This paper presents the results of extensive technical and statistical analyses of more than 650 matrix stimulation jobs. Seventy-eight failures were identified with absolute reliability by second stimulations successfully repeated on the same pay zones of the same wells. Only one significant parameter of the stimulation design differentiated the first failed stimulations from the second successful ones. This paper illustrates these failure/success factors and gives novel field-stimulationresponse graphs.

This paper describes the theory and application of a new matrix stimulation diversion technique, maximized pressure differential and injection rates (MAPDIR), that uses injection rate as the key parameter to obtain sustained, planned bottomhole differential pressure, t.p, levels. The technique, applicable wherever Darcy's law is valid (in ·sandstones and in unfractured or microfractured carbonatic reservoirs), favorably replaces diverting agents and extends to long intervals (> 100 ft) the main advantage provided by mechanical zone isolation for "selective stimulation," which is maximized t.p. The use of slightly viscosified stimulation fluids can further extend the range of application and improve the economics by reducing both the amount of hydraulic power and the fluid volumes required for stimulation. A multilayered reservoir case illustrates the application ofthe MAPDIR technique. A new log-log plot of skin evolution vs. injected-acid volume shows significant damage removal trends and provides important information for matrix stimulation treatment optimization. Field experience includes the successful treatment of horizontal pay intervals of up to 460 ft.Because the real objective of diversion techniques is the effective removal of damage from the whole exposed pay, the meaning of"diversion" in this paper is not narrow (i.e., applicable only to diverting agents and ball sealers) but includes "placement" and "coverage" techniques, such as mechanical isolation, viscosified acids, and MAPDIR.

Most of the failures of matrix stimulation treatments can be imputed to incorrect field procedures. To improve the success ratio of the jobs, the tailoring of field stimulation procedures must be strictly linked with the clear identification of the target and with the optimization of the design. Objectives are set to meet different well situations, and require specific design and field procedures. Maximizing the injection pressure and rate during matrix stimulations has proven to be a very successful technique, particularly effective over extensive pay zones. Introduction Many interesting theoretical and laboratory studies aimed at optimizing the matrix stimulation treatments and their success ratio have been carried out in the past. Deep consideration has been given to the knowledge of the chemical reactions between the numerous components of the involved systems, and the various types of formation damage which may occur. Also close attention has been paid to optimizing the composition of the stimulation fluids with respect to the reservoir rock lithology, by means of proper selection of additives and by considering the effects of temperature and other important chemical and physical parameters. Such studies have resulted in a great benefit to the potential of matrix stimulation treatments. However, inadequate field procedures may frustrate the technical and economical efforts of the engineers, geologists, lab researchers involved in the optimization of the design phase. Field experience combined with an accurate post-job evaluation of more than 500 jobs performed over the past 8 years, has shown that out of 53 well documented failures, 41 were due to incorrect field stimulation procedures (Tables 1, 2). Often in the field the program of a stimulation job is handled as a recipe, the job itself is performed with lack of motivation. This is probably due to a misunderstanding of the ultimate goal; thus, some important items requiring additional rig time (such as wellbore preparation, tubing cleaning, availability of a coiled tubing unit at the well site, proper placement of the acid, use of correct injection procedures, quality control) are neglected because of "short term" economic considerations. Unfortunately, "long term" economics are often adversely affected by this type of philosophy. P. 111^

The paper presents an innovative approach to the development of a knowledge-based system aimed at support matrix stimulation design. The basic role played by preliminary context evaluation is enhanced. Context evaluation Involves assessing whether or not the stimulation must be carried out, through technical-economical feasibility analysis and risk assessment. A conceptual model and a system architecture have been specifically defined for this domain. allowing fast and expressive system prototyping. Extensive exemplifications of system reasoning performances are provided. performances are provided. Introduction The large internal demand for matrix stimulation jobs compared to the small number of expert available. the failures caused by lack of skill and inadequate operation control have led to the decision to develop a tool devoted to the broad support of matrix stimulation activity. The target objectives of such a tool are as follows: 1 - outline of detailed procedures for matrix stimulation planning, for selection of chemicals, equipment and service companies and for supplier and service quality control; 2 - data analysis and post-job evaluation; 3 - support for less-experienced users to avoid wasteful and uneconomical matrix stimulations and to design at expert level those which are necessary and cost effective. Knowledge-based systems (KBS) offer a solution to reach those goals. They are well-suited for domains of application where expertise plays a fundamental role In achieving optimal results. Therefore the huge amount of expertise involved in matrix stimulation design makes it the typical kind of domain where this computer technology could be successfully applied. Of the few KBS that have been previously developed in this field, all have highlighted the complexity of the domain. Cram et al. (1986) first Investigated the possibility of using KBS for matrix treatment design. They focused mainly on the selection of optimal chemicals, disregarding the process design. Alegre et al. (1988) dealt with formation damage diagnosis by defining classes of causes, mechanisms and related types of damage. Jia-li Ge et al. (1989) similarly approached damage diagnosis by focusing mainly on handling data uncertainty, using fuzzy logic. In this paper we present MAST (MAtrix Stimulation Treatment), a knowledge-based system developed for improved damage diagnosis and matrix treatment design MAST not only helps with correct damage diagnosis and supports the selection of chemical types and quantities, but it also evaluates economical opportunities and plans the appropriate operations sequence. We recognize that the availability of a domain expert is a crucial point in making a KBS fully reliable. Therefore MAST capabilities have been enhanced by the adoption of a design and evaluation method developed in-house: the key factors increasing the success ratio of matrix stimulations jobs have been identified extensive statistical analysis on more than 650 matrix stimulation jobs. carried out over the past 11 years in 9 countries, allowed us: to understand the reasons for past 11 years in 9 countries, allowed us: to understand the reasons for failures, to obtain a full scale lab (i.e. field) response, to compare lab-test Information with field Indications, and to identify and validate specific techniques aimed at optimizing matrix stimulation design and performance. performance. Currently MAST is the prototype stage and it covers only damage diagnosis. The validation phase is underway and. at the present time, the system has been successfully tested on a present time, the system has been successfully tested on a significant set of field cases. The system is implemented using a commercial knowledge engineering environment, based on an object-oriented paradigm and Common-Lisp programming language. The user interacts with the system using a mouse and programming language. The user interacts with the system using a mouse and a window-based interface. 2 PROBLEM SOLVING IN MATRIX STIMULATION The MAST architecture, shown in fig. 1, reflects the complex line of reasoning used by the expert for optimum matrix stimulation design The expert considers the following groups of Input data: 1 - well objective (i.e. production and/or recovery optimization, economical production rate, etc.); 2 - well data (i.e. bottom-hole flowing pressure. bottom hole temperature, net pay thickness, etc.); 3 - field and history data (i.e. reservoir lithology, mineral contents, static pressure, permeability. porosity, previous treatments. production history); 4 - laboratory results (i.e. acid sensitivity, wettability, formation packing, formation cementation, etc.).

This paper presents a knowledge-based system (KBS) for the identification of problems in producing wells. The KBS is based on a general Well Problems Analysis (WPA) methodology that entails three main activities: verification of a problem's existence, identification of a preliminary set of possible problems, refinement of the most plausible problem(s). The relevant experts' knowledge has been formalized by means of a classification approach based on the organization of all problems in a hierarchy of classes and, by associating the corresponding symptomatology with them. The system, validated at the company headquarters, is currently in use in the operating districts. The system is a part of a more comprehensive project, called PROGRESS (pRoduction Optimization inteGRated Expert SystemS), aimed at providing a decision support system for well production optimization.