Uncertainty analysis using experimental design and response surface techniques has been extensively used in the field of reservoir simulation. This study outlines an innovative workflow to generate multiple realizations of forward stratigraphic modelling of three Lower Cretaceous reservoirs from onshore Abu Dhabi. Forward stratigraphic modelling is a deterministic technique that simulates basin infill providing a better understanding of vertical and lateral facies distribution and connectivity in sedimentary basins. During the course of forward modelling a variety of environmental and stratigraphic parameters are used. Due to the uncertainties of these parameters it is critical to assess their impact on the development of the basin fill. The experimental design and response surface techniques have been innovatively applied at reservoir scale to enhance the understanding of major controlling parameters on carbonate production and to produce alternative facies distribution scenarios in the study reservoirs. The methodology used in this study was based on running multiple simulations, through varying key input parameters. The best stratigraphic models were then selected based on calibration quality and geological consistency. Calibration quality was assessed by two user defined quantitative functions called Thickness Calibration Indicator and Rock Texture Calibration Indicator. The initial step in the workflow identified uncertain environmental parameters (e.g. eustasy, carbonate production versus depth, carbonate production versus time, wave parameters, gravity and wave transport and erosion rates) from a manually calibrated reference case and ranges of values for each parameter defined based on the knowledge of geology over the area. Latin Hypercube experimental design was then used to define a set of simulations to allow an efficient and uniform sampling of the entire uncertain domain. Sensitivity analysis was then performed on simulation responses (texture and thickness calibration indicators) using the technique of nonparametric Response Surface Modelling (RSM). The influence (quantitative and qualitative) of the impacting parameters on responses was studied to identify the most influential parameters as well as the ranges yielding good calibration indicator values. A further set of simulations was then launched that considered the most influential parameters and their precise ranges. Non critical parameters were assigned with the constant values from the reference case model. These simulations generated a series of well calibrated models. A filtering of simulations with high calibration indicator values and good geological consistency was then performed to choose acceptable multi-realizations. Finally, thickness and texture confidence properties were mapped based on the selected multi-realizations and the reference case. Sensitivity study on three Lower Cretaceous reservoirs from onshore Abu Dhabi successfully addressed the uncertainty associated with forward stratigraphic model input parameters. Sensitivity analysis was performed using Experimental Design and RSM. This was applied to enhance the understanding of the major controlling environmental parameters on carbonate production for individual sequence with each of the study reservoirs.

A giant brownfield re-development project with long horizontal wells was initiated to arrest production decline mainly caused by a lack of pressure support and free gas influx from the large gas cap. Key value drivers for the project are developing an understanding of the layers with regards to gas breakthrough, and achieving capital efficiency through low-cost well delivery, better planning and technology applications. Firstly, the field has been segmented based on the analysis of multiple factors influencing the free gas production. It considers geological aspects such as the study of depositional environment and diagenesis, structural elements such as high permeability streaks and fractures, dynamic behaviors such as the water injection efficiency, gas cap expansion or coning. Secondly, numerical simulations were then run in order to rank the sectors based on the expected model performance, compare them with real data categorization, and test the effect of the new proposed development schemes such as water injection at gas-oil contact and long horizontal wells equipped with downhole control valves. It was found that each sector has a specific production mechanism and appropriate developments were recommended and then tested in the simulation. For instance, high permeability streaks play a significant role on the development of some sectors instigating a big difference of maturity between sub-layers, early water or gas breakthrough. Also, the inefficiency of water injection is one of the biggest issues of the field. Most of the water injectors are located too far from the oil producers, and have a low injectivity due to the often degraded facies in the aquifer because of diagenesis. This leads to a lack of pressure support that is counterbalanced by the gas injection, ending up with a lot of high GOR wells and a bad sweep from the top of the structure as the gas tends to by-pass the oil. Simulation work showed that several remaining zones are safe for immediate development and should be prioritized for development in the near future. On the other hand, some of the mature layers prone to gas and water breakthrough need a boost for development, such as water injection at gas-oil-contact, artificial lift, low pressure system, GOR relaxation. Tight and undeveloped reservoirs are improved by implementing long horizontal drains.

