A recent series of tight gas discoveries in the Amin format ion of the greater Fahud area represents some of the most exciting exploration success of this decade in the Sultanate of Oman. The structures have been evaluated as containing very significant amounts of gas locked in a challenging deep and hot environment requiring hydraulic fracture stimulation. Recently, horizontal well trials started taking place in two of the structures aiming for testing efficiency of this type of completion and further evaluation of formation deliverability. Successful completion of horizontal laterals would open new horizons in this challenging environment. Achieving this goal is not possible without thorough evaluation of reservoir conditions followed by completion and stimulation. Horizontal well performance in a tight gas reservoir is largely controlled by the number of hydraulic fractures placed along the lateral and their spacing and conductivity. Designing a reservoir access strategy might not be a trivial task, either, when the well trajectory intersects several productive vertical layers and the reservoir properties are changing laterally. Manual selection of intervals and perforations could be susceptible to mistakes and may be perceived as subjective at times, while also being time and effort consuming. The workflow based on reservoir quality (RQ) and completion quality (CQ) developed in North America for unconventional resources for optimizing completion decisions brings engineering to this process for stage and cluster selection in horizontal sections. This project applies the same reservoir-centric RQ/CQ workflow integrating all available data and creating specific criteria and cutoffs applicable to a specific tight gas field in the Sultanate of Oman.
Innovation and advances in technology have enabled the industry to exploit lower-permeability and more-complex reservoirs around the world. Approaches such as horizontal drilling and multistage hydraulic fracturing have expanded the envelope for economic viability. However, along with enabling economic viability in new basins come new challenges. Such is the case in the Middle East and North Africa regions, where basin complexity arising from tectonics and complicated geology is creating a difficult geomechanical environment that is impacting the success of hydraulic fracturing operations in tight reservoirs and unconventional resources. The impact has been significant, including the inability to initiate hydraulic fractures, fracture placement issues, fracture connectivity limitations, casing deformation problems, and production impairment challenges. Completion quality (CQ) relates to the ability to generate the required hydraulic fracture surface area and sustained fracture conductivity that will permit hydrocarbon flow from the formation to the wellbore at economic rates. It groups parameters related to the in-situ state of stress (including ordering, orientation, and amount of anisotropy), elastic properties (e.g., Young's modulus and Poisson's ratio), pore pressure, and the presence of natural fractures and faults. Collectively, this group of properties impacts many key aspects determining the geometry of the fracture, particularly lateral extent and vertical containment. Heterogeneity in CQ often necessitates customizing well placement and completion designs based on regional or local variability. This customization is particularly important to address local heterogeneity in the stress state and horizontal features in the rock fabric (e.g., laminations, weak interfaces, and natural fractures) that have been identified as key contributors impacting the success of hydraulic fracture treatments. Given the observation that a wide range of CQ heterogeneity was creating a complex impact on hydraulic fracture performance, CQ classes were introduced to characterize the risk of developing hydraulic fracture complexity in the horizontal plane and the associated impact on well delivery and production performance. They indicate the expected hydraulic fracture geometry at a given location and are analyzed in the context of a wellbore trajectory in a given local stress state. CQ class 1 denotes locations where conditions lead to the formation of vertical hydraulic fractures, CQ class 2 denotes locations where conditions lead to the formation of a T-shaped or twist/turn in the hydraulic fracture, and CQ class 3 denotes locations where conditions lead to the formation of hydraulic fracture with predominantly horizontal components. Wellbore measurements indicate that these CQ classes can vary along the length of the wellbore, and 3D geomechanical studies indicate that they can vary spatially across a basin. By understanding this variability in CQ class, well placement and completion design strategies can be optimized to overcome reservoirheterogeneity and enable successful hydraulic fracturing in more challenging environments. This paper introduces the novel concept of CQ class to characterize basin complexity; shows examples of CQ class variability from around the world; and provides integrated drilling, completion, and stimulation strategies to mitigate the risks to hydraulic fracturing operations and optimize production performance.
