Rock-Mechanics Considerations in Fracturing a Carbonate Formation

Abstract

This article, written by Assistant Technology Editor Karen Bybee, contains highlights of paper SPE 102590, "Acid Fracturing or Proppant Fracturing in Carbonate Formation? A Rock Mechanic's View," by H.H. Abass, A.A. Al-Mulhem, M.S. Alqam, SPE, and K.R. Mirajuddin, SPE, Saudi Aramco, prepared for the 2006 SPE Annual Technical Conference and Exhibition, San Antonio, Texas, 24–27 September. Acid fracturing is used to improve well productivity in acid-soluble formations such as limestone, dolomite, and chalk. Proppant fracturing is an alternative option used in carbonate formations. There is no quantitative method to determine whether acid fracturing or proppant fracturing is an appropriate stimulation method for a given carbonate formation. Laboratory experiments were performed with full core samples to examine the effect of elastic, plastic, and viscoelastic rock behavior on fracture conductivity for acid- and proppant-fracturing treatments. Introduction Hydraulic fracturing (acid or proppant) is used to create a conductive fracture in the formation to improve well productivity. The induced fracture tends to close because of 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 leave open pathways upon closing in addition to the wormholes and channels created from the fracture into the formation. Fracture conductivity is generated by the quantity of rock removed and the rock-removal pattern. Depending on the pattern of the 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. Although longer acid contact with the formation results in more etched surface and thus higher fracture conductivity, formation compressive strength is reduced. Claims have been made that at high reservoir temperatures, fast acid reactions in formations containing high concentrations of calcite result in acid-fracture lengths much shorter than propped fractures. The suggestion was made that for reservoirs with a minimum horizontal stress (fracture-closure stress) greater than 5,000 psi, proppant fracturing is the optimum stimulation method because etching caused by fracture acidizing cannot support such high stress. In chalk formations, it has been shown that proppant fracturing yielded better results than acid fracturing. Fracture length from acid fracturing and proppant fracturing will be different because of their dissimilar fracture mechanics. In proppant fracturing, the fracturing gel is not reactive with the formation and can penetrate deeper when compared with acid fracturing for a given fracturing-fluid volume, especially at high reservoir temperature. Therefore, it is expected that proppant fracturing will create longer fractures. The full-length paper presents a rock-mechanics view of fracture closure of propped and acid-etched fractures that describes the following.In acid fracturing, fracture closure extent is the result of asperities embedment, asperities crushing, and viscous flow (creep).In proppant fracturing, fracture closure extent is the result of proppant embedment, proppant crushing, and proppant flowback.