fax 01-972-952-9435. AbstractThis paper describes ongoing efforts to improve the performance of Nimr wells by identifying and shutting off thief zones and summarizes results to date. The presence of a combination of unstable displacement, strong aquifer drive, complex geological settings and non-matrix permeability events has resulted in "scatter-shot" new well performance with progressively higher average initial water cuts. Although not a traditional fractured system, Underbalanced Drilling (UBD) data have shown that rogue fractures and bypass zones play a key role in the movement of water through Nimr reservoirs. The concept is to use UBD to identify "non-matrix" water bearing features (rogue fractures, bypass/thief zones), and shut them off using different shut-off techniques, including a new selective shut-off completion technology called EZIP. It is expected that this will lead to improved water cut behavior, and higher oil production rates and ultimate recoveries from new wells.A sector model of the reservoir was used to evaluate the impact of the different fracture shut-off techniques. Using the unstructured grid capability of the simulator, rogue fractures intersecting the wellbore are introduced at arbitrary locations, and the impact of shutting them off either along fracture length or along wellbore face is investigated. The simulator model provided a means to quickly study several sensitivities such as dimensions, number and orientation of fractures, initial water saturation, reservoir pressure, etc. The results of the model showed that properly designed fracture shut-off methods could delay water cut by as much as 600 days. Equally importantly, the model showed that shutting the fractures along their length (by cement or gel squeezes, for instance) has almost no beneficial impact on the water cut behavior of the wells during production.
TX 75083-3836, U.S.A., fax 01-972-952-9435. AbstractWe present an algorithm and an approach for the coupling of a 2-phase, 2-dimensional transient reservoir simulator using a control volume finite element approach, and a 2-phase transient mechanistic multiphase wellbore flow model to dynamically characterize reservoirs while drilling underbalanced. This approach succeeds in accounting for storage effects in both the wellbore and reservoir. The coupling is achieved at the wellbore flowing pressure for given injection parameter and back pressure time functions, and the inverse problem is posed as one of minimizing the difference between predicted and observed return parameter time functions. A genetic algorithm (GA) approach, coupled with a one dimensional search scheme is used to achieve optimization. This paper focuses on one aspect of dynamic reservoir characterization-the prediction of permeability as a function of drill depth. Permeability results are obtained for an example problem and the accuracy and limitations of the proposed approach are discussed. Numerical problems in dealing with the grossly different time scales of reservoir and wellbore behavior, and practical limitations of reservoir characterization during underbalanced drilling are described. Improvements to the algorithm and future work are also discussed.
This paper describes ongoing efforts to improve the performance of Nimr wells by identifying and shutting off thief zones and summarizes results to date. The presence of a combination of unstable displacement, strong aquifer drive, complex geological settings and non-matrix permeability events has resulted in "scatter-shot" new well performance with progressively higher average initial water cuts. Although not a traditional fractured system, Underbalanced Drilling (UBD) data have shown that rogue fractures and bypass zones play a key role in the movement of water through Nimr reservoirs. The concept is to use UBD to identify "non-matrix" water bearing features (rogue fractures, bypass/thief zones), and shut them off using different shut-off techniques, including a new selective shut-off completion technology called EZIP. It is expected that this will lead to improved water cut behavior, and higher oil production rates and ultimate recoveries from new wells. A sector model of the reservoir was used to evaluate the impact of the different fracture shut-off techniques. Using the unstructured grid capability of the simulator, rogue fractures intersecting the wellbore are introduced at arbitrary locations, and the impact of shutting them off either along fracture length or along wellbore face is investigated. The simulator model provided a means to quickly study several sensitivities such as dimensions, number and orientation of fractures, initial water saturation, reservoir pressure, etc. The results of the model showed that properly designed fracture shut-off methods could delay water cut by as much as 600 days. Equally importantly, the model showed that shutting the fractures along their length (by cement or gel squeezes, for instance) has almost no beneficial impact on the water cut behavior of the wells during production. UBD was critical to the development and implementation of the shut-off technology. UBD was used to characterize not only the bypass features, but also their effectiveness and water production potential. Indeed, the authors argue that UBD is the only approach whereby static and dynamic characterization can be achieved while drilling. Based on interpretive analysis of data gathered during UBD, together with petrophysical data, non matrix features and their potential were identified, and used to decide upon the most effective shut-off strategy. The concepts were tested in a number of wells where bypass features producing water were identified. The first well used abandonment plugs to shut off the whole lower section of the well, while the others have used EZIP technology to shut off the features. Results from the tests have been very encouraging, and confirm the key conclusions of the dynamic modeling. Based on the study and well results, the Nimr Asset team is embarking upon a long term campaign of using UBD to characterize the reservoir and optimally complete the wells to improve water cut behavior in wells. The paper discusses the modeling, the reservoir characterization enabled by UBD, as well as the results from completed wells. The authors conclude that a combination of fit-for-purpose reservoir modeling, UBD enabled reservoir characterization and appropriate shut-off technologies can lead to significant value creation in fields like Nimr.
