wells produce under limited drawdown (high P.I), the prospect is that only a limited amount of the drain length will clean up and produce. One may get the desired oil rate at the wellhead, but what is happening down hole ? - the reservoir flows through a limited length resulting in high local fluid velocity - the high local velocity may induce borehole weakening and sand production - high velocity and sand production may induce screen wash out and well plugging (already experienced in the North Sea and the Gulf of Mexico) - limited area opened to flow induces a P.I reduction and a greater pressure gradient for radial flow which can cause early gas or water coning if the contact level is close. The challenge is to find the right combination of actions that will lead to (i) an easy and readily removable cake (external AND internal) and (ii) without creating additional damage. This paper describes the necessary laboratory approach prior to designing the drilling fluid : - the necessity to have a reproducible core plug preparation (Swi), - the importance of mastering solids size versus sandstone permeability, - the necessity to adapt linear flow (core plug) to radial flow conditions (borehole), - the type of damaging and cleaning-up laboratory procedures leading to a representative comparison between fluids, - the relative inadequacy of the return permeability (RP) concept in the case of open hole if it is misused, - the necessity of analysing parameters like Flow Initiation Pressure, and the difficulty in analysing the Lift Off Pressure. The above considerations are illustrated by the analysis of an extensive laboratory testing on OBM . Guidelines are drawn for the best jamming ratio compromise and a closer analysis of the drainage pressures is recommended. The necessary synergy between the drilling phase and the completion phase is discussed as the best performance of a properly designed Drill In Fluid can always be ruined by an inadequate completion procedure. For instance, do we have to clean open hole drains drilled with an OBM ?