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One of the great challenges facing our industry is accurate prediction of well inflow. Conventional methods have been too cumbersome and imprecise and have suffered from a lack of accuracy and clarity. Informed use of computational fluid dynamics, fine scale modelling and improved computing power enable far more accurate prediction of the impact of formation damage and thus of well performance. The accurate prediction of well performance helps with appraisal of development prospects, well planning and reliable prediction of true well and field value. If we know what the outcome of our actions and of our well designs will be, then we can make sensible and informed choices on damage impact and mitigation. Results from laboratory simulations of drilling and completion operations were generated from "standard" return permeability testing. The detailed data obtained, and its millimetre scale resolution was incorporated in to a well specific model. The damaged and undamaged states were examined using flow rate predictive computational fluid dynamics. The impact on flow in a single and dual permeability reservoir interval were calculated. In a specific example presented, the case for underbalance drilling was clearly made as the impact of overbalance drilling was predicted to have a severe impact on well productivity. The results of accurate and detailed laboratory simulation of formation damage have been translated using innovative software applications to give a prediction of well performance. This is the first time that computational fluid dynamics has been employed to predict well performance based on high quality laboratory testing. In future the laboratory tests will be designed to yield data most useful for the model and the model and grid scale will continue to be adjusted based on the specific challenge and objective. The detailed workflow used to achieve more accurate well performance prediction will be outlined in the paper. Introduction Modeling of permeability, pressure drops and formation damage in the near wellbore is an art that has suffered from lack of input and lack of tools in the past. The input deficit has largely been driven by poor or incomplete measurement and knowledge of formation damage mechanisms. These are not very often identified or quantified and some of the basic assumptions and rules of thumb for damage are simply incorrect. Even when properly understood and measured, existing industry software, inflow performance relationship models are inadequate tools to capture the detail and complexity of damage distribution. They generally accept only a very simple input of damage magnitude and thickness. Often data is altered in order to match the model. This is entirely the wrong way around. In building the workflows to model and understand damage, our guiding principal was that the model must alter to capture the damage rather than the other way around. Previous authors have attempted to model damage and create near wellbore inflow models which incorporate flow restrictions to a fine scale (Bennion et al 1996, Burton et al 1997, Han et al 2005, Yildiz 2004, Qutob and Ferreira 2005). These could be termed very near wellbore models of damage, most of which are numerical 2D models. Our objective here is to create a process through which complex very near and very, very near well inflow models can be created in 2D, 3D and 4D. The overriding objective is accurate prediction of well productivity and injectivity which will enable thorough well and completion designs and enable focus on real production barriers rather than speculation and heresay.

Conventional well inflow modeling fails to capture the complexity of flow in to wells from asymmetric near wellbore permeability distribution. In order to simulate flow in to horizontal wells with varying degrees of damage along the well and around the circumference a complex numerical model is required. Computational fluid dynamics enables fluid flow in to the well to be modeled. Along a horizontal well the magnitude and depth of mud cake, filtrate invasion and formation damage will vary. Longer exposure times at the heel of a well and lower drawdown at the toe will lead to different formation damage, clean-up and inflow patterns along the well. In addition, gravity can have a profound effect on damage distribution at the base, middle and top of the well. Thicker mud cakes will tend to develop at the base of the well and whilst there may be more erosion on the low side, this will only serve to compound the damage by leading to deeper mud invasion. Inevitably, formation damage will have an asymmetric distribution along and around the well. Asymmetric models of fluid flow in to horizontal wells have been created to reflect the damage heterogeneity. Results of the models have shown that even in homogenous reservoirs, the vast majority of inflow in to the well is derived from the high side of the heel of the well. Modeling shows the potential impact of consistent chemical clean-up and of inflow control along the horizontal well. By fully capturing the magnitude and impact of damage in conventional wells using computational fluid dynamics, the value of well drilling and completion options such as underbalance drilling and hydraulic fracturing can be evaluated. This paper presents the first use of complex three dimensional computational fluid dynamic models to predict and mitigate against asymmetric formation damage in horizontal wells.

Computational Fluid Dynamics (CFD) has been used to model fluid behaviour in reservoirs, near wellbores and wells. More recently the CFD has been applied to more diverse challenges such as sand transport, coupled mechanical modelling of failing wellbores and gas well clean-up. CFD has been applied to model the crushed zone around perforation tunnels and also to predict well performance base on laboratory testing. Where the reservoir is involved in these models they are fully coupled and flow is enabled deep in the reservoir and in to every point along the well. Capturing the simultaneous flow of fluids in the reservoir and in the well is essential to predict performance in wells and reservoirs with complex geometry. Cross flow between reservoir layers and the flow of fluids through and along the well length are real phenomena which if ignored, can lead to poor prediction of well performance. CFD modelling relies on fundamental physical properties and enables changes in flow restriction such as formation damage or matrix stimulation to be captured. This paper presents some recent case histories where model results are verified by real well performance and where traditional analytical model results are compared to fully coupled CFD reservoir and well models. The impact of formation damage is observed to produce very different results, depending on the model applied. Case histories illustrating the application of innovative, rigorous modelling of formation damage from different drilling and completion fluids and practices are presented. Further advances that enable sand failure and sand transport to be modelled are also illustrated. The case is made for use of CFD or similar modelling processes where complex reservoirs or complex wells are considered. IntroductionCFD has well tried and tested applications in many industries including the oil and gas industry. Here it has principally been used to model fluid flow in surface pipelines and to examine fluid flow through and around tubing and equipment. For example CFD is used in the design and development of new drill bits and in the development of Inflow Control Devices (ICDs). More recently there has been an increasing trend to move CFD in to the well and the complexity of completion and reservoir geometries (Byrne et al 2010). This evolution has been enabled by the advance in computational power and speed and by improvements in CFD software packages. CFD enables modeling of the physics of fluid flow through restrictive media and so is ideally suited to modeling the flow of fluid or fluids through reservoirs, completions and wells. Previous papers have presented some of the earliest work in this field but even in a few short years significant progress in the accuracy and the scale of modeling has been made. Complex phenomena such as unloading liquid completion fluid from gas wells, sand failure and the impact of changing wellbore shape on fluid flow and complex well geometries have now been modeled and used to assist in the design of optimum wells. A case study is pr...

In order to accurately predict and assess the impact of formation damage on well production or injection models of near wellbore are frequently employed. These models can range in detail and accuracy from basic analytical models to comprehensive numerical models. But which is most appropriate for a specific reservoir or well type? This paper presents a review of the principle options available and their potential applications. In addition a vision of potential future developments in modelling is presented. A constant challenge to the formation damage community is predicting the impact of damage and the value in its reduction or elimination. Thus modelling the impact of damage is usually undertaken in order to answer the question "what does this mean for my well?". Many different model types have been employed over the years. The different approaches, their logic and potential applications in different well and reservoir types are presented together with a brief history of near wellbore modelling. With continuing dramatic advances in computational power and in software, the future of reservoir, near wellbore and well modelling has previously unimagined potential. Some of this potential is also explored and its' applicability discussed. The paper provides a comprehensive overview of near wellbore and formation damage modelling and should provide a useful first entry in to this subject for novices and will no doubt draw constructive responses from the various near wellbore model advocates.