Field presented here is located in the southern part of the Arabian Gulf approximately 135 km north-west of Abu Dhabi city. This giant heterogeneous carbonate field consists of multi-stacked reservoirs. The presented reservoir is highly fractured, it measures 9 km by 11.5 km. The reservoir has an original oil in place estimated at 2,240 MMstb of 35°API oil with saturation gas of 400 SCF/bbl. The reservoir pressure is +/− 2,700 psi and the sealine pressure in the field is +/− 1100 psi. The wells completed in T reservoir are unable to flow naturally against the high sealine pressure. Some wells are producing against by-pass line at 600 psi. Crestal gas injection was introduced to maintain the reservoir pressure. To produce the reservoir at its potential, it is required to use some artificial lift techniques. ESP was finalized to install for overcoming the high sea line pressure. As mentioned earlier the T reservoir is naturally fractured and has crestal gas injection, which lead towards 3,500 – 4,000 SCF/bbl and beyond the ESP limit. This requires some solution to handle the gas. A collaborative team of engineers was assigned to design and meet the challenge of such a premier application. The team conducted a detailed and comprehensive analysis of the T reservoir's fracture and fault characterization: the aim was to deliver an optimal well design meeting the requirement of ESP gas handling with minimum cost. A unique, fit-for-purpose dual completion (4-1/2" × 2-3/8") was finalized. The rigless ESP will be run through 4-1/2" tubing and 2-3/8" tubing will be utilized for gas handling and re-injecting gas in the sealine at surface. The dual completion will allow to handle high GOR through short string, which will lead towards the long ESP runlife. Before commencing the full development plan for T reservoir, this will be a pilot for better understanding the reservoir and its behavior. Rigless ESP was selected due to the advantages compared with conventional ESP: POOH and RIH of retrievable ESP parts through a conventional slickline unitPumps can be replaced during the well life without rig workover.Low OPEX cost.
The giant heterogeneous carbonate field presented here consists of multi-stacked reservoirs and is located in the Arabian Gulf approximately 135 km north-west of Abu Dhabi. The reservoir named "T" measures 9 km by 11.5 km, with large accumulation of 35 °API oil with initial gas oil ratio of about 400 scf/STB. The current reservoir pressure is around 2,700 psi; many of wells are unable to flow naturally against the high sealine pressure, due to low productivity and relatively low GOR. To produce these wells, "artificial lift" or lower sealine pressure are required. A collaborative team of Reservoir Engineers, Petroleum Engineers and Geoscientists was assigned to find a sustainable and cost-effective solution to produce reservoir "T" in order to evaluate its potential. The team conducted a detailed and comprehensive study of the field starting from reservoir "T" and then expanded to the other reservoirs. As a result, the proposal of an "Auto Gas Lift" (AGL) pilot was formulated to use gas from the reservoir "C" (underlying reservoir T) to artificially lift the oil produced from reservoir T. AGL is a cost-effective artificial lift system, directly replacing for conventional gas-lift equipment, gas compression facilities, gas transport pipelines and ancillary equipment. This technique has been identified as the most suitable for such mature offshore field, where existing platforms have limited spare load and space capacity and could not accommodate gas-lift compression facilities or ESP topside equipment. The first pilot completion has been designed. It consists of a perforated downhole high GOR zone from which gas is bled into the tubing at a rate controlled by a downhole gas lift valve. The gas produced from high GOR reservoir C will allow reservoir T to flow by reducing the hydrostatic head of the fluid column in the well. Artificial lift has not been implemented yet in the field. However, several artificial lift techniques, such as Electrical Submersible Pumps or conventional gas lift, are foreseen in long term development plans. "AGL" technique, if successful, could represent a cost-effective solution for further appraisal of this reservoir, without waiting for the implementation of large-scale artificial lift techniques.
Sourceless well logging (logging without using traditional chemical sources) has become more attractive in certain locations because of evolving government regulatory and HSE requirements. LWD sonic tools provide opportunities to operators for both well placement and formation evaluation. For the first time in the UAE, an LWD acoustic tool was added to a bottomhole assembly (BHA) to acquire sourceless porosity measurements to help stay in the porous sub-layers in the target reservoir. In comparison to more traditional nuclear measurements, LWD sonic has a superior depth of investigation that is less affected by standoff and a driller-friendly BHA free from stabilizers. Based on the evaluation of offset data, which indicated excellent correlation of sonic vs. density/neutron measurements, we decided to provide shear porosity in real time owing to the sequence stratigraphy and corresponding energy partition. A post-job comparison with density/neutron data acquired in a wipe run was also conducted to verify the sensitivity of real-time acoustic porosity measurements for both well-placement and formation-evaluation purposes. Sonic-derived porosities were found to be instrumental in the real-time decision making needed to keep the well the in higher porous sub-layers. Current developments, including real-time azimuthal sonic together with considerations of integrating acoustic and NMR measurements in both petrophysical modeling and field applications, show promise in providing reliable sourceless porosity estimation in these formations. This case history delivered a BHA free from radioactive chemical sources. Safe drilling objectives as well as maximized productivity per unit lateral length were achieved despite the potential risks associated with the faults that were observed in the pilot hole. Introduction With a global focus on exploring oil and gas with minimum environmental impact, the regulations in the region are very strict on adhering to zero harm to the environment. One of the most stringent laws in place is with regards to the abandonment of radioactive sources in the well in the event of a stuck pipe. ADMA OPCO is an active player in helping service companies develop and improve on their “sourceless” alternative technologies to reduce potential risks associated with stuck pipe. Real-time acoustic measurements were used for well-placement purposes in Field A, which is one of the giant fields located in offshore Abu Dhabi. Oil was discovered in 1958, and production began in 1962. Down-flank water injection started in 1973 followed by crestal gas injection in 1994. The Arab reservoir in Field A was formed from regressive cycles of sedimentation divided into four highly heterogeneous sub-reservoirs (labelled in ascending order from A to D (Fig. 1)). The principal oil-producing reservoirs are Arab zone C and Arab zone D, whereas Arab zones A and B still remain undeveloped. Based on the core description, the Arab reservoir section is mainly composed of three lithologies, namely, anhydrite (purple shading), dolomite (green), and limestone (light blue), as shown below in Fig. 1.
