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Most oil fields worldwide are stored in carbonate rocks, which vary in rock and fluid properties. Heterogeneity, thickness, and depth play vital roles in the selection of specific acid stimulation to improve the productivity and injectivity of the wells from near-wellbore damaged formation resulting from overbalance drilling. This paper describes the integrated workflow that guides petroleum engineers in the selection of optimum acid stimulation techniques and technologies for a specific field.Specifically, this work discusses the stimulation strategies in the form of a stimulation workflow that considers various aspects to achieve the stimulation objectives. The stimulation objectives are also streamlined for the well objectives, which mainly consider the well completion approach. Considering the well completion and well objective, the stimulation strategies highlight the conveyance selections, fluid selections, and the methodologies. Guidelines are provided as examples for fluid specification. Some comparisons of the stimulation fluids are also shown to consider the advantages and disadvantages of each application. Example simulation and case studies show various applications in UAE.Numerous new technologies and different acid types have been tested worldwide on different carbonate rocks whereas very limited information exists on a tailored acid stimulation workflow that can be fit to the needs of a specific field. The developed workflow is an integrated holistic guidance based on worldwide best practice of acid stimulation and can facilitate and guide technical expertise for production and cost optimization in different oil and gas fields. The latest stimulation technologies are highlighted for the reference on the case studies.

The United Arab Emirates oil and gas reservoirs are continuously intersected with a growing number of horizontal wells and longer drains at varying bottomhole static temperatures. This results in a variety of naturally flowing and more challenging wells where stimulation is required for sustainable flow. Hence it became important to not only rely on plain acid systems for production gain, but to also include more sophisticated acid stimulation systems that can provide improved results in more challenging environments where plain acid may be found lacking. These results were recently achieved via the introduction of single-phase retarded acid (SPRA) as well as viscoelastic diverting acid (VEDA) in inactive wells offshore. The application of SPRA and VEDA was subsequent to extensive laboratory testing including core flow tests, solubility tests, and emulsion tendency testing to the performance of these blends against existing acid recipes such as plain HCl and polymer-based diverting acid. These tests proved that a combination of SPRA and VEDA would allow maximizing lateral coverage in heterogenous reservoirs due to the chemical diversion capabilities from thief zones without imposing further damage that polymer-based diverted acids may cause. The combined SPRA and VEDA would also enhance acid wormhole penetration due to the reduced rate of reaction caused by acid retardation. Such tests were supported with software simulations that provided acid dosage, pumping rate, and pumping method sensitives. Proposing SPRA and VEDA at higher pumping rates enabled the delivery of previously unattainable production influx at sustainable wellhead pressures. In addition, 28% acid content typically used for dolomitic reservoirs was considered unnecessary as 20% retarded acid proved sufficient in such environments. This allowed bullheading treatments, which was previously not possible due to the restriction on pumping 28% acid content across wellheads to avoid causing corrosive damage. Other treatment parameters such as volumes, rates, and acid/diverter sequence and ratio were also adjusted for optimal wormhole penetration across all zones using a fit-for-purpose carbonate matrix acidizing modeling software. The success of SPRA and VEDA was clear in post-treatment evaluation for the cases of previously shut-in wells. These wells were able to produce sustainably at the required tubinghead pressure (production line pressure) after unsuccessful attempts to flow prior to stimulation. The novelty of this paper is the assessment between legacy carbonate stimulation results in UAE using plain HCl acid and polymer-based diverting acid (PDA) and using SPRA and VEDA in shut-in or inactive wells. It also highlights the game-changing solutions that suit the increasing challenges observed in offshore inactive wells including well placement, lithology, bottomhole static temperature, and permeability contrast.

Globally, dolomite formations are important reservoirs for oil and gas. Acid stimulation is commonly used to extend the life of carbonate reservoirs, and a good understanding of the fluid performance is essential for effective treatment design. Three acids, hydrochloric acid (HCl), emulsified HCl, and a single-phase retarded acid based on HCl, were assessed for their ability to create wormholes in Silurian dolomite under laboratory conditions using a standard core flow experiment. Select cores were imaged by X-ray computed tomography to visualize the wormhole morphology. Similar experiments in Indiana limestone was used as a control. The core flow experiments showed that the pore volume to break-through (PVbt) values for the retarded acids in Indiana limestone were less sensitive to changes in temperature overall than unmodified HCl. For Silurian dolomite though, the opposite is observed. HCl has uniformly high PVbt values at lower (200 °F) and higher (325 °F). The emulsified acid and the single-phase retarded acid are more efficient than HCl, but the difference is smaller at 325 °F. Core images revealed that all three fluids had some degree of wormhole branching at 200 °F and much less branching at 325 °F. By visual inspection, the single-phase retarded acid has less ramification than HCl and the emulsified acid. Overall, the results show that retarded acids should make effective stimulation fluids for dolomite reservoirs.

