Carbonates exhibits diverse flow characteristics at pore scale. Petrographic study reveals micro-level heterogeneities. Thin sections are key to assess reservoir quality although these are images and interpretations in text format. Thin section microscopic analysis is descriptive and subjective. To an extent, optical point counting is routinely used quantitatively to estimate porosity, cement, and granular features. Overall, thin section descriptions require specialist human skill and an extensive effort, as it is repetitive and time consuming. Thus, a manual process limits the overall progress of rock quality assessment. There is no recognized method to handle thin sections for direct input with conventional core data due to its image and descriptive nature of data. An automated image processing is one of the emerging concepts designed in this paper to batch process thin sections for digital reservoir descriptions and cross correlating the results with conventional core analysis data. Thin section images are photomicrographs under plane polarized light. Initially, denoise and image enhancement techniques were implemented to preserve elemental boundaries. Computational algorithms mainly, multilevel thresholding and pixel intensity clustering algorithms were programmed to segment images for extracting elements from segmented regions. The extracted elements were compared with original image for labeling. The labeled elements are interpreted for geological elements such as matrix, pores, cement, and other granular content. The interpreted geological elements are then measured for their physical properties like area, equivalent diameter, perimeter, solidity, eccentricity, and entropy. 2D-Porosity, polymodal pore size distribution, mean pore size, cement and granular contents were then derived for each thin section image. The estimated properties were compared with conventional core after calibrating with laboratory NMR data. The whole process is automated in a batch process for a specific reservoir type and computational cost is analyzed for optimization. 2D-porosity is in excellent agreement with core porosity, thus reducing uncertainty that arises from visual estimations. Scale related issues were highlighted between 2D porosity and core porosity for some samples. Polymodal pore size distributions are in good correlation with NMR T2 distribution compared to MICP distributions. The correlation coefficient was understood to be equivalent to surface relaxivity. A digital dataset consisting of 2D porosity, eccentricity, entropy, mean pore size, cement and grain contents is automatically extracted in csv format. The digital dataset, which was previously in text format in conventional analysis, is now a rich quantitative dataset. This paper demonstrated a unique and customized solution to extract digital reservoir descriptions for geoscience applications. This significantly reduced the subjectivity in visual descriptions. The solution presented is scalable to large number of samples with significant reduction in turnaround and effort compared to conventional techniques. Additional merit is that the result from this method has direct correlation to conventional core data for improving rock typing workflows. This paper presents a novel means to use thin section images directly in digital format in geoscience applications.

To develop a mature onshore carbonate field in Abu Dhabi and reduce the footprint and cost, an artificial island has been built in shallow water that can accommodate drilling rigs and extended-reach wells. This paper presents a case study of the longest onshore well drilled in Abu Dhabi. Planning to drill such a deep well starts long before execution, using offset well data and extended-reach drilling (ERD) engineering. There were formation and reservoir challenges due to the uncertainty in the earth model in the horizontal section of the well. Hence, it was very challenging to maintain contact with the thin reservoir intervals, without approaching the boundaries. In addition, the limited power available to drive the drillstring and maintain circulation drove the ERD engineering team to find optimum solutions, including drillstring and bottomhole assembly (BHA) design. Furthermore, there was a known risk of differential sticking, which meant that the use of radioactive sources in the BHA was undesirable. The well was planned to be drilled in two runs, using nuclear measurements in the first run and non-nuclear measurements in the second. A well-placement methodology and workflow was developed and integrated with the geological understanding of the target layer. Analysis of offset horizontal wells resulted in the delivery of an optimized BHA design, including careful selection of logging-while-drilling (LWD) technologies, to mitigate the geological challenges. The BHA also included a new generation of intelligent, fully rotating, high-dogleg, push-the-bit rotary-steerable system, to geosteer the well in the thin target layer while maintaining the planned target trajectory with minimum borehole tortuosity by means of real-time drilling optimization. The extended-reach horizontal section was drilled successfully, and the geosteering objectives were achieved with 100% reservoir contact over a 20,000-ft interval, targeting a thin carbonate layer and overcoming the complex geological environment. The well was drilled to a record depth of 32,300 ft. The new intelligent rotary steerable system with automatic cruise control helped to eliminate any well-profile issues, minimize wellbore tortuosity, and maintain aggressive drilling parameters. The nuclear and non-nuclear LWD measurements, including NMR, helped to reinforce understanding of the reservoir properties along the entire section. This success has opened the door for drilling more challenging wells. In addition, it has proved that proper planning and execution can shift the boundaries further and gave confidence to drill even deeper.

