In a matrix acidizing treatment, it is extremely important that the appropriate acid formulation be used, that no damage be left in the reservoir, and that no excess acid be injected. Nowadays, this can be easily accomplished with the aid of computerized real-time monitoring techniques, which provide on-site evaluation of the stimulation results. We applied a real-time monitoring technique to a series of acidizing treatments to evaluate stimulation performance and to optimize subsequent treatments in the same area. From the measured surface pressure and injection rate, a computer program calculated the bottomhole pressure and estimated the skin factor as the treatments progressed. In this way, the responses of the wells to stimulation were evaluated. Real-time monitoring of the evolving skin factor showed when further acid injection resulted in no further stimulation, leading to reduced acid volumes in subsequent treatments. By applying this real-time monitoring method, we have improved acidizing practices in two fields, located in the northern part of Brazil. In a series of injection well treatments, acid volumes were reduced by 25% from standard practice, with no loss in acidizing efficiency. We were also able to change one of the acid compositions from regular mud acid to a more compatible 6% HCl/l.5% HF. In more sensitive production well treatments, the real-time monitoring of skin factor may be even more useful by ensuring total damage removal or in identifying deleterious effects of overtreatment. More importantly, this technique will also indicate when design changes such as increased acid volume, different acid formulation, or different injection rate schedule are needed to reach the treatment objectives. Introduction For many years, the design of acid volumes and concentrations for a matrix acidizing treatment could not be evaluated in the field. The only indication of success was the increase in rate, because well tests were seldom performed and no one could tell whether or not the damage was completely removed. Nowadays, there are several methods to monitor damage evolution during acid injection, aimed at stopping it as soon as damage remove has been completed; i.e., when the damage ratio equals one or skin factor equals zero. The model used in this work was developed by Hill and Zhu. It consists of plotting the inverse injectivity versus the superposition time function, from which the skin factor may be derived. To apply this technique, a simple program, called UTRTM (University of Texas Real-Time Monitoring) was developed. It monitors the skin factor in real-time, using input data such as formation permeability and initial skin. This method allows the operator to take control of the treatment and to decide when to stop acid injection or if it is necessary to increase the volume initially designed. Besides, the results of one stimulation help improve the design of subsequent treatments in the same field, in terms of volumes and concentrations. This way, the introduction of this technology ensures the fulfillment of matrix acidizing objectives and fosters the selection of the most adequate formulations for the various types of formations that require acidizing Case Histories Bottomhole Pressure Calculation. The method relies on the accurate calculation of bottomhole pressures from surface pressure. Friction losses and the hydrostatic pressures of each phase inside the tubing are accounted for, using the values of density and viscosity supplied by the user. To do so, the position of each injected fluid inside the tubing is tracked by the program, as illustrated in Fig. 1. The bottomhole pressure is given by (1) where hj and fj are the hydrostatic and frictional pressure drops of sequence j, ps is surface injection pressure, and pwf is bottomhole pressure. P. 573^

Recent models for sandstone acidizing have shown that the secondary reaction of fluosilicic acid (H2SiF6) with clays plays a significant role in the removal of formation damage. The conventional wisdom is that possible precipitation during the secondary reactions can adversely affect treatment success. However, this paper presents laboratory experiments with Berea cores and two Brazilian sandstone cores, as well as with field tests, that show that fluosilicic acid can be injected, by itself, into a sandstone reservoir without causing any damage. In fact, mixtures of fluosilicic acid, with a proper amount of hydrochloric acid (HCl) or an organic acid such as acetic acid (HAc), have been able to improve the performance of two Brazilian water-injection wells by removing clay damage. These two wells, which were injecting 11 and 15 m 3 /d, respectively, are sustaining injection rates at or above the desired quota of 30 m 3 /d 5 months after the treatments. The treatments were monitored with a real-time monitoring program, which showed that the skin factor has dropped from about 30/40 to zero. Besides having a lower cost than conventional hydrofluoric acid (HF) mixtures, the main advantage of the new acid system is that it reacts mostly with clays and feldspars, but it almost does not react with quartz. Thus, it can dissolve damage without weakening the rock structure, which makes it suitable for deep damage removal. The low cost of H2SiF6, which is a byproduct of sodium fluoride manufacturing, also makes it a viable option for routine acidizing operations that normally would require the use of an HF acid system.

The northeastern region of Brazil is the largest onshore oil producer in the country. However, regional oil production is still much less than offshore production in other Brazilian basins. Therefore, customized low-cost technologies are routinely employed locally to deal with issues, such as perforation plugging, scaling, and sediments production, for example. Heavy oil produced in this area builds up paraffin and asphaltene deposits over time, further decreasing productivity of the fields. This paper discusses applying a true fluidic oscillator (TFO), an enhanced rocker tool (ERT) that synergistically combines fluidic, acoustic, and chemical effects, to enhance the action of a fluid treatment on the formation. Usually, this tool is used in conjunction with coiled tubing (CT) equipment. Because the operator considered a CT operation too expensive for the needs of this region, the tool was run into the well with a regular work string as a lower-cost alternative. Three similar wells in the Potiguar Basin were selected and treated with diesel and butyl glycol through the TFO with significant improvement in estimated permeability and oil production (an increase of 20% to 240%) after treatment. Overall, the tool enhanced the treatment effectiveness by amplifying the contact area between fluids and formation. This demonstrated the applicability of this cost-effective solution for stimulation of low-productivity wells.

TX 75083-3836, U.S.A., fax 01-972-952-9435. AbstractRecent models for sandstone acidizing have shown that the secondary reaction of fluosilicic acid (H 2 SiF 6 ) with clays plays a significant role in the removal of formation damage. The conventional wisdom is that possible precipitation during the secondary reactions can adversely affect treatment success. However, this paper presents laboratory experiments with Berea cores and two Brazilian sandstone cores, as well as field tests, which show that fluosilicic acid can be injected, by itself, into a sandstone reservoir without causing any damage at all. Quite the contrary, mixtures of fluosilicic acid with a proper amount of hydrochloric acid (HCl) or an organic acid such as acetic acid (HAc) have been able to improve the performance of two Brazilian water injection wells by removing clay damage. These two wells, that were injecting 11 and 15 m 3 /d, are sustaining injection rates at or above the desired quota of 30 m 3 /d, five months after the treatments. The treatments were monitored with a real-time monitoring program, that showed that the skin factor has dropped from about 30/40 to zero. Besides having a lower cost than conventional HF mixtures, the main advantage of the new acid system is that it reacts mostly with clays and feldspars, but almost does not react with quartz. This way, it can dissolve damage without weakening the rock structure, what makes it suitable for deep damage removal. The low cost of H 2 SiF 6 , which is a byproduct of sodium fluoride manufacturing, makes it also a viable option for routine acidizing operations, that would normally require the use of an HF acid system.

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