Decision Criterion for Acid Stimulation Method in CarbonateReservoirs: Matrix Acidizing or Acid Fracturing?

Most wells in carbonate reservoirs are stimulated. Due to low cost and simpler operations, acid stimulationmethods are usually preferred if they are sufficient. Matrix acidizing can effectively stimulate carbonatereservoirs, 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 effectivein high permeability, high confining stress, or soft rocks. There is a combination of parameters, amongthem permeability, confining stress, and rock geomechanical properties, that can be used as a criterion todecide between matrix acidizing and acid fracturing which is the best acid stimulation technique for a givenscenario. 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 volumeof acid for both operations. The productivity of the acid fractured wells is estimated using a fully-coupledacid 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 finallycalculates the well productivity by simulating the flow in the reservoir towards the acid fracture. Using thissimulator, 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 givenamount of acid. The productivity of the matrix acidized wells is estimated using the most recent wormholepropagation models upscaled to field conditions. Results are presented for different types of rock and reservoir scenarios, such as shallow and deepreservoirs, soft and hard limestones, chalks, and dolomites. For each type of reservoir rock and confiningstress, 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 generalindustry 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 arepresented. For example, for harder rocks or shallower reservoirs, acid fracturing is more efficient up tohigher permeabilities than in softer rockers or at deeper depths.This method provides an engineered criterion to decide the best acid stimulation method for a givencarbonate reservoir. The decision criterion is presented for several different scenarios. A simplified conciseanalytical decision criterion is also presented: A single dimensionless number that incorporates all pertinentreservoir properties and determines which stimulation method yields the most productive well, withoutneeding any simulations.