When Do Surfactant-Based Acids Work as Diverting Agents?

Abstract

The purpose of matrix stimulation in carbonate reservoirs is to bypass the damaged zones and increase the effective wellbore area. This can be achieved by creating highly conductive flow channels known as wormholes. A further injection of acid will follow the wormhole path where the permeability has increased significantly, leaving substantial intervals untreated. This problem can be significant as the contrast in permeability increases within the target zones. Diverting materials such as surfactant-based acids play an important role in mitigating this problem. Several papers published in the literature aimed to verify the diversion capability and the performance of the self-diverting acids. However, most of the parallel-coreflood experiments, if not all, used relatively short cores (2 to 6 inches in length).In this paper, acidizing experiments were conducted using two 20-inch long cores with different permeabilities. Carbonate cores were used with a permeability of 5 to 150 md and the total flow rate was varied from 3 to 20 cm 3 /min. The initial contrasts in permeability between the two cores ranged from 2 to 15 fold. To characterize the wormholes generated, a computerized tomography (CT) was used to generate 3-D images to describe the shape of the wormholes in both cores.Several periods were identified from the shape of the flow rate distribution entering each core. Acid injection rate was found to influence the efficiency of surfactant to divert acid. Acid diversion was noted to be most efficient at low flow rates (3 cm 3 /min). No significant diversion was noted at higher flow rates (> 6 cm 3 /min). Also, no significant diversion was noted at high initial permeability ratios at least for the given core length. In conjunction with the experimental study, an analytical model was developed to verify the experimental results obtained with regular acids. IntroductionThe stimulation process starts by injecting an acid into the formation below the fracture pressure, which will result in dissolving the carbonate minerals. The dissolution patterns created are known as wormholes. A relatively close permeability ratio is essential to ensure proper placement of acid to stimulate the target zones. When such condition is not part of the characteristic of the candidate formation, diversion is necessary for better acid placement. In this case, diverting materials play an important role to equalize the flow, allowing untreated zones to be stimulated and thus benefit the overall productivity of the treated well. Several techniques have been implemented with the aim of diverting the stimulation fluids to the target zones. These procedures can be classified into two main categories: mechanical and chemical.Extensive laboratory testing has been conducted to evaluate the diversion of a viscoelastic surfactant-based acid systems. Chang et al. (2001) conducted multi-core flood testing, incorporating a post acidizing computed tomography (CT) scan that showed the self-diverting acid successfully diverted acid from the high permeabili...

Application of Nanoparticles in Stimulation: A Review

“…The diversion ability was evidenced by the increase in the down hole pressure during the injection of the diverter and the change in bottom hole injection temperature. Extensive laboratory experiments were conducted by Al-Ghamdi et al to evaluate the efficiency of VES-based HCl in carbonate stimulation using a parallel coreflooding system. − They showed that, when straight 15 wt % HCl was used, the acid flowed through the high permeability cores and created dominant wormholes and did not break through the low permeability cores. They tested the VES-based HCl acid system and they showed that it was effective in diverting the acid flow in permeability contrast (ratio of high permeability to low permeability rocks).…”

Study on the Diverting Characteristics of Diverting Acid Based on Digital Rock Physics

“…13 Al-Ghamdi et al found through experimental research that achieving better diverting efficiency is closely related to the injection displacement of diverting acid and, based on experimental data, established an analytical model for the velocity distribution of flow inside wormholes and the optimal displacement. 14 Liu et al further conducted 3D wormhole expansion research under linear flow conditions, and according to the minimum resistance seepage principle, in reservoirs containing karst caves and fractures, acid solution is more inclined to flow into such media, believed to be because large-scale karst caves and fractures are key actors affecting wormhole development. 15 Liu et al established a mathematical model to describe the VES diverting behavior through experiments, and the simulation results confirmed that the diverting behavior depends on the residual acid area and the viscosity of the residual acid.…”

Synthesis, Characterization, and Viscosification of Amidosulfobutaine and Zwitterionic Gemini Surfactants

“…In the recent past, several studies attested the efficacy of VES-based system as acid diverters (Al-Ghamdi et al, 2010;Gomaa et al, 2012;Yu et al, 2010). Chang et al have reported the use of carboxybetain VES as selfdiverting acid systems, which successfully diverted acid from high-permeability core into the low-permeability core (Al-Ghamdi et al, 2014).…”