A Case Study of a Successful Matrix Acid Stimulation Treatment In Horizontal Wells Using a New Diversion Surfactant in Saudi Arabia
- Shaizad Chatriwala (Saudi Aramco) | Kay E. Cawiezel (BJ Services Company) | Hisham A. Nasr-El-Din (Saudi Aramco) | Yousef Ahmed Al-Rufaie (Saudi Aramco) | Yeyha Al-Tameimi (BJ Services Company)
- Document ID
- Society of Petroleum Engineers
- SPE Middle East Oil and Gas Show and Conference, 12-15 March, Kingdom of Bahrain
- Publication Date
- Document Type
- Conference Paper
- 2005. Society of Petroleum Engineers
- 1.10 Drilling Equipment, 4.1.2 Separation and Treating, 5.8.7 Carbonate Reservoir, 1.6.9 Coring, Fishing, 2.7.1 Completion Fluids, 1.8 Formation Damage, 2.5.2 Fracturing Materials (Fluids, Proppant), 1.6 Drilling Operations, 4.2.3 Materials and Corrosion, 3.2.4 Acidising
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Matrix acidizing of horizontal and multi-lateral wells is a challenging taskfor several reasons. First, oil reservoirs are heterogonous in nature and thepresence of high permeability streaks is common. Proper diversion technique isalways required to better distribute the injected acid across the target zone.Secondly, stimulation of horizontal wells requires injection of large volumesof acids. This will require using an effective corrosion inhibitor package thatcan protect down hole tubulars and coiled tubing, especially in sourenvironments. Thirdly, recovering large volumes of spent acid is a concern,especially in tight formations. Acid treatments should be designed to addressthese issues in a cost effective way.
Currently, there are a limited number of chemicals that can divert the acidinto the formation. This paper introduces a new viscoelastic surfactant whichwas used to form stable foam during matrix stimulation of a seawater injector.The length of the target zone was 1,500 ft, and the average permeability of thecarbonate was 700 mD. A 1.75-inch coiled tubing was used to better distributethe acid in the open hole section (6.125-inch in diameter).
As with previous viscoelastic surfactant systems, the surfactant moleculesform structures in solution in the presence of salts. These structures enhancethe viscosity of solution significantly and can be broken by dilution withinjection water or by adding mutual solvent (3-5 vol%) to thepreflush/postflush stages.
Unlike previous viscoelastic surfactant-based systems, the new surfactantbuilds viscosity faster and its gel breaks down in a shorter period oftime.
This paper examines the results of extensive lab testing that led to thedevelopment of this improved self diverting system. It will also give detailsof the first field application of this system. Following the acid treatment,the well injection rate improved by five folds, from 16,000 to 80,000 BPD.Benefits and improvements introduced by the new diversion technique will bediscussed in detail.
Effective diversion is the key for the success of carbonate matrixstimulation treatments, especially for long horizontal wells. Conventionalstimulation treatments include stages of regular acid with suitable divertersincluding: foam,[1,2] gelled and in-situ gelled acids.[3-5] However, severalconcerns were raised regarding the use of polymer-based fluids in matrixacidizing treatmentst.[6,7] To overcome some of the problems encountered withpolymer-based fluids, viscoelastic surfactant-based acid systems have beensuccessfully used over the last few years.[8-14]
Simple inorganic salts cause the molecules of viscoelastic surfactants toform long worm-like or rod like micelles. Entanglement of these micellesgenerate a 3-D structure in solution which increases the viscosity of thesolution. The resulting high viscosity creates temporary plugging of highpermeability streaks within the matrix, forcing the treating fluids intountreated zones. The viscosity breaks down when hydrocarbons contact the fluidduring flow-back. For injectors a mutual solvent solution is pumped to breakdown the viscosity of the surfactant gel.
The objectives of this work are to: (1) Examine the properties of variousbrines, foams and spent acids that were prepared using a new class ofviscoelastic surfactants, (2) Investigate the stability the diversion abilityof the new surfactant system, and (3) Use the new surfactant to restore theinjectivity of a seawater injector in a carbonate reservoir.
Viscosity of the New Surfactant in Heavy Brines Procedure
The high-density brine fluids were prepared by adding the surfactant tocalcium chloride and calcium chloride/calcium bromide high-density brines. Thesurfactant was mixed into the brine using a standard Servodyne mixer with ahighefficiency paddle mixing at 500 rpm. The brine was heated to 130°F toaccelerate mixing of the surfactant into the brine and to remove small airbubbles.
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