Reactivity of San Andres Dolomite

Abstract

Summary The San Andres formation is routinely stimulated with acid. Althoughnumerous acidizing simulators are available to aid in treatment optimization, existing reactivity data were generated with quarried rock rather thanformation samples. This paper presents reactivity data for five San Andresdolomite samples. These data can be used in most fracture-acidizing-designsimulators to allow more accurate simulation of the acidizing process. Introduction The use of various fracturing methods for stimulation of oil and gas wellshas become common in the petroleum industry. Fracturing treatments areperformed on wells of various potentials to increase production. Concurrentwith the desire for increased production is the need to optimize treatmentdesigns and to predict production is the need to optimize treatment designs andto predict the expected posttreatment well response. This is true regardless ofwhether the stimulation method is hydraulic fracturing with propellants orfracture acidizing. In fracture acidizing, the created propellants or fractureacidizing. In fracture acidizing, the created fracture conductivity is a resultof the action of acid on the exposed formation face. Kinetic parameters-such asacid type and strength, reaction temperature and time, and formationreactivity-affect the amount of rock removed during the acidizing process. Notonly is the total amount of rock removed important, but an estimate of therelative amounts removed in each element of the fracture must be made to allowprediction of the magnitude of production improvement that might be achievedafter a given treatment. A number of experimental and theoretical modelscurrently are used to help predict the distance that live acid will penetratealong a hydraulically induced fracture and the amount of reaction that willoccur in each fracture segment. Two factors affect the rate at which acid injected into a fracture willspend: the surface reaction rate and the mass-transfer rate. The surfacereaction rate is a measure of the rate at which the acid-rock reaction proceedsat the fracture surface. The mass-transfer rate is a measure of the rate atwhich acid is transported from the bulk acid solution to the fracture face. Either, or both, of these factors may control the live-acid penetrationdistance. The parameters affecting mass transfer in fracture acidizing operations havebeen studied extensively. The limited reactivity data available, however, weregenerated with quarried rock rather than formation core samples. To develop abroader data base and thus to improve acidizing treatment design, a study wasinitiated on actual formation samples. The San Andres dolomite was selected asthe first case study. Reactivity of the San Andres dolomite was studied through the use of coresamples from several fields, including the Wasson, Slaughter, and Levellandfields. Experiments were conducted with a rotating disk system, a proven andindustry-accepted procedure. The data are presented in a manner consistent withclassic acidizing theory; therefore, they can be used in mostfracture-acidizing-design simulators. Reaction Kinetics HCl is the most common acid solution used in fracture acidizing. HCl reactswith dolomite as follows: 4HCl +CaMg(CO2) - CaCl +MgCl +2H O+2CO. In this reaction, 4 mol of HCl react with 1 mol of dolomite to form calciumand magnesium chloride, CO2, and water. The HCl/dolomite reaction is considereda heterogeneous reaction because it occurs between a liquid and a solid. Aheterogeneous reaction takes place at the interface between the two phases, inthis case at the fracture face. Reaction rate has the units of moles transformed per unit time per unitvolume. For heterogeneous reactions, it is appropriate to per unit volume. Forheterogeneous reactions, it is appropriate to define the reaction rate in termsof the interfacial area available for reaction. In oilfield terms, the reactionrate, or flux, is a measure of the amount of rock dissolved from a givenfracture area in a given period of time. The reaction rate of HCl and dolomitecan be described by the following equation:where u=flux or reaction rate(gmol/cm s); C =acid concentration at fracture surface (gmol/cm);K=reaction-rate constant; and n=reaction order. The units K vary depending onn, which is dimensionless. The term "reaction-rate constant" is a misnomer because K varieswith temperature. It is generally accepted that rate constants riseexponentially with temperature and that the temperature dependence can bedescribed by the Arrhenius law expression: (2) where E =activation energy (cal/gmol); R= 1.987 cal/gmol K; T=temperature(K); and K =pre-exponential factor, sometimes called the frequency factor, which is assumed to be temperature-independent. Experimental Aspects of Kinetic Studies The chief significance of rate functions such as Eqs. 1 and 2 is that theyprovide a satisfactory framework for the interpretation and evaluation ofexperimental kinetic data. Determination of the reaction-rate expression is atwo-step procedure. First, the concentration dependence (reaction order, n) isdetermined at a fixed temperature. Then the temperature dependence of thereaction-rate constant is evaluated to give a complete reaction-rateexpression. To determine reaction-rate constants and reaction orders, it is necessary todetermine reactant or product concentrations at known times and to controlenvironmental conditions of the reaction. If the HCl/dolomite reaction isevaluated with various acid strengths and under isothermal conditions, then thelogarithmic form of Eq. 1 can be used to interpret the data: (3) A log-log plot of u vs. C will yield a straight line with the slope equal ton and the intercept equal to ln(K). If the reaction-rate constant is determined at a series of temperatures, then the logarithmic form of Eq. 2 can be used to evaluate the temperaturedependence of K: (4) A log-log plot of K vs. (1/T) yields a straight line with the slope equal to(E/R) and the inverse log of the intercept equal to K. Once n, K, and E have been determined, the rate expression can be formulatedin terms of both the temperate- and concentration-dependent terms as: (5) Experimental Procedures In many instances, particularly in attempts to measure surface kinetics, observed reaction rates will not be restricted totally by either the surfacereaction rate or the mass-transfer rate. For this reason, experiments todetermine reaction kinetics must be conducted in such a manner that the kineticexpressions are the only unknowns. The experiment should be designed so thatthe rate of acid transfer to the surface is known.