This paper presents the implementation of the dual continuum approach (dual porosity/permeability), into a PEBI grid based, general-purpose reservoir simulator. Experience shows that not all parts of a reservoir will typically require dual porosity/permeability treatment. Sometimes numerical or other restrictions limit the use of a dual porosity/permeability model. The approach presented in this paper takes these aspects into consideration. It allows creating dual porosity/permeability cells wherever the geological attributes (facies, rock type, fracture intensity) imply to do so. The remaining - unfractured - part of the model will be treated like conventional single porosity areas. Therefore, the number of grid blocks and the CPU time required for the flow simulation are minimized. Fracture and matrix-fracture transfer properties (including shape-factors) are derived from the geological information and fracture intensity. All of this information and data is stored on a cell by cell basis. Furthermore, the presented method does not restrict any of the superior gridding capabilities of the simulator like local grid refinement, fault or well modeling. The equation system resulting from all grid blocks will be solved fully coupled. The flow simulator is based on a multi-purpose, compositional formulation, and copes with systems ranging from single-phase to multi-phase, multi-component fluid flow. Several examples illustrate the usefulness and flexibility of this modeling approach for accurate and economic simulation of fractured or partially fractured reservoirs. Introduction The existence of fractured reservoirs is known since many years. In fact, one may claim that all hydrocarbon reservoirs are fractured. The question remains, however, whether or not those fractures are important for fluid flow (i.e. form a fracture network) and if their existence justifies use of a dual porosity approach in the simulation. Only when fractures with sufficient length of penetration, connectivity and spacing occur, do their effects become important. To assess fracture contributions accurately, a wide range of geological, petrophysical and engineering determinations are required on large as well as small scales and their complex interaction must be investigated. A classification of fractures is typically based on either descriptive criteria (natural fractures vs. induced fractures, macro fractures and micro fractures, open and closed fracture) or geological criteria (origin of fractures, stress state, folding, stratigraphy, etc.). For the purpose of reservoir simulation, the classification after Nelson1,2 or modifications of it3 are used most commonly. This classification defines the following types of fractured reservoirs:Type 1: Fractures provide the essential porosity and permeabilityType 2: Fractures provide the essential permeabilityType 3: Fractures provide permeability assistance to an already producible reservoir Only with the first type is the accurate calculation of fracture porosity, fracture width and spacing of paramount importance. The estimation of these properties will predict whether initial flow rates can be maintained or will drop rapidly within short time. With this type of fractured reservoirs, the producible hydrocarbon fluid volumes reside within the fracture network and the application of a dual porosity approach must be questioned. For reservoir types 2 and 3, where the main storage volume resides in the matrix rock volume, early knowledge of matrix-fracture interaction is extremely important. It will determine how efficiently the matrix can be drained by the fracture system. The accurate determination of the fracture porosity will be of less significance in those cases.