Exploration drilling indicated that the Natih B formation in Oman may have a significant amount of oil locked in a challenging low-permeability, laminated, carbonate reservoir. The first completed vertical wells suggested that stimulation treatments were required for economic production. The operator applied acid fracturing stimulation techniques to establish baseline production. To further improve well performance, proppant fracturing stimulations were scheduled for several vertical and horizontal wells. Mechanical earth models were constructed to help improve understanding of fracture geometry. The earth models revealed stress inversion between layers, differentiating stress regimes into strike-slip and thrust. Areas of different stresses within the same formation would lead to different hydraulic fracture shapes (e.g., regular planar fractures in the case of strike-slip mode or horizontal fractures in the case of thrust mode). Predicting these geometries would impact the fracture design and completion strategies. In addition, the pay zone was bounded by water-bearing zones at the top and bottom, introducing a risk of water production if any of the wells exhibited excessive fracture-height growth. Several measures helped survey fracture-height extension during initial diagnostic treatments. In the formation's first horizontal well, uncertain stress regimes and lateral unconformities, such as natural fractures, complicated the hydraulic fracturing treatments. To overcome these uncertainties and optimize stimulation, we needed to accurately model natural and hydraulic fracture interaction. A mechanical earth model from vertical wells in the vicinity, and the horizontal wellbore itself, were combined with natural fracture network data and the staging and perforation design.
Interpretation of logs from an exploration pilot well and a lateral drilled from the pilot in the Late Cretaceous Natih formation in the Sultanate of Oman was used for designing a multistage hydraulic fracturing treatment. A high-tier logging suite including borehole image, advanced dipole sonic, geochemical, and triple combo data was acquired in both wellbores. The objective of the pilot hole was to select the best landing point in terms of reservoir quality (RQ) and completion quality (CQ) so that a horizontal well could be drilled and multistage stimulations performed in the organic-rich Natih B source rock. In contrast to much of North America, significant tectonic forces are frequently present in this region. The geomechanical setting might thus strongly affect hydraulic fracture initiation, propagation and proppant placement. It therefore plays an important role in lateral landing point selection. Borehole images, integrated with petrophysical and geomechanical log properties, were used to identify the optimum landing zone. Breakouts as well as longitudinal and transverse drilling-induced fractures were identified on the pilot borehole images over the Natih Formation, indicating a large horizontal stress anisotropy and a compressional tectonic setting. An interval from which vertical hydraulic fractures would initiate at low initiation pressure and grow vertically to contact intervals with good RQ was selected as the target lateral landing point. Image and dipole sonic data were acquired in the horizontal well, and both longitudinal and transverse induced fractures were identified. Those data were used to selectively place hydraulic fracturing stages. Diagnostic injection tests on each stimulation treatment confirmed low fracture initiation pressures and the creation of vertical hydraulic fractures, thus validating the selection of both the landing point and the location of the hydraulic fracture initiation points. All treatments were successfully placed to completion. This paper demonstrates that a workflow based on the combination of image and dipole sonic logs in both a pilot well and a lateral drilled from the pilot enables the creation of vertical hydraulic fractures at moderately low initiation pressures and successful placement of stimulation treatments in the lateral. This technique shows promise for effective hydraulic fracturing in regions where significant tectonic forces are present.
An integrated fracturing services (IFS) project between Petroleum Development Oman (PDO) and Schlumberger started in November 2009 and spanned a 7-year period until November 2016. It steered the local fracturing industry from standalone operations to an integrated package, offering collaborative services between hydraulic fracturing, coiled tubing, and well testing services. The synergies between the various services allowed for a more streamlined decision-making process between the operator and the service company. Ultimately, this led to faster well delivery times, lower non-productive time, and maximum utilization of personnel. The integrated approach was primarily based on communal resource pooling which was available for any of the services on location at any given time. At its prime, the IFS project comprised of 4 fleets deploying almost 300 employees across the Sultanate of Oman's northern and southern fields. The coordination process was centralized and handled by a dedicated team in charge of logistics, transportation, personnel, and resources. This centralization process was essential for the IFS workflow, and it was primarily managed by the service company in collaboration with the operator's well engineering team. Tangible results were achieved, and the IFS model added a new dimension to the fracturing industry in the Sultanate. It allowed the operator to align its well development operations with its production goals and delivery times. It also provided the service company with an opportunity to bridge operational and communication gaps, which would have been harder to achieve if other vendors were involved. These high-level results were coupled with operational successes by the fracturing, coiled tubing, and well testing crews. The novelty of this project is in the dynamism of all available communication channels, as well as the elimination of operational lag due to the reduced number of vendors in any given location. The success of the IFS program makes it PDO's first-choice fracturing model, especially during the current downturn when lowering operational costs is of utmost importance.
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