Stimulation fluids are used for near-wellbore clean-up and either removal or bypass of formation damage or for improvement of the effective permeability. In carbonate reservoirs, typical formulations are most commonly based on HCl or organic acids, such as acetic or formic acid, which increase connectivity between the reservoir and wellbore by dissolving the rock matrix itself. However, how this occurs greatly influences the effectiveness of stimulation for a given amount of dissolution. By far the least effective stimulation mode is face dissolution, as this has very little benefit on inflow and can lead to deconsolidation and collapse of the near-wellbore area. This paper examines the selection of chemicals to reduce face dissolution and improve the efficiency of chemical treatments in carbonates via much more effective formation of conductive flow channels or wormholes.

When acidizing carbonates, the morphology of the resultant wormholes is controlled by rock morphology, composition and heterogeneity, pump injection rate, temperature, and both physical and chemical properties of the stimulation fluid formulation. Effective fluids create long wormholes that penetrate away from the wellbore face, with only limited branching, thus (i) changing the inflow from simple radial flow to modified flow into the wormholes and (ii) bypassing near-wellbore formation damage if the dominant wormholes are sufficiently long.

Reservoir condition Pore Volume to BreakThrough (PVBT) core flood tests were performed: initially applying a typical acid treatment at various injection rates and then comparing these with identical tests with a novel additive included. Fluid effectiveness was assessed based on measurement of PVBT. Micro Computed Tomography (CT) imaging and density difference mapping were used to visualize the wormholes formed. Rates of penetration were gained from differential pressure data combined with consideration of peak elution time of Ca2+ from the analysis of effluent samples.

In tests performed on comparable outcrop limestone core samples, with the same injection flow rate, temperature, and acid concentration, the blended stimulation fluid performed very similarly in the presence and absence of the additive in terms of PVBT and Time to BreakThrough (TBT), showing that the stimulation fluid's performance was not hampered by the presence of the additive. However, while post-test micro-CT imaging of the core plugs revealed that the wormhole morphology was very similar in each case (as might be expected given the consistency in PVBT and TBT), there was a substantial reduction in the extent of undesirable face dissolution observed in presence of the additive. The effect was more pronounced at lower flow rates; poorer chemical transport typically leads to greater face dissolution problems. With the additive, there was also a substantially lower concentration of calcium ions in the effluent for a given set of conditions, despite the stimulation being similarly effective.

Hence, new chemistries have been identified that reduce face dissolution during stimulation compared with acid alone. The additives’ ability to reduce calcium-ion concentration may also reduce the potential for re-deposition of CaCO3 when the spent acid mixes with formation water during flowback. The beneficial effect of this additive has been clearly shown by utilizing a detailed programme of core floods and complementary analysis. This demonstrates the potential for core flood studies, when utilized properly, to advance knowledge and aid development of effective stimulation strategies for field application.

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