This paper covers a case study of successfully applying innovative NMR logging technology for fluid typing using continuous measurement from longitudinal and transverse relaxation two dimensional (2D) maps (2D T1T2app), helped in reducing fluid typing uncertainty in different clusters of one of the recent discovered gas-condensate Cretaceous stacked carbonate reservoirs of Abu Dhabi field. Generally, the reservoir fluid type in gas-condensate reservoir is confirmed by observing a representative fluid sample in laboratory, but at times collecting representative sample becomes challenging especially in tighter formations or unstable wells or rugose holes. The advanced logging techniques, such as Nuclear Magnetic Resonance (NMR) and Downhole Wireline Formation Testers (FT) with pump-out fluid sensors, can be extremely beneficial in resolving fluid types in downhole reservoir condition for such complex fluid regimes and is also applied in this recently discovered carbonate field. NMR continuous T1T2 (simultaneous longitudinal relaxation-T1 & transverse relaxation-T2) logging & 2D fluid characterization methods at closer interval have been very important and useful data to assist in differentiating condensate, gas, and water in each of the reservoir in the static condition. Free gas (methane) occupies a unique location in a 2D T1T2app map with a relatively long T1 and short T2 signature. Dead oils or low GOR oils typically have a low T1T2 ratio and condensates are identified by their fairly large T1T2 ratio. The fluid signatures from the 2D maps are then quantified to compute individual fluid volume for hydrocarbons and water. Reservoir fluid typing from NMR 2D T1T2app not only helped in optimizing formation tester PVT sample points, but was also found in agreement with fluid analysis results from formation tester sensors where reservoir fluids were sampled from a greater distance from wellbore after longer duration of pump-out period. The NMR Fluid typing using 2D-T1T2 app has helped identifying gas-condensate-water fluid types in static condition before sampling or testing. This has helped to understand regional fluid distribution in tighter reservoirs especially where fluid gradient has not helped due to unreliable formation pressure data. By applying NMR fluid typing in difficult gas-condensate fluid regime validated further by fluid sampling and testing, has provided confidence in placing perforation at correct depth for testing, improving regional fluid distribution and bringing timely value to optimize engineering techniques for fast track appraisal & development program.

As a part of Country strategy to fulfill growing demand in gas energy in the country, development of Arab formations started. This paper shares the actual case history of successfully completed evaluation of Arab formation in one of the Company fields. High temperature (300F), high H2S concentrations (up to 37% H2S) and presence of elemental Sulphur dictated usage of very exotic downhole corrosion resistant alloy (CRA) material. Tightness of carbonate formation (less than 1 mD) pushed to drill 10,000’ horizontal section to meet project objectives. High reservoir pressure reaching 5,700 psi in combination with all conditions above resulted in high level of risk during initial project evaluation and required very robust HSE and Integrity systems in place with no chance to failure. Arab formation evaluation project started in 2014 and was performed in several steps, with gradual increase of the complexity of the each next stage: from vertical well to horizontal, from sampling to longer duration testing, from carbon steel completion to fit-for-purpose inhibition and high grade CRA materials. Efficient collaboration among multidisciplinary teams, continuous management support and clear communication channels between all stakeholders proved to be a key to success for this challenging project. The project included involvement of high capacity rig to fulfill extended reach depth (ERD) well requirements, specific well intervention techniques while drilling and testing, usage of special inhibitor for high sour wells and finally inspection of the recovered completion. All Company available best practices and technical competencies applied in this project allowed to overcome all the challenges and achieve appreciable sustainable gas rates, meeting all set objectives with no HSE incidents and failures. Findings and lessons learned received are used to tailor next stage of the project to ensure most efficient scenario of field development to support country strategy in increasing energy potential. The content of the paper gives a good understanding to the readers on the key aspects and main challenges that they may face during initial evaluation stage of the tight gas high pressure high temperature (HPHT) carbonate formations with high H2S content. The document could be used as a local considering current government strategy of massive attraction of new business partners for the exploration of new gas and gas condensate blocks in the region.

In tight Cretaceous carbonates with complex reservoir and fluid typing, Nuclear Magnetic Resonance (NMR) petrophysical solutions has become a one of the integral logging technology to provide reliable and robust solution through systematic acquisition and processing methodology. This paper covers a case study from two carbonate fields of UAE applying innovative NMR log analysis techniques to understand varying complex reservoir porosities, permeabilities, identifying irreducible water, movable hydrocarbon and water in complex tectonic-cum-geological area, and identifying difficult gas-condensate fluid regime, validated by fluid sampling and testing to support optimizing well placement, and bringing confidence for fast track appraisal-cum-development program. The Nuclear Magnetic Resonance (NMR) Logs and its derivatives bring lots of value addition to overcome challenges in reservoir and fluid characterization for tight, complex and heterogeneous carbonate reservoirs. In the early life of exploration and fast appraisal stage of any carbonate field, measuring NMR log data for porosity typing (clay, capillary & free fluid), permeability variations, irreducible water saturation, identifying presence of movable water in unknown fluid contacts and resolving hydrocarbon typing through continuous T1T2 (simultaneous longitudinal relaxation-T1 & transverse relaxation-T2) with two dimensional (2D) fluid characterization to assist in differentiating condensate, gas, and water in reservoir static condition are such useful technical information coming out-off single logging tool to properly access reservoir and field potential. NMR data like porosity, rock matrix density and irreducible water saturation continuous profiling is already filling the gap for routine core analysis and capillary pressure data. These outputs from NMR can be easily integrated with other logs and geoscience data to plan perforation and testing in such challenging reservoirs. Despite NMR having shallower depth of investigation, with proper log acquisition plan, a good NMR data can be acquired and interpreted to get reliable interpretations for rock and fluid characterization, overcoming borehole and its fluid influences. Further all these NMR valuable data can be used for the application of reservoir rock typing in such complex carbonate reservoirs, if required. In today’s lower oil price and cost optimization phase, NMR logging is becoming popular acquisition tool providing most of time efficient petrophysical answers to reservoir and fluid characterization. The NMR log analysis products namely porosities (clay, capillary & free fluid), permeability, irreducible water, and fluid typing in difficult gas-condensate fluid regime validated by fluid sampling and testing has helped in trusting this systematic approach for fast track appraisal-cum-development program in these exploration-cum-fast appraisal carbonate fields. The NMR based log evaluation has been integrated with other logs and geoscience data to bring clarity on reservoir characterization, fluid typing and fluid contact to update the static model accordingly.

