Acid Fracturing or Proppant Fracturing in Carbonate Formation? A Rock Mechanic's View

Abstract

Acid fracturing is performed to improve well productivity in acid-soluble formations such as limestone, dolomite, and chalk. Hydrochloric acid is generally used to create an etched fracture, which is the main mechanism for maintaining the fracture open during the life of a well. Proppant fracturing is an alternative option that has been applied in carbonate formations. In certain areas, proppant fracturing has been used as a standard stimulation method for carbonate formations. There is no quantitative method to provide an answer of whether acid fracturing or proppant fracturing is an appropriate stimulation method for a given carbonate formation. In proppant fracturing, proppant is used to sustain the effect of the minimum horizontal stress from closing the fracture. In acid fracturing the etched, non-smooth, surface with sufficient roughness should leave open channels upon closing. The effect of elastic, plastic, and creeping deformations in acid fracturing and the proppant crushing and embedment in proppant facturing, on reducing fracture permeability is investigated. The viscous effect, creeping, is a slow displacement that incurred over a long period of time. The creeping effect on fracture closure following an acid fracturing treatment is demonstrated in this paper. Laboratory experiments have been performed to simulate acid and proppant fracturing treatments. The effect of elastic, plastic and viscoelastic rock behavior on fracture conductivity was studied for acid and proppant fracturing treatments, using full core samples. Comparison of acid vs. proppant fracturing conductivity in carbonate formation is also presented. Introduction Hydraulic fracturing (acid or proppant) is used to create a conductive fracture in the formation to enhance well productivity. The induced fracture will tend to close due to the effect of the minimum horizontal stress. Fracture closure is controlled by elastic, plastic, and viscous rock properties. In acid fracturing the etched, non-smooth, fracture surfaces would leave open pathways upon closing in addition to wormholes and channels created from the fracture into the formation. Fracture conductivity is generated by the quantity of rock removed and the pattern of rock removal. Depending on the pattern of natural fracture system, acid solubility of the formation, magnitude of the minimum horizontal stress, and reservoir temperature, acid fracturing vs. proppant fracturing should be evaluated to select the most effective stimulation treatment for a given formation. Interesting observations relavant to stimulation of carbonate reservoirs, have been reported in the literature. Fracture conductivity does not increase with increasing amounts of dissolved rock1. After successful application of proppant fracturing in a chalk formation, it was concluded that proppant fracturing yielded sustained production rate and became the standard stimulation treatment2. Chalk formations are usually soft with Brinell hardness less than 10 Kg/mm2 and therefore creeping is pronounced. The effect of increased effective stress, due to reservoir depletion, on fracture and matrix permeabilities, was reported3. Proppant's importance in sustaining fracture conductivity in carbonate formation was demonstrated4.

Development of Acid Fracturing Model for Naturally Fractured Reservoirs

“…As was mentioned previously, the efficiency of acid fracturing stimulations strongly depends on the ability of fracture to retain conductivity under closure stress. The mechanism of fracture closure is controlled by elastic deformation, compressive failure of contact points (asperities crushing), and viscous flow, also known as creeping effect (Abass et al 2006). The elastic deformation happens as a result of increasing effective minimum horizontal stress due to the reservoir depletion.…”

Impact of Diversion on Acid Fracturing of Laminated Carbonate Formations: A Modeling Perspective

“…A propped fracture is an alternative stimulation technique wherein acid is replaced with proppant slurry. Abbas et al and Suleimenova et al showed that a propped fracture is better at sustaining conductivity at higher closure stresses. Nevertheless, acid fracturing is a cheaper option that does not suffer from screenout complications.…”

Carbonate Rock Chemical Consolidation Methods: Advancement and Applications

“…18 The extraction of hydrocarbons results in increased stresses on the fracture surface, which, combined with the elevated temperatures, causes gradual plastic deformation of the reservoir rock (rock creeping). 19,20 Creeping, in turn, leads to reduction of the fracture width (conductivity reduction), presented in terms of a failure of asperities and embedment (indentation) of proppant into formation rock. 21−23 Researches have shown that the mechanical characteristics of the reservoir rock (hardness, Young's modulus), fracture surface roughness, proppant properties (type, size, concentration), and acid properties (type, reaction time) are critical factors that affect the decline of the fracture conductivity.…”