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.
Liquid loading is experienced in high water-cut wells operating against high sealine pressure in an offshore mature field without artificial lift. This phenomenon is usually not captured by VLP/IPR representation built into reservoir simulation or steady-state well models, which results in overestimation of future oil production. Key value drivers for the project are developing an understanding of the liquid loading based on critical fluid velocity, well completion and reservoir characteristics and study its impact on simulation forecasts. Firstly, the liquid loading condition has been correlated to different well parameters such as productivity index, GOR and water-cut. The correlation is built from routinely acquired flow test data showing wells ceasing to flow due to unstable flow regimes and fluctuating sea line pressures. Additionally, extensive well modelling using Prosper software was carried out to assess the proper fluid correlation method. Secondly, that correlation was converted into a response surface model to make the link between the unstable flow regime conditions and the impacting parameters such as well PI, GOR, well geometry. Thirdly, the unstable flow correlation was implemented in the simulation model with a script of conditional events in order to flag wells with unstable flow and shut them if no activation condition is applied such as artificial lift or THP reduction. Currently, it is estimated that about 4 wells are becoming unable to flow every year due to the mentioned loading issues. These loaded wells require vessel/barge to carry out unloading which involves significant operating cost in an offshore environment. This complex behavior is normally overcome by dynamically coupling the subsurface models to the surface production system. However, in a giant field with hundreds of wells from multiple reservoirs this can be complex and resource demanding. Application of the correlation in the simulation resulted in about 80 wells with unstable flow conditions to be closed by year 2040, whereas they would have otherwise continued to flow if regular VLP curves were used. This method served to improve the model accuracy and to increase the assurance in forecasts predictability, with regards to water-cut evolution. This study was a key driver input for deciding on the acceleration of a major debottlenecking facility related to medium pressure system at surface. Sustainability of field plateau, improved wells availability, reduced in-active wells count and associated reactivation resources are tangible benefits of the mentioned study.
A new production development scheme was proposed for one of the major offshore fields in the Gulf. To assist building assurance of the delivery of this scheme, subsurface uncertainty evaluation was initiated; firstly based on an in-house approach and later through the use of commercial softwares. The proposed study is primarily focused on dynamic reservoir uncertainties. The objective of this study is to evaluate the impact of the reservoir uncertainties on plateau duration period in the prediction phase. The workflow, based on experimental design and response surface, advantages and outcomes of this study are presented and its limitations discussed. This study was completed over a short period of time, thanks to an optimization of the CPU resources. This was a key advantage obtained while carrying large models with long history exceeding 50 years. The study resulted in successfully delivering probabilistic profiles (P90, P50, P10) in order to assure the production delivery of the proposed development scheme and enables to develop risk mitigation plans. A ranking of the most influential uncertainty parameters with quantification of their interactions is obtained. Consideration is also given to the history match quality, which results in reducing the parameters distributions and highlights the value of the data acquisition. Introduction ADMA launched a study to assist building assurance of the delivery of new production development scheme and to assess the risk of applying such strategy. The objective of this study is to evaluate the impact of the reservoir uncertainties on plateau duration period in the prediction phase. This study come up with a probabilistic forecast (P90, P50, P10) instead of a single deterministic forecast as usual. First, ADMA launched the study in-house in two phases. The fist phase is simple and based on mono-parameter sensitivity at a time method but requires a large number of runs and doesn't evaluate the impact of interactions between parameters. 13 parameters are used and 4 runs are generated for each parameter. The second phase is based on the Monte Carlo analysis. After assigning a probabilistic law to each uncertain parameter, n realizations of the uncertain parameters are picked to obtain a sample of n reservoir simulations. This method is expensive in terms of number of reservoir simulation runs (at least 100 runs for 10–15 parameters). Secondly, ADMA called for two companies and their commercial softwares (X and Y) to tune the study further. ADMA's optimized methodology is based on experimental design and response surface modelling to model flow simulations. The same 13 uncertain parameters have been considered for this Field study. Those parameters have been re-evaluated and constrained by historical data in order to reduce uncertainty range and they were assigned a new distribution law. Using new ranges and distribution laws, uncertainties are propagated and probabilistic production forecasts are obtained. The work flow of this study is shown in figure 1.
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