TX 75083-3836, U.S.A., fax 01-972-952-9435. AbstractFor a gas reservoir, the intake pressures are available at a delivery point and the operator is obligated to supply gas honoring such downstream pressure requirements. As a result, it is imperative that the reservoir deliverability prediction be coupled with the pressure drop in the surface network. Additionally, this coupled system should not only honor the imposed delivery point pressure constraints but also the historical attributes of pressures and rates. In this paper, a modified Gauss-Newton method is utilized in conjunction with a non-linear parameter estimation algorithm to history match a surface-reservoir coupled gas reservoir simulation.Control Volume Finite Element (CVFE) based reservoir simulator is amenable to unstructured grids to resolve near wellbore and high activity (high permeability channels, fractures) flow areas. The pressure drop in the surface network is modeled using Weymouth's equation of steady state pipeline flow. More importantly, the total system is solved in a coupled fashion thereby enabling the solution to be controlled by surface pressure constraints. In addition, both surface and reservoir decision variables are estimated using a modified Gauss Newton algorithm in the assisted history matching step.Examples from dry gas reservoir demonstrate the usefulness of this methodology. First, the necessity of coupling surface and reservoir models is highlighted. Next, the computational efficiency of the assisted history matching algorithm as compared to perturbing critical reservoir and surface attributes in a heuristic manner is underscored.There are three significant contributions of this paper, namely (a) an assisted history matching algorithm proves efficient in comparison to arbitrary perturbation of decision variables (b) a coupled reservoir-surface model renders a more accurate pressure prediction of the total system and finally, (c) an unstructured grid simulator provides the platform for accurate and cost-effective reservoir modeling alternative.
In this paper, we illustrate an approach to underbalanced drilling (UBD) candidate selection based on damage assessment and characterization using dynamic reservoir simulations. A 2-phase, 2-dimensional transient reservoir simulator (eRes UBD) using a control volume finite element approach is used to reflect the reservoir conditions. Because of its unstructured grid formulation, the simulator is ideally suited for investigating near-wellbore phenomena, such as drilling-induced invasive damage. In addition, the simulator is augmented to capture dynamic formation exposure, thus enabling drill-ahead capability. Overbalanced drilling conditions are mimicked using the drill-ahead capability of the simulator by specifying an arbitrary depth-vs-time profile, and the resulting formation fluid distribution is investigated, with specific attention to near-wellbore saturation. The impact of invasive damage on productivity is then examined by putting the well into production. Additionally, the extent of damage, flowback period and return permeability are characterized through perturbing critical reservoir and fluid variables. Representative sector simulation runs exhibit the longer term benefits of UBD as compared to conventional counterparts. The methodology described in the paper can ultimately guide the screening and selection of UBD candidates, and is an important part of value-based selection of UBD candidates. Introduction The impact of damage and the need for its mitigation, remediation and/or elimination have received considerable attention in the literature as well as in drilling and production practice. Underbalanced Drilling (UBD) has emerged in recent times as an important technique for the elimination of the most pervasive form of damage- that caused by drilling overbalanced. Drilling induced formation damage can take many forms, some of which are:Invasive drilling fluid causing phase blockage thereby attenuating relative mobility of hydrocarbon phaseSolids invasion occluding pore throats of the exposed formation thereby reducing the permeability in the near-wellbore region of a well. Solids come from drilling, muds or from crushing of near-wellbore rock.Uneven flow through the entire length of the horizontal well because of one or both of the aforementioned reasonsOverbalanced drilling (OBD) can result in stress alteration and be the cause for unstable wellbore and permanent degradation of flow behaviorInfluence of the above effects may also cause unnecessary fracture initiation and propagation thereby resulting in preferential flow conduit for displacing fluids. For reservoir applications of UBD, it is important to assess the type of formation damage and the possible effect of UBD on its mitigation. Thus, estimation and quantification of conventional overbalanced drilling induced damage is of value in UBD candidate screening. In this work, we introduce the concept of using dynamic simulations in quantifying drilling damage, and the comparative influence of UBD on productivity when such damage is mitigated. Recent work on formation damage caused by overbalanced drilling is reviewed. A 2-phase, 2-D transient reservoir simulator has been modified to enable consideration of near-wellbore damage (through the use of unstructured grid formulation) as well as drilling ahead to mimic overbalanced drilling. Using these two features, the effect of invasive damage on productivity is illustrated for an example. Coupled with core-analysis based assessments, the methodology described here can provide a quantitative basis for the selection of UBD candidates.
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