Method for Effective Fluid Diversion when Performing a Matrix Acid Stimulation in Carbonate Formations
Authors: MaGee J. , Buijse M.A. , Pongratz R. 
Published: December 2007

Abstract
This paper describes the use of in-situ crosslinked acid (ICA) for fluid diversion during a matrix acid stimulation treatment in carbonate formations. ICA is a gelled acid system with relatively low base-gel viscosity that crosslinks when the acid spends. The highly viscous crosslinked gel creates a permeability barrier and subsequent fluid stages are diverted to other sections of the zone. The chemistry of ICA is described and results of acid flow tests in radial cores, in which ICA is compared with gel led acid, are presented and discussed.
ICA was used in combination with the "Maximum Pressure Differential and Injection Rate" (MAPDIR) technique as a diverter system in a number of acid stimulation treatments in the ARABC and ARAB-D formations in the Idd El Shargi field, offshore Qatar. Alternating stages of 15% HCl and ICA were pumped. Good diversion was indicated by the bottomhole pressure response when the ICA reached the perforations. Three treatments are discussed in detail: IS-58D, 1S-61D, and IS-63D. Plots of the downhole pressure response during the treatments are shown and production data and the results of pressure buildup tests are discussed. Skin changes during the treatment are calculated from the pressure response and used to analyze the diverter effectiveness.

Introduction
Correct downhole placement and optimal zonal coverage is one of the most important factors for successful matrix acid stimulation treatments. When injected into a well, acid will follow the path of least resistance and preferably enter the sections of the zone with the highest permeability and/or lowest skin. If the method of fluid diversion is not applied, a large fraction of the acid will flow into the least damaged sections, while the sections with most damage only accept a small fraction of the acid. An effective diversion method must be applied to help ensure complete zonal coverage and removal of damage from the entire producing interval.
A number of methods and techniques are commonly used for fluid diversion: mechanical isolation by packers, ball sealers, diverting agents such as oil soluble resins or benzoic acid flakes, and viscosified fluids and foam. Recently Paccaloni introduced the MAPDIR diversion technique. The most attractive feature of this technique is its simplicity: fluids are pumped at the maximum rate possible without exceeding the formation fracturing pressure. The MAPDIR technique is especially suited for diversion in carbonates. In a carbonate formation, efficient diversion of acid fluids is important because the acid-carbonate dissolution reaction rate is fast. When a fast reacting acid such as HCl is pumped into a carbonate rock, highly conductive, branched channels or "wormholes" are created that bypass damaged areas and effectively transport the acid deeper into the formation. Therefore, wormhole generation is the preferred dissolution process in matrix acidizing carbonate reservoirs. Without fluid diversion, however, the rapid increase in effective permeability caused by wormholing will lead to even more fluid entering the least damaged sections of the interval. The end result will be poor zonal coverage and an increase in the permeability contrast.
ICA was developed as a system to prevent fluid loss in fracture acidizing and as a diversion system in matrix acid treatments in carbonates. ICA is a thin gelled acid with a viscosity of approximately 20 cp that forms a highly viscous crosslinked gel when the acid spends in the formation and the pH increases to a value of about 2. The crosslinked gel will effectively stop any further fluid invasion and divert subsequent acid stages to different parts of the zone. The crosslink will break upon further spending and at a pH value of 4, the viscosity of the fluid will return to its original value. The ICA system is ideally suited for acid diversion in long intervals, and a number of treatments have been successfully performed in long horizontal openhole completions in the North Sea and Middle East.
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