Fluid-Velocity and Reaction-Rate Effects During Carbonate Acidizing: Application of Network Model

Abstract

Summary Coreflood experiments were performed and a network model developed to study flow and matrix dissolution in carbonates during acidizing treatments. Wormholing in carbonates is controlled by the relative rates of flow and dissolution, which are characterized by a Damkohler number, NDa, for the process. Acid penetrates only as a result of wormholing, and well-defined channels will not form if NDa is either very high or very low. Effects of acidizing differ markedly between limestone and dolomite because of differences in the mineral dissolution rates. Introduction Carbonate reservoirs commonly undergo acid stimulation treatments either to remove near-wellbore damage (matrix treatments) or to create artificial flow channels in low-permeability media (fracture treatments). Matrix treatments can be effective if fracturing is undesirable, such as in water-injection wells or near strong aquifers for where fracturing is ineffective, such as in soft chalks. But acid consumption is usually so rapid that matrix effects may be limited to the very-near-wellbore area. In low-permeability reservoirs, fracturing is a more common stimulation method. Fractures greatly extend the distance of acid penetration, and the result is improved treatment effectiveness. The productivity or injectivity increases realized from matrix treatments in carbonates depend directly on the depth to which live acid penetrates and to which damage is removed away from the wellbore. While matrix acidizing can be very effective in sandstones where significant acid penetration is possible, the depth of penetration in carbonates is typically extremely limited because acid is consumed rapidly, before significant penetration is possible. Estimates of the effective radius of the stimulated zone in carbonates range from 1 to at the most several feet [0.3 to ∼ 1 m], although damage beyond these distances certainly can severely affect the performance of a well. Acid is consumed more rapidly during matrix treatments in carbonates than in sandstone for several reasons. First, the carbonate dissolution rates are typically very fast. Limestone dissolution (Eq. 1) is mass-transfer-limited at temperatures above 32 degrees F [0 degrees C], while dolomite dissolution (Eq. 2) is mass-transfer-limited above about 302 degrees F [150 degrees C]: ..........................(1) and .........................(2) Second, the amount of material available for dissolution is much greater in carbonate rock, which is usually nearly pure. In sandstone, only interstitial materials are dissolved while the quartz itself is quite inert. Finally, matrix treatments in carbonates are usually performed only when fracturing is undesirable or ineffective. If injection pressures must be low to avoid fracturing, the low fluid velocities result in decreased acid penetration A more common stimulation treatment in carbonates is fracture acidizing. Fracturing is often necessary because carbonate matrix permeability may be very low or even absent. During fracture treatments, a high fluid injection pressure is necessary to create the fracture and to maintain fracture conductivity during injection. High fluid pressure in the fracture results in acid flow, or leakoff, from the fracture to the surrounding rock. Dissolution resulting from leakoff increases the rock permeability near the fracture face. Thus, if uncontrolled, leakoff can become more and more severe as injection continues. Leakoff limits the distance down the fracture to which live acid can reach, thus reducing treatment effectiveness. Acid attack and fluid leakoff at the face of a fracture during a fracture-acidizing treatment are completely analogous to near wellbore dissolution during matrix acidizing. Several studies have looked at the effects of matrix acidizing in carbonates, each emphasizing that permeability increases because channels, called wormholes, form in the rock as a result of dissolution. Fig. 1 is an example of a wormhole in a limestone core. Wormholing is the primary mechanism of permeability increase in carbonates and thus controls both matrix treatment effectiveness and acid leakoff from fractures. To predict the response of a well to a matrix acid treatment, to optimize treatment design, and to control leakoff from fractures effectively, it is important to know what factors affect wormholing. Currently, little is known about wormholing, including both the magnitude of the permeability increases and under what conditions and to what extent wormholing will occur. Early work in this area predicted wormholing qualitatively. A more recent experimental and modeling study claimed to predict wormhole behavior quantitatively. The experimental results in this case are based on a plaster/water system, and the process modeling was highly simplified. A constant rate of wormhole propagation was predicted in the linear system, and this is not what is observed in acid/ carbonate systems. Agreement was reported between the model and the plaster results, indicating that this experimental system did not properly mimic the more complex behavior of the actual acid/ carbonate system. Experimental Laboratory coreflood experiments were performed to simulate reservoir acidizing conditions and to create acid-etched channels for further study. Indiana limestone with 2- to 5-md permeability and two types of dolomite with 1- and 5-md permeability were used. Aqueous HCl was injected axially through cylindrical cores of each material at a constant rate, and partial dissolution occurred according to Eq. 1 or 2. Dissolution resulted in a decrease in the fluid pressure drop across the core. and the permeability was calculated as a function of the fluid volume injected with Darcy's law. A Ruska proportioning pump was used to ensure absolutely pulse-free fluid injection. Effluent pH was monitored to track acid breakthrough, in some cases, effluent mineral concentrations were also monitored. The experiments were conducted at room temperature, and a minimum pressure of 1,000 psi [6890 kPa] was maintained in the core by including a backpressure regulator downstream of the core. High backpressure is required to maintain single-phase flow as gaseous reaction products are generated. The exact experimental technique and the apparatus used are described elsewhere. Acid concentrations studied were 0.125 N and 1.0 N aqueous HCl. Bulk acid concentrations higher than 1.0 N were not used because of the excessive pressure required to keep the CO2 in solution. The size and number of wormholes visible on the flow faces indicate that both the fluid injection rate and the rate of mineral dissolution have a strong effect on stimulation efficiency and on channel structure. To further the study of these structural effects, metal castings were made of the channels. The acidized cores were dried, and molten Wood's alloy was then injected to fill the channels. SPEPE P. 56^