Decision Criterion for Acid Stimulation Method in Carbonate Reservoirs: Matrix Acidizing or Acid Fracturing?
Authors: Palharini Schwalbert Mateus , Aljawad Murtada Saleh , Hill Alfred Daniel , Zhu Ding 
Published: February 2020

Abstract
Most wells in carbonate reservoirs are stimulated. Due to low cost and simpler operations, acid stimulation methods are usually preferred if they are sufficient. Matrix acidizing can effectively stimulate carbonate reservoirs, often resulting in skin factors on the order of -3 to -4. In low confining stress and hard rocks, acid fracturing can yield better results than matrix acidizing. However, acid fracturing is less effective in high permeability, high confining stress, or soft rocks. There is a combination of parameters, among them permeability, confining stress, and rock geomechanical properties, that can be used as a criterion to decide between matrix acidizing and acid fracturing which is the best acid stimulation technique for a given scenario.
This study compares the productivity of matrix acidized and acid fractured wells in carbonate reservoirs. The criterion used to decide the preferred method is the largest productivity obtained using the same volume of acid for both operations. The productivity of the acid fractured wells is estimated using a fully-coupled acid fracturing simulator, which integrates the geomechanics (fracture propagation), pad and acid transport, heat transfer, temperature effect on reaction rate, effect of wormhole propagation on acid leakoff, and finally calculates the well productivity by simulating the flow in the reservoir towards the acid fracture. Using this simulator, the acid fracturing operation is optimized, resulting in the operational conditions (injection rate, type of fluid, amount of pad, etc) that lead to the best possible acid fracture that can be created with a given amount of acid. The productivity of the matrix acidized wells is estimated using the most recent wormhole propagation models upscaled to field conditions.
Results are presented for different types of rock and reservoir scenarios, such as shallow and deep reservoirs, soft and hard limestones, chalks, and dolomites. For each type of reservoir rock and confining stress, there is a cut-off permeability below which acid fracturing results in a more productive well; above this cut-off permeability, matrix acidizing should be preferred. This result agrees with the general industry practice, and the estimated productivity agrees with the results obtained in the field. However, the value of the cut-off permeability changes for each case, and simple equations for calculating it are presented. For example, for harder rocks or shallower reservoirs, acid fracturing is more efficient up to higher permeabilities than in softer rockers or at deeper depths.
This method provides an engineered criterion to decide the best acid stimulation method for a given carbonate reservoir. The decision criterion is presented for several different scenarios. A simplified concise analytical decision criterion is also presented: a single dimensionless number that incorporates all pertinent reservoir properties and determines which stimulation method yields the most productive well, without needing any simulations.