TX 75083-3836, U.S.A., fax 01-972-952-9435. AbstractDue to the complex nature of carbonate reservoirs, reservoir characterization often leaves many uncertainties. Finding the right balance between risks associated with these uncertainties and optimum stimulation makes acid fracturing and matrix acidizing treatments challenging. The stimulation objectives become even more difficult in highly slanted, layered, naturally fractured reservoirs exhibiting high permeability contrast. In this environment, adequate fluid diversion and leak-off control have always been the key design elements for stimulation. Achieving diversion and leak-off with a degree of control to make treatments applicable to a wide range of reservoir uncertainties presents challenges.A novel, polymer-free degradable diversion system has been in use for the last three years in the largest carbonate reservoirs of the world, namely the Khuff formation in Saudi Arabia's Ghawar field. The self-diverting fluid combines viscoelastic surfactant in HCl with degradable fiber technology. The fluid develops viscosity as the acid spends, while the fibers bridge across perforation tunnels and fissures to form a filter cake. Because the fibers completely degrade with time and the spent fluid breaks when it comes into contact with hydrocarbons or solvents during flowback, the fluid temporarily limits injectivity into thief zones without damaging the reservoir.More than 50 wells have been stimulated with this fluid system covering a wide range of single and multi-stage matrix acidizing and acid fracturing treatments. The controllable nature of diversion from well to well and on-the-fly adjustment capabilities of the fluid system have successfully ensured stimulation performance despite the uncertainties of carbonate reservoirs in Saudi Arabia. In essence, this novel fluid became the standard insurance policy for stimulation treatments of carbonate formations where the permeability models are inherently underestimating the contrast due to difficulties of placing natural fractures and quantifying their impact.

Offshore operations are extremely expensive because of the operational environment and the necessary infrastructure. In this environment, emphasis is placed on high-efficiency operations based on specially tailored solutions combining available resources with new technologies. This results in a significant impact on operational efficiency by lowering costs and ultimately increasing hydrocarbon production. To introduce greater efficiencies in offshore operations, a horizontal openhole candidate well was selected to be equipped with a permanent completion system that would enable multiple fracturing treatments. Later, it was determined that by using a novel viscoelastic polymer-free surfactant-based fluid, the entire operation could be performed in a single operation, adding additional savings to the process and improving efficiency. Interpreted openhole images and advanced sonic logs were used to determine the optimum completion configuration and to select favorable fracture initiation points and treatment designs. Because a specialized fracturing vessel tailored for operations in the Black Sea was not available, a supply vessel was used. The vessel had all required fracturing equipment rigged up and secured on decks. To enable sufficient fracturing fluid volume for placing three propped fracturing treatments in a single pumping operation, a polymer-free fracturing fluid was formulated and mixed with seawater continuously. This novel multistage fracturing system was introduced in Europe for the first time. Results indicate a sustained increased production. Because of this success, additional wells are scheduled to be stimulated using same approach in the following months. Introduction The Lebada Vest field is situated ~95 km offshore Romania in the Black Sea. This field was discovered in 1984 and put on production in 1993. Since then, numerous vertical oil and gas wells were drilled and completed (Fig. 1). The wells were produced initially in natural flow and later equipped with gas lift to enhance ultimate hydrocarbon recovery. The target reservoir is a Cretaceous-age formation located at depths of ~1,900-m true vertical depth (TVD) composed of varying shale, sandstone, and carbonate content. The laminated pay zone is generally formed by streaks with permeability ranging from 0.1 md to 2.0 md and average of 0.8 md. Reservoir rock porosity ranges between 15% and 22%. Bottomhole static pressure (BHSP) at ~1,850 m true vertical depth sub sea (TVDSS) sub sea is ~220 bars and bottom hole static temperature (BHST) is 93°C.

Acid fracturing treatment performance is largely determined by the achieved effective etched fracture length. Evolution of fracture length during such treatments leads to progressively increasing the acid leakoff rate up to a point when the fracture stops extending. Zonal coverage and fluid loss control in naturally fractured carbonate reservoirs with high permeability contrast are the key challenges during acid fracture treatment.Nonreactive and reactive polymer based fracturing fluids and diverters were historically accepted as systems that could efficiently control fluid leakoff. The performance of such fluids relies on wall building fluid loss additives, such as polymers. Their deposition on the fracture face forms filter cake that decreases fluid leakoff into the formation. Filter cake on the etched fracture wall could cause skin. Nondegradable particulate fluid loss additives used in naturally fractured reservoirs can be a good leakoff control tool; however, particulates could permanently shut natural fractures off and obliterate their production contributions. Finding the right balance between induced fracture damage and conductivity is a challenge, and avoiding this damage by using nondamaging fluid with major fluid leakoff control properties is the logical problem solution.A novel fiber laden polymer-free self-diverting acid system was introduced in Saudi Aramco as an acid fracturing diverter to control fluid leakoff, and enhance the diversion process by combining the aspects of both particulate and viscosity based diversion techniques. The fluid system has a distinct advantage in that it does not contribute to formation damage because the viscoelastic surfactant will breakdown upon contact with hydrocarbons, and the fiber will degrade with time and temperature.More than 25 acid fracture treatments using the novel acid system have been successfully implemented in gas bearing carbonate reservoirs in Saudi Arabia. Unlike the approach used in acid fracture treatments using conventional fluid systems, the degree of diversion was dynamically adjusted to maintain the treating pressure above the fracturing pressure throughout the entire period in these treatments. The bottom-hole pressure (BHP) measurement confirmed superior fluid leakoff control leading to an outstanding diversion performance with excellent gas production increments.This paper provides details about treatment design, field implementation, and post-stimulation performance for two out of the more than 25 wells treated using this novel acid system.