Recently multilaterals wells are drilled in selective reservoirs in ADNOC Onshore fields for development of hydrocarbon gas in Abu Dhabi. The targets are commonly thin multilayered carbonate reservoirs. The development strategy of multilaterals has significantly enhanced the reservoir contact aiding higher drainage. Multilateral wells can significantly reduce well counts while still achieving robust recoveries. Benefits of using multilaterals included the acceleration of gas production and the reduction in CAPEX/OPEX . The reservoirs are targeted by drilling duel and triple lateral holes. In triple lateral wells, a main horizontal borehole is drilled followed by laterals on either side of the main borehole. There are challenges of drilling multilaterals as the multilayered reservoir subunits are thin and selective drainage is planned in each sub layers in 6" horizontal sections. It is imperative to have a detail well planning with industry standard drilling technology to deliver successful multilaterals. During the well planning stage the laterals are planned in 60-80deg azimuthally apart from main both, on either side. The laterals drilled by performing open-hole sidetrack from the main bore, which apparently has a challenge of success. During initial stage of the campaign, lessons learnt from applied practices has encouraged an innovative way for successful multilateral drilling. Wells are landed on top of the target reservoir. 6" horizontal main bore hole is drilled with creating three humps in the initial part of the well trajectory. The humps are high inclination short section of well path with high Dog Leg Severity. These humps are strategically created at selected points with 300- 500 ft gap from each other and are used to kick off open-hole sidetrack of the lateral holes. Recorded real time inclination (RTI), Image log data and porosity of main bore along the humps are the key factors considered before executing the side track. MWD-LWD triple combo along with image log are used in the main bore and tools without radioactive source are used while drilling lateral holes to offset the risk of stuck-up of BHA carrying radioactive source tool. A detailed step wise operational procedure has been identified and introduced for the success of this development strategy. The high-confidence, successful open hole sidetracking strategy has aided maximum reservoir contact in 6" section with minimum risk and rig time and has substantially contributed to offset additional cost and rig time in all the multilateral wells in ongoing gas development drilling. This multilateral drilling and field development strategy has been a combined effort of geoscience and directional drilling and have paved way for successful open hole sidetrack campaign with proven standard procedures.

Hydraulic fracturing is the industry-proven technology for efficient exploitation of tight reservoirs. This study evaluated the technology by drilling horizontal wells through acid fracturing the low permeability gas reservoirs of a lower cretaceous carbonate formation, located onshore Abu Dhabi. 1D Geomechanical modeling was critical in determining fracturing strategy. Containment of fracture height growth was the most critical aspect of the fracturing process, as the target units were stacked within heavily depleted reservoirs. The challenge was that the rock strength, poroelastic behavior and stress paths vary significantly through the various formations overlying/underlying to target reservoirs. In addition, the depositional nature of the target reservoirs meant inherent variations in rock mechanical properties. Hence, a continuous profile of in-situ stresses and other rock mechanical properties was essentially mandatory for addressing the challenges for optimal placement of the horizontal drain hole and assessing fracture height growth. Existing rock mechanical measurements, dipole sonic and image logs, as well as in-situ stress measurements were reviewed for 400ft (TVD) interval above and below the target reservoirs. Significant data gathering was managed in the vertical pilot hole of the first well to fill data gaps. A custom-built workflow was performed for 1D Geomechanical modeling in the first well, integrating poroelastic modelling with failure models. The study integrated drilling-induced and production-induced Geomechanical aspects into the 1D stress profile, including depletion-induced poroelasticity, shear anisotropy, and rock mechanical heterogeneity. Wellbore failures observed in vertical pilot hole and the in-situ stress measurements from offset wells were used for validating 1D Geomechanical model. This work resulted in a rigorous 1D-stress profile that contributed to initial fracture modelling, whilst de-risking the fracture height growth into the depleted reservoirs and optimizing the choice of drain hole location within sub-units of the target reservoirs. The fracture gradient and breakdown pressures derived from 1D stress profile were found to be in very close agreement with that measured from the minifrac conducted in vertical pilot hole of the first well. This paper presents a purpose-driven workflow designed specifically for these circumstances. The merit of the workflow lies in its systematic and methodical approach, providing solutions to various Geomechanical problems relevant to the target reservoirs, as well as the depleted reservoirs above and below. The results are beneficial for analogous fields where hydraulic fracturing is required to improve recovery of low permeability reservoirs in mature fields

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.