The Maari Field in the offshore Taranaki Basin, New Zealand was discovered in 1983 by the Moki-1 exploration well (figure 1). Appraisal activities and development studies undertaken by various oil companies over the subsequent two decades failed to identify a viable development concept with an acceptable risk profile. Key factors delaying development were distance from existing infrastructure, field size in association with oil price and critically the high wax / low pour point nature of the oil. The Maari Joint Venture identified a strategy to develop the field and produce the previously stranded oil by means of innovative technology applications.The self-installing wellhead platform and the FPSO were installed in April and May 2008, respectively but the jack-up rig to drill the five horizontal production and three deviated water injection wells was delayed by three months due to winter weather. First Oil from the field was on 25 th February 2009, a milestone achieved some 25 years after its discovery.A number of new technologies and pioneering applications were put into practice with the Maari field development:• The self installing wellhead platform. At the time Maari WHP was the largest self-installing platform of its kind.• The production wells have a thermal design with both passive (thermal gel and thermal cement in the annuli) and active (electric down hole heating system) elements. • Drilling while casing.• The electric submersible pump assemblies were designed to handle free gas conditions down hole in the horizontal section due to the reservoir fluid being close to its bubble point. • The injection water is heated to reservoir temperature to avoid wax precipitation in the reservoir and injected above the 'fracture pressure'.Two smaller near field satellite oil accumulations were also drilled and completed, including a ~8,000m horizontal ERD production well targeting a deeper reservoir in separate structure to the south of Maari. Numerous challenges, such as scale, hydrogen sulphide, corrosion, completion designs and wax, have been encountered and largely overcome during the production phase.The paper will detail the innovations, new technologies, major learnings and experiences from the development concepts to production of this originally 'stranded' asset.

The offshore Field-A Gas Development Project is an integral part of the Abu Dhabi Deep Gas Project targeting untapped hydrocarbon reserves with priority given to gas reservoirs to satisfy an increasing gas demand and sustain economic growth of Abu Dhabi Emirate. Well test data from Field-A Khuff Formation cannot be modeled solely using a matrix based porosity system and requires the presence of fracture network to give a "mixed" porosity system to explain the observed flow test results. To assess this a subsurface study was initiated to incorporate fractures within the matrix porosity based 3D reservoir model and to examine different methods of populating fracture distributions across the geologic model. The ultimate aim of this work was to more accurately predict gas rates to better understand and mitigate rate uncertainties across a range of development scenarios.

Accurately placing a horizontal appraisal well within an interbedded reservoir sequence presents a wide range of challenges especially when there is a lack of nearby control wells. These challenges relate to uncertainties in the formation (dip, reservoir continuity & porosity development) and reservoir fluids (contact depth, transition zone height). In order to achieve the appraisal objectives it was critical to successfully intersect certain zones within the reservoir sequence and ascertain their hydrocarbon flow potential along with quantifying key reservoir properties and fluid boundaries. This data was essential for defining and optimizing the subsurface components of field development planning including well count, expected flow rates and in- place / recoverable resource estimations. In this particular application the target reservoirs are porous gas saturated carbonates developed within an interbedded Jurassic aged limestone. Well placement in the subject well had the primary objective of intersecting five HC bearing zones while maintaining a safe distance from a conductive zone interpreted to be water saturated. As part of the pre-well planning, 3D real-time multiwell reservoir modelling and its updating capabilities with appropriate LWD measurements for Proactive Geosteering and Formation Evaluation was planned. Based on forward response model from offset well data along with drilling engineering and data acquisition requirements, an LWD suite consisting of RSS, Gamma Ray Image, High Resolution Resistivity Image (Fracture and Fault identification), Neutron, Density and 16 sector Density image along with a Deep Azimuthal Resistivity measurement for early detection and avoidance of conductive/water zones was utilized. This tool is capable of early detection of conductive zones that could indicate either transition zone saturations or water saturated porosity beneath a gas-water contact (GWC). Application of the Azimuthal Resistivity measurements along with the realtime updates of the subsurface model helped place the appraisal well within the hydrocarbon column and also established the top of the low resistivity "Wet Zone". Importantly, these results were later confirmed with production logs acquired as part of well testing operations.

