The determination of the existence and composition of azeotropes is important both from theoretical and practical standpoints. An important test of the veracity o f thermodynamic models is whether or not known azeotropes are predicted, and whether or not they are predicted accurately. Model parameters can be ne tuned by comparing the model predictions can be used as starting points for experimental searches for actual azeotropes. These azeotropes often present limitations in process design which m ust be known, and their determination strictly from experiment alone can be expensive.There are two main di culties in solving the problem. The rst is the fact that the equations derived from most thermodynamic models are highly nonlinear, which m a y make nding any azeotrope a nontrivial exercise. In addition, there is the question of whether or not all of the azeotropes have been found, or of being certain that there are no azeotropes if none have been found. Because of these di culties there has been much recent i n terest in the reliable computation of azeotropes, focused primarily on the prediction of homogeneous azeotropes. For example, Fidkowski et al. 1993 have present a homotopy continuation method for the calculation of homogeneous azeotropes. The primary drawback of this technique is that it can not guarantee that all azeotropes have been found. More recently, Harding et al. 1997 have reported a global optimization procedure based on a branch and bound approach using convex underestimation functions that are continuously re ned as the range where azeotropes are possible is narrowed. This technique does provide a guarantee that all azeotropes have been found. Harding et al. 1997 have developed appropriate convex underestimating functions for several speci c thermodynamic models.We describe here a new approach for reliably nding all homogeneous azeotropes of multicomponent mixtures. The technique is based on interval analysis, in particular the use of an interval Newton generalized bisection algorithm. The method can determine with mathematical certainty all azeotropes for any system. The technique is general purpose and can be applied in connection with any thermodynamic models. No model speci c convex underestimating functions need be derived. In this paper, the technique is described in detail, and then tested on several problems. Results of the test problems indicate that the method can e ciently and reliably determine all homogeneous azeotropes for multicomponent mixtures.

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Recently, a robust new computational technique, based on interval analysis, has been developed for solving the difficult nonlinear problems arising in the modeling of phase behavior.This technique can be used, with mathematical and computational guarantees of certainty, to find the global optimum of a nonlinear function or to enclose any and all roots of a system of nonlinear equations. As shown in the applications here to phase stability analysis and to the location of homogeneous azeotropes, it provides a method that can guarantee that the correct result is found, thus eliminating computational problems that may potentially be encountered with currently available techniques. The method is initialization independent; it is also model independent, straightforward to use, and can be applied in connection with any equation of state or activity coefficient model.

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This paper describes a very successful acid stimulation treatment performed in AGIP's Trecate-Villafortuna Field. The matrix acidizing treatment used in-situ crosslinked acid (ICA) as the diverting agent. The treatment is unique because it represents the highest temperature application ever attempted for such a system and falls under the definition of high-pressure high-temperature (HPHT). The design process included temperature simulations, detailed laboratory testing, and a review of acid formulations that were used successfully in the Trecate-Villafortuna Field and elsewhere. Temperature simulations indicated that cooldown from the bottomhole temperature (BHT) of 180°C to at least 150°C could be achieved despite the high treating pressures that limited injection rates. Even after cool down, serious concerns about corrosion and the effectiveness of the ICA system still existed. Laboratory support included fluid optimization for high-temperature application of the ICA. The flow tests enabled the selection of the most appropriate base acid systems and demonstrated that the ICA system would indeed function at the predicted high temperatures. Success of the treatment must also be attributed to the operational planning and close attention to experience gained from previous stimulation treatments. The execution of the treatment used all of the components considered to be state-of-the-art in matrix acidizing treatment execution and evaluation: pre stimulation injection tests, spotting of acid with coiled tubing (CT) to help reduce injection pressures and improve zonal coverage, the use of the Maximum Pressure Maximum Rate Diversion Technique (MAPDIR), and real-time treatment pressure monitoring. The paper will present job procedures and a detailed treatment pressure analysis. It will also give details on the changes in injectivity and the Productivity Index (PI) before and after stimulation. Introduction The HPHT Trecate-Villafortuna well discussed in this paper produces oil from a naturally fractured dolomite reservoir at a depth of 6000 m. In this well, a new horizontal 220-m section was drilled and completed as open hole. The goal of the acid treatment was to remove the near-wellbore mud damage and to improve the permeability of the horizontal drain. The high pressure at 6000 m and the bottomhole static temperature (BHST) of 182°C, classify the acid treatment as HPHT. During the treatment design phase, two major issues had to be addressed:Potential problems associated with the HPHT character of the well: high acid-rock reaction rate, cross linking chemistry, and corrosion of tubular goodsProper diversion and optimal zonal coverage of the entire 220-m payzone High-temperature acidizing poses a number of problems during treatment design and execution, which are not normally encountered during treatments at lower temperatures.1,2 The high acid-rock reaction rate requires the use of a retarded acid system to ensure that acid will not all spend on the formation face (compact dissolution) but will penetrate deeper into the formation. Protecting the tubulars against acid corrosion is another serious challenge at high temperatures and requires careful selection of the acid fluids and inhibitor package design. In the following sections of this paper we will discuss these issues in more detail.