With the expanding offshore rig activity in United Arab Emirates leading to an increased number of horizontal wells and longer drains, coupled with changing rock fabrics, it became imperative to diverge from the existing stimulation methods to deliver more consistent and reliable results. These results were achieved via the introduction of single-phase retarded acid (SPRA) and viscoelastic diverting acid (VEDA) to both active and shut-in wells offshore. The introduction of SPRA and VEDA was possible after extensive laboratory testing including core flow tests, solubility tests, emulsion tendency testing, and corrosion inhibition tests to evaluate and benchmark the performance of these blends in comparison to the existing acid recipes such as plain HCl and polymer-based diverted acid. These tests showed that a combination of SPRA and VEDA would allow maximizing lateral coverage and enhance acid penetration due to the reduced rate of reaction and chemical diversion capabilities from thief zones. Combining the introduction of SPRA and VEDA with a shift to bullheading and higher pumping rates enabled the delivery of previously unachievable production results at sustainable wellhead pressures or even well revival of shut-in wells. In addition, reduction of acid content for dolomite stimulation was possible due to the implementation of acid retardation, which also allowed protecting wellheads from exposure to higher acid concentrations while bullheading. Treatment parameters such as volumes, rates, and acid/diverter sequence and ratio were then adjusted for optimal wormhole penetration across all zones using a new carbonate matrix acidizing modeling software. Post-treatment evaluation for the cases of previously shut-in wells has proven the success of the SPRA and VEDA combination. Shut-in wells that were unable to produce sustainably at the required tubing-head pressure (production flowline pressure), were able to produce sustainably with a 100% increase in production compared to prestimulation testing. Similarly, for gas wells, the combination of SPRA and VEDA resulted in a 50% increase in production at a similar bottomhole pressure. In addition, water injectors have also exhibited sustainably increased levels of injectivity compared to prestimulation levels, leading to better sweepage. The novelty of this paper is the comparison between historical carbonate stimulation results in UAE using plain HCl acid with polymer-based diverted acid and using SPRA with VEDA. It also sheds light on the game-changing solutions that suit the ever-increasing challenges observed in offshore oil and gas wells including well placement, lithology, permeability contrast, and type of hydrocarbon within the various target sublayers.

Treating deep hot carbonate reservoirs, such as those found in the Arabian Gulf, presents a series of complex and related challenges to achieve effective and uniform stimulation. Due to the elevated temperature and heterogeneous formation, achieving good reservoir contact with an acid system along the entire interval of interest requires robust treatment fluids that can withstand the harsh environment. Recently, a novel single-phase retarded acid (SPRA) system and an engineered degradable large-sized particulate and fiber-laden diverter (LPFD) were introduced in a well in the Arabian Gulf, yielding strong results for the stimulation treatment. The SPRA, a 15% HCl-based acid system, showed excellent performance in a high-temperature environment (320°F). The fluid delivered similar friction pressures to unmodified 15% HCl, wormholing performance equivalent to emulsified acid without encountering the issues of fluid quality with respect to emulsion stability, and much higher dissolution power than organic acids and chelating agents. The pressure drop after the first acid stage was over 1,000 psi in about 60 min. After the second stage of acid, the pressure drop was close to 1,000 psi in about 30 min. Previous stimulation jobs in the region indicated a need for a significant amount of traditional diversion materials to achieve an effective plugging of the leakoff zones. A novel degradable LPFD system was introduced, achieving a significant increase of injection pressure (~1,000 psi) across the perforations. In addition to the effect on the diversion pressure, the implementation of the LPFD system has helped to reduce the footprint in offshore operations, has simplified materials handling, and has delivered the most efficient diversion performance in bullhead operations compared to other diverters. This article presents a novel method of stimulating deep hot offshore wells by combining an efficient SPRA and a unique degradable LPFD. These methods represent a step change to current practices and can be considered for effective stimulation in challenging carbonate formations.

Hydraulic fracturing is one of the most effective methods of improving the deliverability of low permeability wells. Hydraulic fracturing in a rich gas condensate field has earned cost benefits by reducing number of infill wells, producing condensate drop out from reservoirs and increasing overall productivity of oil and gas. The target development well was drilled and completed in September 2013 and post completion testing rates as well as pressure build up analysis indicated severe formation damage issues and requirement for hydraulic fracturing. In the same field hydraulic fracturing was carried out earlier in other wells but yielded mixed results. In some of the wells 2-3 times increase in productivity was observed while in some wells, the results were below expectations, despite placing ~100,000 lbs of proppant. In one of the well, formation break down did not occur. Utilizing prior frac experience it was decided to optimize the upcoming frac job but at the same time not to compromise on the frac quality. Extensive pre-job evaluation and planning which included review of available logs and previous frac data, development of 1D Mechanical Earth Model (MEM), frac modeling, implementation of Channel fracturing technique, optimization of fracturing chemical and proppant volumes was done to get maximum benefit of this major well stimulation job. As a result of these efforts frac jobs of the target sandstone reservoirs was completed successfully and the well was put on production in minimum clean-up and post frac testing time. As a result of above effort in planning and optimization of frac job at the target well, significant increase in production was observed which exceeded the operator expectations and raised the confidence level in 1D MEM application for hydraulic frac design (i.e. geomechanics) and the new frac technique by the service company. Based on post frac results, these techniques are being utilized in the upcoming wells in the target field.