Current offshore operations are extremely expensive because of the operational environment and the necessary infrastructure. In this environment, emphasis is placed on high-efficiency operations based on specially tailored solutions combining available resources with new technologies. To introduce greater efficiencies in offshore operations, a horizontal openhole candidate well was selected to be equipped with a permanent completion system that would enable multiple fracturing treatments. Later, it was determined that by using a novel viscoelastic polymer-free surfactant-based fluid, the entire operation could be performed in a single pumping operation, improving efficiency and adding additional savings to the process. Because a specialized fracturing vessel tailored for operations in the Black Sea was not available, a supply vessel was used. The vessel had all required fracturing equipment rigged up and secured on decks. To enable sufficient fracturing fluid volume for placing three propped fracturing treatments in a single pumping operation, a polymer-free fracturing fluid was formulated and mixed with seawater continuously. This novel multistage fracturing system combined with polymer-free viscoelastic surfactant fluid system prepared with seawater was applied in the World for the first time. Results indicate a sustained increased production. Introduction The Lebada Vest field was discovered in 1984 and put on production nine years later. This field is situated in the Black Sea, ~95 km offshore Romania. Since then, numerous vertical oil and gas wells were drilled and completed. The wells were produced initially in natural flow and later equipped with gas lift to enhance ultimate hydrocarbon recovery. The target reservoir is a Cretaceous-age formation located at depths of ~1,900-m true vertical depth (TVD) composed of varying shale, sandstone, and carbonate content layers. The laminated pay zone is generally formed by streaks with average permeability of 0.8 md. Reservoir rock porosity ranges between 15% and 22%. Bottomhole static temperature (BHST) is 93°C and bottomhole static pressure (BHSP) at ~1,850 m true vertical depth sub sea (TVDSS) is ~220 bars. To increase the hydrocarbons recovery, operating company decided to drill a horizontal well and target an un-drained part of the reservoir. Furthermore, to achieve greater efficiency in offshore operations and after evaluating completion options, including open hole, cemented liner with perforations, and slotted liner, Operating company decided to complete the well openhole and equip it with a permanent completion system that enabled the placement of multiple fracturing treatments (Barba and Shook 2004). Later, it was also determined that by using a novel Visco-Elastic Surfactant-based (VES) fluid, the entire operation could be performed in a single sequence, achieving additional savings in time and related costs.

Acid stimulation of carbonate formations using hydrochloric (HCl) acid-based systems to improve hydrocarbon production or water injection has been a common practice since the early days of the oil and gas industry. The challenge of achieving full zonal coverage with the stimulation fluid still persists. This task is even more difficult in highly slanted layered, naturally fractured reservoirs exhibiting high permeability and porosity contrast.During acid stimulation treatments, the fluid has a natural tendency to take the path of least resistance, penetrating layers with the highest porosity and permeability while little or no acid is injected into lower-quality zones. Excessive fluid loss to these highquality thief zones results in a nonhomogenous distribution of stimulation fluids across the targeted interval. These higher-quality zones are essentially over stimulated, allowing them to contribute the most to post-treatment production. This leaves neighboring lower-quality zones unstimulated, preventing them from reaching their maximum potential.A novel fiber-laden polymer-free self-diverting acid system was introduced in Saudi Aramco to improve zonal coverage across the entire interval of interest in vertical and highly slanted wells during stimulation treatments. This diversion system combines fiber and a self-diverting acid, which utilizes a nondamaging viscoelastic surfactant that gels as the acid spends. The combination of the self-diverting acid and fiber enhances the diversion process by combining the aspects of both particulate and viscosity-based diversion techniques. The fluid system has a distinct advantage in that it does not contribute to reservoir damage as the viscoelastic surfactant will break down upon contact with hydrocarbons, and the fiber will dissolve with time and temperature.The fiber-laden self-diverting acid system has been applied in new developments and producing gas wells in the Khuff reservoir of Saudi Aramco with impressive results. The diversion and acid reaction signatures were evident in the acquired treating pressure plots during acidizing with the fiber-laden polymer-free self-diverting acid system. The initial results indicated substantial pressure responses and excellent gas production performance.