The hydraulic fracturing technology is widely applied in tight reservoirs, including shale reservoirs, as one of the established reservoir stimulation methodologies to enhance the productivity. Even though the hydraulic fracturing is currently a common technique, there are remaining challenges in offshore fields with the high degree of geological and geomechanical uncertainties. In offshore hydraulic fracturing operations, key issues are the limited deck space for the required equipment on-board and economical aspects of the surface equipment including stimulation vessels due to the limited number of dedicated offshore stimulation vessels in the world. In addition, the limited reservoir information brings uncertainties in the hydraulic fracture design and causes difficulties in finalizing the operation plans from the timing and logistics point of view.This paper contains the first part of the two successive parts of a case study will be shown on successful optimization and productivity enhancement of actual offshore hydraulic fracturing for a deep tight gas reservoir with considerably limited formation data and under a high-pressure and high-temperature (HP/HT) environment. This successful operation was recognized as a landmark in this region, in terms of the first hydraulic fracturing operation in the offshore Abu Dhabi.In this paper (part 1), we describe how the flexible hydraulic fracture design led to an efficient productivity enhancement. The hydraulic fracture design was optimized by the integrated data acquisition strategy and the successive flexible adjustment from the design stage at office to the actual main treatment at wellsite. The relevant fracture design components like proppant usage and size can be optimized, based on sensitivity studies assuming not only all possible geological and geomechanical circumstances but also the actual pre-frac well test and data-frac results. In part 2, the key factors will be highlighted on this successful hydraulic fracturing result against the difficulties from operational point of view (Al Ameri et al. 2014).The work flow and successful strategy in our hydraulic fracturing design and execution can be applied to other offshore tight-sand gas reservoirs including those under HP/HT condition. The optimized design of hydraulic fracturing provides an effective operation and enables more economical field development for the tight reservoirs.

The first application of hydraulic fracturing in the offshore Abu Dhabi was executed safely and successfully. This achievement will be a valuable foothold to expand the field development target toward more challenging reservoirs such as deep tight sand in this region. There were huge amount of operational difficulties to carry out this hydraulic fracturing due to various operational restrictions, limited data availability and high-pressure & high-temperature (HP/HT) condition. Finally, these difficulties were successfully overcome by an intensive designing study on well completion, surface equipment, and operation associated with hydraulic fracturing for a tight gas reservoir.In this paper (Part 2), the key factors that led this trial to the first successful hydraulic fracturing in the offshore Abu Dhabi are described against the difficulties from operational point of view such as well completion design, arrangement of fracturing and surface testing equipment and acquired lessons learnt. However hydraulic fracturing design optimization from subsurface point of view is discussed in the other paper described by Kuroda, et al. (2014) as Part 1. Well completion design and equipment arrangement were optimized to overcome an extremely wide range of pressure and temperature condition with multi stage fracturing. Fracturing and testing equipment of high specification were arranged on the limited space of both the off-shore jack up rig and the fracturing vessel, and then these well-prepared equipment contributed to safe and accurate operation. Acquired lessons learnt will contribute to other offshore tight-sand gas reservoirs development in the offshore Abu Dhabi. These outcomes will be especially applicable in this region to optimize offshore hydraulic fracturing for tight HP/HT reservoirs in order to enhance the well productivity and enable economical development of marginal fields.

Producing hydrocarbon directly from the early mature source rock is becoming a subject of interest for Abu Dhabi National Oil Company (ADNOC) in recent years. The maturation modeling done in ADNOC for the Cenomanian Shilaif formation indicated the presence of early mature kitchens in the offshore and southern onshore synclines.Another maturation modeling showed that the Oxfordian Diyab Formation is also prospective mainly in the northwestern offshore area of Abu Dhabi.Several conventional stimulation attempts were made in order to produce hydrocarbon from these source rock intervals encouraged by strong hydrocarbon shows while drilling and high hydrocarbon saturation from interpreted logs. Unfortunately, no successful results were obtained during all previous trials.A volumetric calculation showed that the estimated volume of the hydrocarbon available in these new areas is significant and once a successful production method is implemented, an economical hydrocarbon recovery can be obtained.In 2011, ADNOC drilled one well in the onshore area of Abu Dhabi. A secondary objective of the well was to test productivity of Diyab source rock and for the first time in Abu Dhabi a new approach was planned and implemented to produce these source rock intervals.Due to logistical issues, the original testing design was not excuted, and a re-designed program was performed with the available equipments on site to prove the concept of producing hydrocarbon from such formations.Promising results of gas and condensate were obtained and assuming much better results if the well is drilled in the optimum potential location in the most northwestern onshore area of Abu Dhabi.In this paper, we will explain both the geological aspects of the potential Diyab source rocks in Abu Dhabi and the planned program, difficulties, executions and results of the new approached stimulation.