The value of the continuing integration of logging-while-drilling (LWD) and directional drilling processes has been more prominent in the current economic environment in terms of optimizing field development costs by means of precise well placement, as well as improved reservoir characterization and drilling performance in real time. A successful horizontal drain was drilled in an undeveloped Reservoir A for the first time in an offshore carbonate sequence, using advanced LWD acoustic and high-resolution microresistivity sensors. The well plan required maximizing the exposure of the most porous body in a thinner sublayer. This sublayer lies directly over a large, developed carbonate reservoir as part of the Upper Jurassic Carbonate sequence located offshore Abu Dhabi. The flow test results during the drillstem test (DST) operation for the first appraisal well in the target reservoir produced at a rate that was greater than expected. Production log data were acquired and integrated with the LWD microresistivity image interpretation. In addition, in this environment, the inferred Rt and Rxo measurements from the LWD azimuthal focused resistivity tool were shown to be more reliable than conventional electromagnetic wave resistivity measurements, which are prone to exhibiting significant polarization, anisotropy, and bed boundary effects. Lessons learned from the first appraisal well in Reservoir A for reservoir characterization and flow unit identification were used and implemented in the planning and successful delivery of the future horizontal wells. Unlike the other reservoir subunits that are deposited within the same sequence, the field development strategy for these undeveloped reservoirs has been under review based on the recent data. The field development strategy used enhancements in well placement, formation evaluation, and production technologies, including extended reach horizontal wells, with maximized reservoir exposure in the sweetest zones, to compensate for the poor petrophysical character and low oil mobility. This case study presents insights into the advanced geosteering and multidisciplinary reservoir characterization processes along these successful horizontal drains drilled in undeveloped Reservoir A and the future horizontal wells. It also demonstrates the integration between the geological and petrophysical interpretation and the use of acoustic measurements and high-resolution microresistivity imaging. This combination has enhanced the understanding of Reservoir A in terms of the unexpected production performance and helped optimize efforts for the future field development plan.
The lower cretaceous carbonate sequence, offshore Abu Dhabi is represented by the third to forth order sequences. Limestone is the dominant lithology for this group of ramp to intrashelf basin sediments. Fracture intensity and density vary vertically along the sequences, controlled by rock texture contrast. Dense layers are heavily fractured compared to the porous bodies throughout these Formations. Two dominant sets of fractures are observed throughout the field, NW-SE and NNE-SSW. Historical well test data indicate strong preferential flow in that same direction compared to less flow in the NW-SE trend (anisotropic drainage behavior). The objective of this study is to demonstrate the capabilities of simultaneously acquired near and far field borehole sonic reflection logging measurements to characterize the present fractures along a dedicated horizontal drain for data gathering. Borehole image log interpretation and other well logs are integrated. Understanding fracture systems using resistivity imaging solely could be challenging due to the limited depth of investigation of the measurement (at the well location). Well trajectory, (open) fracture density and orientation can cause uncertainties in the number of fractures that intersect the borehole. Primary fractures could be abundant away from the borehole but still contributing to flow and reservoir pressure behavior. With a unique extended depth of investigation as well as azimuthal sensitivity, dipole sonic imaging is able to reach tens of meters into the formation and provide fracture intensity and extension information in the far field. A new scale of data integration using near field measurements from monopole sonic imaging, Stoneley wave reflectivity analysis and borehole image interpretation for a comprehensive fractures characterization is accomplished. A set of structural incidents could be detected tens of feet away from the borehole, some seemed to be extending towards the borehole wall itself as seen by the Stoneley reflectivity and the sonic-resistivity borehole imagers. Open fractures are clearly characterized in terms of orientation and aperture, extension inside the reservoir could be recognized, small-scale fractures near the borehole could be discriminated, as well as the closed ones, in addition to the dense stylolite markers. Comparisons with offset cores, seismic and offset well data shows a range of coherence. Most of the fracture clusters were observed at the stylolite boundaries. The main orientation of these fractures are consistent with the present day in-situ stress orientation. The integration of data with respect to resistivity, sonic borehole image and Stoneley wave data from sonic monopole processing are in coherence. Far-field dipole shear sonic imaging adds valuable information to investigate the major carbonate reservoir structural incidents away from the borehole. The value is maximized by integration with the high-resolution borehole image that drew some conclusions on the presence of different sets of fractures distribution and their nature.
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