TX 75083-3836, U.S.A., fax 01-972-952-9435. AbstractTo conventionally drill, complete and produce horizontal and multilateral wells with zero skin is not an easy undertaking. Much attention has been given to "best oil field practices" in both drilling and production processes, but the reality is that formation damage will often have to be addressed sometime during the life time of the well.Formation damage due to drilling and scale formation is common problem in carbonate reservoirs that can be remediated using chemical means. The success of any treatment however, requires a complete understanding of the problem and a solution that will address the majority of the damage. The solution evolves with time and experience. Extended reach intervals with variable permeability complicate the process. Reservoir and fluid characteristics, cleanup fluid chemistry, and operational considerations must always be considered.The acidizing experience and an improved understanding of how to effectively treat long and heterogeneous intervals in carbonate formations is rapidly evolving in Saudi Arabia. Water injectors, oil producers and wastewater disposal wells have been treated and are reviewed in this study. These wells have different configurations including: vertical, horizontal, extended reach, and multilateral with open hole and cased completions. Several acid placement and diversion techniques have been applied and a specialized treatment package was developed based on the latest coiled tubing and chemical diversion technologies. Laboratory studies, lessons learned and specific design guidelines from both successful and less than expected well treatments are highlighted in this work.

In order to treat the total interval in an anisotropic, heterogeneous carbonate formation, acid diversion is necessary. Many diverting agents are commercially available which will plug higher permeability portions of the formation and divert the acid treatment to lower permeability zones. For an oil-productive reservoir solid diverting agents such as wax beads, benzoic acid flakes, or polymers are used. Several problems may be associated with the use of these diverting agents: 1) too much additive may be used which may result in a severe reduction in acid injectivity and diverter may be very difficult to remove after the treatment; 2) flow may not be diverted from one zone to the other, but, rather, flow is equalized between the high permeability zone and the low permeability zone. Recently, viscoelastic surfactants have been used in acid treatments for diversion.1–3 The viscoelastic surfactant contains neither solids nor polymer so clean up is of no concern. The appropriate concentration of surfactant is determined by laboratory flow testing of formation cores under reservoir temperature conditions.2 A parallel flow set up has been constructed which allows independent monitoring of flow parameters for a higher permeability and lower permeability formation core pair. Data demonstrate, through pressure analysis and effluent volumes, that flow is diverted from the higher permeability core to the lower permeability core, until acid break through occurs. Pressure data are confirmed by acid effluent volumes which are collected at regular time intervals throughout testing. In all cases, the effluent volume through the lower permeability core is greater than through the higher permeability core. Core plugs are photographed and imaged by the CT-scan technique before and after testing. Parallel flow testing has also been applied to stage treatments in which a stage of stable foam of viscoelastic surfactant in 2 wt% KCl is injected prior to the main acid stage. Appropriate surfactant concentrations in the foamed stage are determined by flow testing with formation cores. The results of core diversion testing and fluid rheology testing of both the foamed and non-foamed viscoelastic surfactant diverting fluids are discussed. Field case histories of the application of acid treatments, which are designed on the basis of laboratory generated data, are reviewed. Introduction Effective diversion is the key for the success of carbonate matrix stimulation treatments, especially for long horizontal and multi-lateral wells. Conventional stimulation treatments include stages of regular acid with suitable diverters including: foam,3–5 gelled and/or in-situ gelled acids.6–8 However, several concerns have been expressed regarding the use of polymerbased fluids in matrix acidizing treatments.9,10 To overcome some problems encountered with polymer-based fluids, viscoelastic surfactant-based acid systems were developed almost four years ago and have been successfully used over the last few years.1–3,11–15 Simple inorganic salts force the molecules of viscoelastic surfactants to form long worm-like or rod-like micelles.11,13 Entanglement of these micelles generate a 3-D structure, which increases the viscosity of the solution. The resulting high viscosity fluid temporarily plugs higher permeability streaks within the rock matrix, forcing live acid into lower permeability zones. The viscosity breaks down when hydrocarbons (oil or condensate) contact the fluid during flow-back. For injectors, mutual solvent (ethlenegylcol monobutyl ether) is added in the preflush stage to break down the viscosity of the surfactant gel.13 Laboratory testing has demonstrated that lower concentrations of viscoelastic surfactant can provide the viscosity to achieve effective diversion. Lower concentrations obviously make it a more cost effective material and also facilitate cleanup at lower bottomhole temperatures without a breaker. Extensive laboratory testing was conducted to evaluate the diversion of both foamed and non-foamed viscoelastic surfactant-based diversion systems. Various concentrations of foamed and non-foamed viscoelastic surfactant fluids were tested at bottomhole temperatures and under pressure.

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TX 75083-3836, U.S.A., fax 01-972-952-9435. AbstractWhen designing an acid treatment, the engineer often faces the question of what diversion method will be the most effective yet least damaging to the formation. Previous work has been published about evaluation methods, and conceptually diversion theory seems straightforward, but detailed post-job analysis and treatment optimization is often forgotten, especially when post-job production results meet or exceed expectation. Once a particular diverter is selected, the next question often is how much is required? If too small of volume is used, then treatment diversion will not be achieved. If too large of volume is used, then cleanup times and the risk of permanent damage increases.Viscoelastic surfactants have recently become a popular additive for diversion pills, based on their polymer-free chemistry. Various formulations, including foam, have been successfully applied in acid stimulation treatments, but little post-job analysis has been published. Viscoelastic fluid properties vary over a pH range during acid spending on the formation and when foamed, determination of bottomhole properties is further complicated.A tube viscometer and parallel core flow cells were utilized to determine the rheological properties and diversion effectiveness of various formulations of foamed viscoelastic surfactant at bottomhole conditions. An optimum formulation was determined, and this fluid was then applied and evaluated in a field application. A down-hole memory gauge was run during a coiled tubing acid treatment, and delta pressure changes were evaluated to determine the diversion effect.

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