ADNOC onshore tested HPHT sour gas reservoirs with 30% H2S, 10% CO2 to evaluate the reservoir and well potential as part of the efforts in supplying additional gas for meeting country's growing energy needs. Developing these massive HPHT sour gas reservoirs is essential for providing a sustainable source of energy for years to come. This critical project serves the broader national strategy and country aspirations in fulfilling the gas demand over the next few decades to come. Few HPHT sour wells were drilled but only one well could be tested successfully. The other two wells had to be suspended due to HSE /environmental and operational reason as elemental Sulphur was detected. Based on the previous well test and reservoir data, it was decided to use one of the existing well and sidetrack in the Sour reservoir to gain experience about drilling a long horizontal section in the High pressure, high temperature sour condition. A specialized drilling Rig capable of drilling the long horizontal well was selected. Due to nature of the reservoir, specialized sour service drilling tools were selected considered the long departure and long open hole horizontal length of 10000+ ft. Selection of the downhole material for these conditions was itself a challenge as very few vendors or IOC (Internatioanl oil companies) have experience of developing and producing from +30% H2S and +10% CO2. Due to the location of the well, stringent HSE measurements were adapter to ensure zero tolerance for the safety violation in accordance with 100% HSE. The testing of the HPHT sour gas was challenging due to not only HSE issues but also from the environment part too as flaring needed to be minimized in the brown field. Hence, it was decided to Tie-in the well to the nearby facilities. The challenge was that the existing facilities were not design to accept the sour gas. This was overcome by blending the sour gas with sweet gas to meet the existing facilities specs and capacities. After the well was drilled, the +10000 ft. open hole was flowed to clean to ensure all the drilling fluid lost was recovered to test to access well potential and obtain representative data for full field development plan. Drilling, testing and producing the highly sour HPHT gas reservoirs with more than 30% H2S and 10% CO2 along with temperature ranging up to 300 deg F is itself a huge challenge. Over the last few years, ADNOC Onshore have developed considerable expertise in testing the sour wells considering all the safety and environmental aspects. This paper highlights the work progress and the lessons learned during each step of the operation from planning phase to drilling, tie-in the well to the existing facilities after dilution during testing. All the proposed mitigation plans considering 100% HSE while dealing with these appraisal wells in the Arab sour reservoir having +30% H2S and 10 % CO2 were developed and implemented sucessfully.

Objective/Scope; ADNOC onshore has overcome yet another challenge in drilling longest horizontal well 10,000 ft in aggressive environment having ±30% H2S and ±10 CO2 with reservoir temperature around ±300°F. This project is part of the country strategy in meeting energy growth in sour gas wells for the coming future development. Methods, Procedure, Process; the strategy is to enhance the well productivity by minimizing the footprint and drilling long horizontal well in harsh environment by achieving maximum accessible reservoir contact. A detailed well design was generated for each zone separately that touched different aspects from the planning phase to the execution and production in safe operating manner. It required an integrated approach bringing together many different technical and operation solution to achieve the drilling of long horizontal well. The well design was reviewed at each step was agreed One the challenges to start drilling Sour Gas exploration well was penetrating multiple high temperature high pressure reservoirs with minimum geosteering to maintain smooth trajectory thru the structure of the reservoir to enhance well accessibility intervention. The objective was achieved by using rig capable to drill long horizontal well and drilling fluid which is compatible with logging tools that contains low salinity and low solids which assists in enhance the efficiency of the tools and achieving the target of drilling 10,000 ft horizontal sour gas well. The torque and drag calculation were reviewed and accordingly the drilling assemblies were selected. The well was completed with specialized material that will withstand the temperature and pressure changes during production and toxic environment having ±30% H2S and ±10 CO2. Moreover, this was also subjected to comprehensive review of HSE rules and regulations including safety and precautions while drilling. Results, Observations, conclusions; Drilling and developing sour gas well with more than ±30% H2S and ±10 CO2 is an accomplishment. ADNOC onshore has studied the opportunity of drilling long horizontal well achieving maximum reservoir contact with the minimum footprintwhich will assist in reducing the cost of the future wells. Over the past years, ADNOC onshore has developed experience in drilling long horizontal wells in sour wells keeping in mind the safety and environmental aspects. A team of professional expects and support is available to achieve the objective safely & efficiently. Novel/Additive Information; Developing sour gas wells has always been challenge due to the sour environment and accessing deeper horizon that require advance theology. Sour gas opens new marketing channels for ADNOC by maximizing the investment opportunities for the future investors and stakeholders. This will open new cost, maximizing the productivity without compromising the safety and allow drilling long horizontal wells in challenging atmosphere. The paper will describe the various issues faced while drilling sour gas well.