This paper presents an innovative filtering and analysis approach to identify candidates for sidetracking in mature water flooded fields. It targets bypassed reserves to improve production and ultimate recovery from such fields at once. The method is based on production engineering concepts, it is very time efficient and requires only a minimum of data, which makes it in most cases more suitable than other methods. The approach provides a filtering concept to select all wells that might have bypassed reserves in their drainage area and provides a step by step analysis to verify, quantify, and locate these bypassed reserves. Further it provides a comprehensive method to develop a production forecast for a potential sidetrack. It also presents a set of criteria to select the most suitable well to sidetrack. Finally it allows leveraging all associated uncertainties by linking the economic analysis to a Monte Carlo simulation, which is critical for a sound management decision. Whereby, the approach tackles the four most critical factors, how much data are needed, how much resources are required, how long will it take to come to a decision, and what will be the certainty of the output. Recapitulating, the approach presented here allows selecting candidates for sidetracking in mature water flooded fields based on a minimum of data time efficiently, which allows making the right management decision to unlock some of the potential of these kind of fields at attractive ROI and NPV. Introduction The world energy demand is continuously increasing, while the number of newly explored fields is declining regardless of the effort taken and technology invested into exploration as C.J. Campbell and J.H. Laherrère1 explained. Thus the E&P companies are coming back to their brown fields re-evaluating the remaining potentials as C. Sbiti stated in an interview with the Petroleum Economist2. Today already ~70% of the world oil production is coming from brown fields3. Improving both, production and ultimate recovery from these fields is critical for growing beyond this level2. The integration of technologies and conceptual approaches, as this paper presents one, are critical in achieving this goal. Whereby, with the ever dwindling reserves base it is of the up most importance to extract all available production from existing fields, within the economic limits. In brown fields the capital investment of infrastructure are already in place. Reclaiming idling wells by sidetracking is an attractive investment option, which also allows the application of new technology into the field at low cost, when significant reserves are accessed. However, the critical aspect is if bypassed reserves remain in significant amounts to be economical given the lower investment required? This uncertainty makes unlocking the potentials of a brown field is still a reinforcing process. Which means that, as soon as potentials are proven to exist, resources will be made available to pursue them - but usually not before. Thus, identifying and proving the potentials of a field based on a minimum of data time efficiently is a key for unlocking these potentials. Target of the Approach Most of the fields in the world are water flooded. Thus, mature water flooded fields cover the highest percentage of fields and associated remaining potentials4, which makes them especially attractive. In almost any water flood, water passes by the oil on a macroscopic scale due to the heterogeneity of the reservoir. This might lead to reasonable spots of bypassed reserves, which are not accessible through any existing well. Drilling sidetracks from existing wells into these spots increases the production and improves ultimate recovery5 from the field. Achieve these two goals at once makes bypassed reserves in water flooded fields an attractive target for unlocking the potentials of such fields. Benefits of the Approach Initially, only limited resources will be available to evaluate the remaining potentials of a field. Thus, it is critical that the applied analysis approach delivers result time effectively with a minimum of data.

TX 75083-3836 U.S.A., fax 01-972-952-9435. AbstractToday, mature fields are a major interest for many oil companies since their un-recovered potentials are expected to help satisfy the worlds growing oil demand. However, while operating in such fields companies face new challenges.As such, many mature fields show under-hydrostatic reservoir pressure due to depletion, which might lead to no returns to surface during workover operations. No returns to surface make it impossible to recognize any losses as an indicator for potential formation damages or gains as an indicator for a potential kick.The crucial question is therefore how to recognize losses or gains in real time without returns to surface during workover operations?Knowing the depth of the fluid level in the well at any point in time would allow recognizing losses and gains, simply by monitoring all the changes of the fluid level in the well. This paper presents a newly developed measurement approach using a specially designed echometer that allows monitoring the fluid level in the well without returns to surface during workover operations automatically in real time, which is a fundamentally new application for an echometer.All the technical requirements that were identified to be critical for this measurement approach are highlighted in the paper. Further the paper lines out all the associated technical challenges that were faced while implementing this measurement approach. Finally, reliability and functionality of the measurement approach were proven in a series of field tests under various operational conditions.

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This paper was presented as part of the student paper contest associated with the European Petroleum Conference. Abstract This paper gives an overview over the different aspects of transient pressure behaviour in fractured reservoirs. Basically fractured reservoirs can be divided into two different groups. The first consists of reservoirs of single porosity which have been fractured artificially, the second group consists of reservoirs which contain a network of natural fractures and which have been additionally fractured artificially(double porosity fractured reservoir). Further it is possible to make a difference based on the radial dimensions and the boundary conditions of a reservoir. Therefore the systems of infinite and finite reservoirs will also be discussed. These four cases will be presented separately. The different flow regimes will be discussed and also the parameters which influence the pressure behaviour. Further the interpretation of the pressure tests will be discussed by using the type curves which correspond with the results of the flow regimes description. Finally a short brief of the simulation of the two main different fractured reservoirs on a numerical reservoir simulator will be given. Introduction To enhance the productivity of wells which were damaged or were completed in tight formations, hydraulic fracturing (called "HF" in the following text) is a common and often used technique. Hydraulic fracturing as well as acidicing have been used with great success in many reservoirs with low natural permeability. It was realised very early that a HF influences the flow behaviour of a well extremely, so that common pressure test analyses techniques may cause errors. Thus the transient flow behaviour of fractured reservoirs became an important topic. The different analyses of the pressure tests do not only describe the status quo of the flow behaviour after the treatment but describe also the form of the HF. The interpretation of the tests is very important for the production strategy and even for the design of a new HF.