This paper discusses the re-construction of the long-term development plan for an offshore giantfield located in Abu Dhabi with the aim to mitigate the rising challenges in the maturing field. The primary objective is to understand the reservoir behavior in terms of fluid movement incorporating the learning from the vast history while correlating with the geological features. The field has been divided into segments based on multiple factors considering the static properties such as facies distribution, diagenesis, faults, and fractures while incorporating the dynamic behaviors including pressure trends and fluid movements. On further analysis, various trends have been identified relating these static and dynamic behaviors. The production mechanism for each of the reservoirs and the subsequent sub reservoirs were analyzed with the help of Chan plots, Hall plots and Lorentz plots which distinctly revealed trends that further helped to classify the wells into different production categories. Using the above methodology the field has been categorized in segments and color coded to indicate areas of different ranking. The green zone indicates area of best interest which currently has strong pressure support and wells can be planned immediately. The wells in this area are expected to produce with a low risk of water and gas. The yellow zone indicates areas of caution where special wells including smart wells maybe required to sustain production. This area showed relatively lower pressure support owing the location of the water injectors and the degraded facies quality between the injectors and the producers. The red zone highlights areas which are relatively mature compared to the neighboring zones and will require new development philosophy to improve the recovery. The findings from this study were used as the basis for a reservoir simulation study using a history matched model, to plan future activities and improve the field recovery. This study involved an in-depth analysis incorporating the latest findings with respect to the static and dynamic properties of the reservoir. This has helped to classify the reservoir based on the development needs and will play a critical role in designing the future strategies in less time.

Development of Giant carbonate reservoir considerable cap gas and condensate reserves along with oil rim is now being considered as one of the on-hand options to overcome shortage of gas in the long term and to add value to the company assets. This paper presents an overview of the study that was initiated to assess the feasibility of the simultaneous development of gas and oil and to evaluate its impact on the existing oil long term development plan. A compositional simulation study has been conducted by adding the co-development of the gas cap on top of the current Long Term Oil development plan. This development includes water injection at the original gas oil contact level to isolate the gas cap and to enable to develop gas cap independently from the oil rim. In this scenario, drilling of producers in the gas cap is considered and condensate is produced to the dedicated production system. Several prediction scenarios have been investigated to set up a development plan of gas: Blow down or natural depletion,Produced gas recycling,Lean gas recycling. The best scenario has then been selected for further optimisation. (On gas injection recycling ratio, project start-up timing, gas production target, and water injection). After analysing the results, it was found that the blow down of the gas cap provides a good recovery of gas and condensates but has a negative impact on oil reserves. Gas recycling option proves interesting as it maintains pressure both for condensate (hence preventing drop-out) and for oil; therefore impact on Black-Oil recovery becomes minor. Oil recovery is higher with late starting time of the co-development or with high recycling ratio. Oil Recovery Factor depends mainly on the pressure maintenance of the reservoir. However excessive gas injection has a negative effect due to breakthrough of the gas into the oil producers which have GOR limit constraints. Co-development simulation study of the gas cap along with oil, through an inner ring of water injection at gas oil contact shows that the co-development is not only feasible but it will also bring significant value to the company future business plan. Gas cap being semi-isolated from the oil rim, the impact on the oil development plan is minimized. Co-development proves appealing to the different owners of the field since it brings more gas and LPG that is foreseen as valuable source of enriched gas for later EOR or WAG schemes

Forward stratigraphic modelling is based on the deterministic reconstruction of depositional processes in a sequence of time steps moving forward in time. This approach is usually hindered with various, uncertain parameters. Today, uncertainty analysis using experimental design and response surface techniques is commonly used in the field of dynamic reservoir simulation. This study presents the innovative application of these techniques on forward stratigraphic modelling of a giant carbonate field from offshore Abu Dhabi, leading to the generation of multiple realizations to be used as the starting point for better geomodel construction.A variety of environmental and stratigraphic parameters are used, some of which carry an important uncertainty with regards to their range of possible values. It is therefore critical to assess their impact on the development of the basin fill -a tedious exercise for subsurface fields, whereby the only physical data come from well cores. The Experimental Design and Response Surface techniques have been innovatively applied at reservoir scale to improve the calibration of the model and to produce alternative facies distribution scenarios in the study reservoir.The idea behind this approach is, first to perform a global sensitivity analysis with a large number of parameters and simulations, and then to narrow down the uncertain domain in order to select the best stratigraphic models according to criteria of calibration quality and geological consistency. Input data for this model calibration consisted mostly of an extensive sedimentological core study carried out on several wells, and a high resolution sequence stratigraphic analysis. The quality of calibration (simulation vs core data) was assessed by two user-defined quantitative functions called Thickness Calibration Indicator and Rock Texture Calibration Indicator.Following a first manual calibration of a reference case, specific uncertain parameters (e.g. eustasy; carbonate production versus depth; carbonate production vs. time; wave parameters; gravity and wave transport; erosion rates) were selected and their ranges of values defined based on experience and knowledge of geology over the study area. Latin Hypercube Experimental Design was used to ensure a uniformly distributed sampling of the parameters. Sensitivity analysis based on the responses (texture and thickness calibration indicators) was carried out and allowed to identify the most influential parameters as well as their ranges of values yielding good calibration indicator values.A second set of simulations was then launched considering only the most influential parameters and their refined ranges. Other parameters were assigned with constant values used in the reference case model. This generated a collection of various, well calibrated models. A last filtering of simulations with the highest calibration indicator values and good geological consistency was performed to provide a handful of acceptable multi-realizations. Finally, confidence maps were computed based o...