Lactic acid has been examined in various laboratories and applied in the oil field for acid fracturing and drilling-fluid-filter-cake removal, and as an iron-control agent during acid treatments. However, the reaction of lactic acid with calcite has not been addressed before. Determination of the reaction rate and the acid-diffusion properties is a critical step for successful treatments in matrix acidizing and acid fracturing. Therefore, the objective of this work is to conduct a detailed study on the reaction of lactic acid with calcite. Mass transfer and reaction kinetics are reported for the lactic acid/calcite system by use of the rotating-disk apparatus.Disk samples were cut from Indiana limestone or Silurian dolomite and were used in the reaction-rate experiments. The effect of lactic acid concentration (1, 5, and 10 wt%), temperature (80-250 F), disk rotational speed (100-1,800 rev/min), and different inorganic salts on the reaction rate was investigated. The diffusion coefficient of 5 wt% lactic acid was determined at low disk rotational speeds and reported at 80, 200, and 250 F. A model that accounts for the effect of the kinetics of the surface reactions and the transport of reactants and products was developed. The activation energy and the rate constant at 80, 150, and 250 F for the reaction of lactic acid with Indiana limestone were reported. Reaction experiments of lactic acid with dolomite at 150 F over disk rotational speeds of 100-1,800 rev/min, and at 1,500 rev/min over a temperature range of 80-250 F, were conducted and the results were compared with those obtained for the calcite reaction.At 80 F, the reaction of lactic acid with calcite was controlled by mass transfer at low disk rotational speeds (up to 500 rev/min) and was surface reaction limited at higher speeds. At higher temperatures (150, 200, and 250 F), both mass transfer and surface reaction influence the overall calcite dissolution. The kinetics of the surface reaction were influenced by both forward and backward reactions. At 80 F, the surface reaction contributes to 28% of the overall resistance. This dependence becomes much less (13 and 10%) at higher temperatures (150 and 250 F, respectively).The reaction of lactic acid with dolomite at 150 F was mainly controlled by mass transfer up to 1,000 rev/min and by the kinetics of the surface reaction after 1,000 rev/min. At 80 and 150 F, the rate of reaction of lactic acid with calcite was an order of magnitude higher than that with dolomite. At temperatures of 200 and 250 F, the rate of reaction of lactic acid with calcite is twice the rate of reaction with dolomite. The presence of Ca 2þ , Mg 2þ , and SO 2À 4 ions reduced the reaction rate, which is most likely because of the reduction in the concentration gradient of the products. The reduction in the concentration gradient will cause a reduction in the rate of diffusion of the generated calcium away from the surface, and hence a lower rate of dissolution.

Acid stimulation of horizontal wellbores in high temperature and/or low permeability formations is complicated due to the need to balance acid reaction rates with the rate of formation penetration. At high bottom hole temperatures, uncontrolled acid spending rates result in inefficient or even damaging treatments. It is advantageous to add materials which slow the reaction of HCl or use alternative acids which are useful as alternative stimulation fluids, such as organic acids and chelants due to their inherently lower reaction rates. The slower reaction rate with carbonate minerals allows for optimal wormhole growth at the limited rates available in long horizontals. This study investigates the complementary use of acid or chelant systems together with a microemulsion delivery system to achieve the desired performance. These delivery systems can significantly increase the efficiency of HCl and chelant stimulation treatments of low-permeability carbonates by optimizing diffusivity and minimizing pressure to cleanup.Horizontal core flow experiments were conducted by injecting solutions of 15 wt% HCl and 10 -20 wt% alternative acids through low permeability (Ͻ 10 md) limestone cores. High permeability cores were also investigated by injecting 15 wt% HCl through high permeability (150 -200 md) Indiana limestone cores. These experiments were conducted with and without the addition of a microemulsion additive and at core injection rates from 0.2 -20 cm 3 /min corresponding to 0.25 to 5 BPM per 100 ft of a horizontal wellbore.Significant performance advantages were found with the addition of the microemulsion delivery system. The treatment volume required for wormholes to propagate the length of the test core was reduced at all flow rates at or above the optimum rate. Core penetration was more efficient when using the microemulsion, without an increase in pumping pressure. Initiation and extension of wormholes was improved, as compared to the experiments without the microemulsion, in which more conical wormholes tended to form at the inlet face. The reduction in treatment volume necessary to reach breakthrough with the microemulsion present also suggested a more efficient treatment.The complementary use of a microemulsion delivery system with acids and chelants has been shown to enhance acid stimulation treatment potential in long horizontal wellbores with high temperature or low permeability formations. This system can be used to optimize wormhole penetration while maintaining an optimum injection rate across long horizontal intervals where the maximum achievable injection rate is relatively low.

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Matrix acidizing of carbonate reservoirs presents a challenge in the middle east as the horizontal legs become longer some now as long as 30,000 ft with some reservoirs at temperatures above 300 F. In this work we have investigated the diffusivity of acids and alternative acids with various additives vs temperature using a rotating disc apparatus. The pore volume to breakthrough (PVBT) was then measures on various carbonate cores such as Indiana Lime and cores form various Middle East formations. To show the impact of acid and additive selection we have built a horizontal wellbore simulator to predict the impact of lithology, acid type and various additives upon diffusivity and productivity.