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In naturally fractured carbonates, the efficiency of acid fracturing stimulation can be hindered due to the decreased effective length and conductivity of created fractures, and acid loss into natural fractures is one of the main reasons for the reduced efficiency. At the same time, acid that leaks into natural fractures creates additional conductivity that may enhance production from the stimulated well. A model was developed to predict the acid fracturing performance in naturally fractured carbonate reservoir by taking into account the etching of both hydraulically induced and naturally occurring fractures to estimate fracture conductivity and well productivity. The model uses a domain that contains a well and a rectangular reservoir. The well is intersected by a bi-wing vertical hydraulic fracture which is intersected by transverse natural fractures. The model simulates acid injection into the fracture system, acid-rock reaction, and width increase for both hydraulic and natural fractures. At the end of the acid injection, the conductivity of the fracture system is estimated, and the well stimulation efficiency is evaluated by calculating productivity increase and skin factor. This is done by simulating the production flow into the hydraulic and natural fractures using a coupled reservoir model without the need of an external reservoir simulator. In contrast to previously published acid fracturing models which calculate leakoff using the Carter's model, in this study, we developed a model that calculates the leakoff during acid injection by simulating the flow through porous media using a reservoir model, which includes both hydraulic and natural fractures. In contrast to the Carter's leakoff model which assumes that fractures are spaced far enough so that no interaction among the fractures occurs, the new approach allows the natural fractures to interact with each other as acid leaks off and pressure changes in the reservoir surrounding the fractures. The new approach does not impose limitations on fracture spacing, and leakoff rate of individual natural fractures is a function of fracture spacing and location. The other feature of the new model is that the leakoff flow rate does not necessarily decrease with time, unlike what Carter's leakoff model predicts. It was observed that leakoff rate from natural fractures may increase initially as the natural fractures are stimulated. The effects of natural fracture geometry and spacing, reservoir permeability, and treatment conditions on acid leakoff, fracture conductivity and well productivity are analyzed. The role of natural fractures on stimulation efficiency is evaluated by comparing the results with the cases where no natural fractures are present in the reservoir. The model enables a better prediction of acid fracturing performance in naturally fractured carbonate reservoirs, and also simulates more realistic leakoff behavior compared to the conventional leakoff model, which improves the accuracy of the results.

In carbonate matrix acidizing, the critical design parameters are interstitial velocity and pore volume to breakthrough for wormhole propagation (vi, opt and PVBT,opt). Hydrochloric acid (HCl), the most commonly used acid in carbonate acidizing, sometimes shows low efficiency because the injection rate needed for optimal wormhole propagation is not attainable, especially for when long completion intervals, low permeability carbonates, or coiled tubing operations are involved. It also raises safety, corrosion, and environmental concerns. When conventional retarded acid systems are used to overcome these challenges, the efficiency of acid stimulation often suffers from low reaction rate. This study presents testing results of a modified acid system that has controlled reaction rate with favourable wormhole propagation characteristics, especially at low interstitial velocities. Due to the increased activation energy barriers utilized in these modified‐acid systems, it is possible to control the reaction rate of the hydrogen proton and optimize the wormholing effect based on the completion method and formation specifications. Laboratory linear core flooding experiments and acid jetting experiments were conducted to study the wormhole efficiency with the new acid systems. The experimental results showed clear advantages of the modified acid systems. The modified acids have similar or better wormhole efficiency parameters compared with HCl having comparable dissolving power. When combined with acid jetting, further improvement in wormhole growth in low permeability limestone is achieved. In addition to minimizing the hazardous exposure levels and corrosion rates compared with HCl, the new acid system provides the positive aspects of solubilizing ability, thus improving optimal wormhole conditions.

A successful application of the acid fracturing stimulation in naturally fractured carbonate reservoirs is challenged by an increased acid leakoff into natural fractures intersecting a hydraulically induced (main) fracture. This may limit the propagation of the main fracture resulting in conductivity reduction. At the same time, the etching of the natural fractures by the leakoff acid may increase the overall conductivity of the fracture system resulting in enhanced well productivity. The effect of the natural fractures on the efficiency of acid fracturing stimulation treatment is not fully understood. A model was developed to evaluate the acid fracturing performance in naturally fractured carbonate reservoirs.The model simulates an acid fracturing stimulation and production in a vertical well located in the center of a rectangular reservoir. The well contains a bi-wing vertical hydraulically induced fracture intersected by symmetric transverse natural fractures. The model simulates the acid flow