The Permo-Triassic Formation of Khuff in offshore Abu Dhabi has shown a considerable potential as a tight gas reservoir, but the complex mineralogy and the heterogeneity has made formation evaluation challenging for this reservoir, particularly in respect to saturation. Complexities with log interpretation in these tight formations have called for the use the advanced measurements to help interpret more basic logs in a simple but accurate and consistent manner. Deriving porosity from density requires the knowledge of matrix density. In complex Khuff reservoir, with a varying mix of dolomite and anhydrite with some calcite, it is impossible to put a constant matrix density. Photoelectric factor is not enough to resolve the mineralogy and advanced spectroscopy measurement is mandatory to provide matrix properties such as grain density, neutron (thermal and epithermal) and sigma (capture cross-section). These properties are used to compensate the formation density, neutron and sigma for the matrix effect, focusing any residual separation between the corrected porosities onto fluid effects. Matrix-corrected density, neutron and sigma porosity curves can be used effectively to qualitatively interpret presence of gas, which is a big challenge in these tight gas reservoirs and this approach is demonstrated on multiple datasets. Since this is being applied on wireline logs with a depth of investigation shallower than 10-in, uncertainties remain concerning the deep, uninvaded zone water saturation. The deep resistivity measurement is still the only access to deep saturation. To quantify the water saturation from resistivity, besides the knowledge of formation water salinity, the other key information is the Archie parameter m (cementation exponent), which is directly dependent on the texture of the rock. Complex diagenesis on the varying mineralogy of the Khuff has created complex water-phase tortuosity, which needs to be considered. Multi-frequency dielectric measurement in addition to giving a shallow zone water saturation, also provides a water-phase tortuosity parameter, which is then used to improve deep water saturation computation. When facing a complex reservoir, the temptation is high to jump to complex evaluation workflows, neglecting simple, quick-look-type approaches. However, advanced measurements, such as spectroscopy and dielectric dispersion empower this type of simple approach, enabling efficient controls and insights on the evaluation, as will be shown through the examples in this paper. Final evaluation still requires a fully integrated analysis, but it will benefit from the lesson learnt and insights from the quick-look.

The Khuff Formation is a Permo-Triassic aged carbonate unit which reservoirs a highly economic gas resource in several countries within the Middle East. The appraisal and development of Khuff Formation tight gas resources is the subject of increased focus in the offshore UAE. This case study focuses on the appraisal of a particular field in offshore Abu Dhabi and summarizes how the understanding of this complex reservoir has evolved over time. The oldest well penetrating the Khuff Formation in this field was drilled almost 3 decades ago. This well tested gas within the Upper Khuff however appraisal of this resource had to wait until 2017-18 when two appraisal wells were drilled on the discovery. These appraisal wells included a complete suite of wireline logs, image log data, formation pressure measurements and well tests to give a clearer picture of the formation and fluid saturations. Subsequent to the drilling of the recent appraisal wells an integrated study was completed integrating all the processed and advanced answer products in order to determine the key elements controlling gas productivity. This knowledge were subsequently applied to optimise a well drilled and production tested in late 2018. Understanding the production behavior of the Khuff Formation reservoirs intervals has been one of the most critical factors behind the decision to develop this complex reservoir. Certain key answer products are considered critical for identification of completion intervals. These products include; sonic imaging (looking for fractures away from the wellbore), advanced textural analysis from borehole images (porosity classification) and critically stressed fracture analysis from geomechanics. This study led to the conclusion that critically stressed fractures and/or connected pores from images are the best indicators of high gas flow potential, while this flow can become exponentially higher when fractures at the wellbore connect to fractures away from the wellbore. This workflow has now been applied to the most recently drilled well and to other Khuff Formation appraisal projects across the off shore of Abu Dhabi. This is an illustration of how in-depth analysis of all the acquired data in an integrated manner can help in understanding a complex reservoir and lead to better decision-making for the future wells and offset appraisal projects. Lessons are hidden in both success and failure and as long as these lessons are analyzed properly, they can lead to long-term success.