Characteristics of Acid Reaction in Limestone Formations

Abstract

A kinetic model for the reaction of Hydrochloric acid with limestone bas been determined. Reaction order and rate constant for this model were calculated from experiments where acid reacted with a single calcium carbonate plate. Experiments were performed so that acid flow past the plate and mass transfer rate to the rock surface could be calculated theoretically. The resulting model, therefore, accurately represents the acid reaction process at the rock surface and is independent of mass transfer rate. Combination of this model with existing theory allows prediction of acid reaction during acid fracturing operations. A model for acid reaction in wormholes created during matrix acidization treatments is presented along with data for reaction of hydrochloric, formic and acetic acids in a wormhole. Factors limiting stimulation in acid fracturing or matrix acidizing treatments are then discussed. Introduction To predict the stimulation ratio resulting from acid fracturing or matrix acidizing treatments it is necessary to know the rate of acid reaction under field conditions. In acid fracturing treatments, for example, reaction occurs as acid flows through a narrow fracture. Reaction in a matrix treatment occurs during flow through wormholes (channels of roughly circular cross-section) created by acid reaction. In both treatments, a large amount of mixing occurs during flow through the fracture or channel as a result of tortuosity and wall roughness. Reaction rate can be obtained from experiments, or predicted by theoretical calculations that accurately model field conditions. In general a theoretical procedure is preferred since it can be used without recourse to laboratory testing. Acid-reaction-rate data have been reported from a number of experiments intended to simulate acid reaction in field treatments. Tests most often used are:the static reaction rate test, in which a cube of limestone is contacted with unstirred acid at a known ratio of rock surface area to acid volume;flow experiments, where acid is forced to flow between parallel plates of limestone; anddynamic tests, whine limestone specimens are rotated through an agitated acid solution. In general, these tests model some aspects of the reaction process, such as area to volume ratio, or acid flow velocity, but do not accurately model all field conditions. To obtain an accurate mathematical model for field treatments, assuming fracture or wormhole geometry is known, it is necessary to characterize acid reaction kinetics at the limestone surface, rate of acid transfer to the surface, and rate of fluid loss from the fracture or wormhole. (Each of these processes is shown schematically in Fig. 1.) processes is shown schematically in Fig. 1.) Reaction kinetics are independent of the geometry in which reaction occurs; therefore, once kinetics have been determined for a given acid-rock system field treatments can be simulated by prediction of the rate of acid transfer to the surface and fluid loss to the formation. Unfortunately, experiments reported to dare do not allow determination of a kinetic model. SPEJ P. 406

Microemulsion Applications in Carbonate Reservoir Stimulation

“…As the Damköhler number was decreased and the dissolution structure changed to ramified wormholes and uniform dissolution, the depth of penetration decreased. This trend is consistent with experimental and theoretical results reported by several investigators, who report that wormholes typically penetrate a few feet (0.3-2 m) radially into the formation [12,13]. At these depths, wormholes stimulate the near-wellbore region (where the majority of flow resistance associated with the radial geometry is manifested) and provide an effective matrix acidizing treatment.…”

“…Early investigators recognized the significant influence of mass transfer on wormhole formation in limestone [12,13]. This influence has served as a basis for many of the theories describing wormhole formation and has led to models of wormhole formation that depend on dimensionless terms such as the Peclet number (ratio of rates of transport by convection to transport by diffusion) [14][15][16] and the Damköhler number (ratio of the overall rate of dissolution to the rate of transport by convection) [10,11].…”

“…However, the treatments are often constrained by the very rapid rate of reaction between HCl and carbonate rock, which results in limited depth of live acid penetration and ineffective flow capacity of the acid-dissolution channels [9][10][11][12][13][14][15][16][17]. The rapid HCl-carbonate rock reaction is limited by the rate of mass transfer of acid to the rock surface (diffusion limited) under common reservoir conditions [18,19].…”

Influence of transport and reaction on wormhole formation in porous media

“…At low pH (values less than about 4 at ambient temperatures), the rate of dissolution has been shohn to be masstransfer limited in a variety of acidic media ranging from HCI to pseudo-sea water (Nierode and Williams, 1971;Berner and Morse, 1974;Lund et al, 1975;Plummer et al, 1978;Sjoberg and Rickard, 1984). In strong acids such as HCI.…”

Pore evolution and channel formation during flow and reaction in porous media

“…An example of this channeling phenomenon in limestone is shown in Figure 1. Several studies have addressed the effects of flow and dissolution in carbonates (Nougaro and Labbe, 1955;Rowan, 1959;Nierode and Williams, 1971;Guin et al, 1971), all observing that permeability increases occur as a result of the highly branched channels, or wormholes, that form as a result of dissolution. These studies have not, however, been able to relate the effects of parameters such as the fluid velocity, the heterogeneous reaction rate, or the properties of the media, to the channel structure, the rate of propagation, and the increases in permeability.…”