Summary In this paper we describe a novel method for water unloading of natural gas wells in mature reservoirs experiencing low reservoir pressures. Current methods for water unloading from gas wells have at least one of the drawbacks of restricting gas production, requiring external energy, using consumable surfactants, or being labor intensive. The proposed design offers a new approach to water unloading that does not restrict or interrupt gas production. It can operate without external energy, and uses no consumables. Virtual and physical simulators have been developed and the full-scale version of the concept has been studied in test wells to demonstrate the feasibility and performance of the new water-unloading concept. An industrial-grade preproduction prototype was tested successfully in a test gas well to validate this study.
Introduction The electrical resistivity of dilute aqueous salt solutions has been studied for a number of years, but very few data exist on concentrations above 0.1 molar. Normal groundwaters commonly are near 0.1 molar, while most geothermal and oilfield fluids are at least several molar (Table 1). Thus, the interpretation of electrical measurements in geothermal areas at present is based mainly on extrapolation of lower temperature and lower concentration data. Such extrapolation may introduce serious errors into the interpretation of geothermal reservoir characteristics determined from electrical measurements. This paper presents new experimental data and an improved descriptive model of the electrical properties of brines as a function of temperature properties of brines as a function of temperature from 22 to 375 deg. C and concentration from roughly 3 to 26 wt% while under 31 MPa hydrostatic pressure. Data and models are given for brines composed of the chlorides of sodium, calcium, and potassium, and their mixtures. Comparison of the older log interpretation formulas to the new models illustrates an order of magnitude improvement in accuracy with an overall fit to within 2%. Resistivity Dependence Upon Temperature Some researchers have postulated that the electrical resistivity of fluid saturated rocks follows the temperature dependence of the saturating fluid in the absence of conducting minerals or significant surface conduction along altered pore walls. This assumption resulted from the success of a simple empirical formula relating the resistivity of a rock to the resistivity of the fluid filling the pores of the rock: Pr=FPw, where Pr= resistivity of clay-free, nonshale material that is 100% saturated, Pw= resistivity of saturating solution, and F= formation resistivity factor. A number of investigators have derived formulas that add the temperature of the saturating fluids. Experimental observations have shown that some rocks obey these formulas while others do not. 17–19 Part of the problem is the inadequate knowledge of Part of the problem is the inadequate knowledge of the resistivity dependence on temperature for the solution that fills the rock pores. We have found empirically that the best fit of the resistivity data to temperature is pw=bo+b1T-1 +b2T+b3T2+b4T3, where T is temperature and coefficients b are found empirically.
Summary Formation evaluation with well-log data is primarily a prediction tool for the purpose of identifying porous and permeable zones containing movable hydrocarbons. When the lithology is complex or the productivity is controlled by fracture density and the welllog data are limited in scope, underestimation of productive intervals can occur. This paper is a unique study on relating more than 28 years of actual production history to the initial well-log responses. This retrospective approach is the basis for presenting a diagnostic approach to predict producibility and reserves for other wells in the field under study or in other neighboring fields with similar geology. The focus of the study is well-log responses in the Monterey formation, offshore California. The geology of this formation is regarded as a prime example of a heterogeneous matrix system and a lithology-dependent fracture network. The lithology of the Monterey formation is heavily influenced by the diagenetic stage of biogenic silica (comprising the bulk of the geologic column), the detrital content, and the calcareous component. The particular field selected represents the typical lithological and diagenetic stages observed in surrounding fields. As such, the proposed approach has merits for application to other fields. Geographically, the Monterey type sediments extend from Baja California all the way along the Pacific Rim to Japan.1 Extension of the lithological patterns observed and the productivity correlations in surrounding fields can lead to eventual development of a more general correlation for widespread exploratory or development wells in this type of formation.
Abstract Dual fracture models are examined as a more realistic alternative to dual porosity models for the representation of naturally fractured reservoirs. A major component of the fracture system is the network of microfracture which by virtue of their lower permeability respond somewhat later than the macrofractures. A delineation of microfracture response versus matrix response is made using the proposed conceptual models. It is demonstrated that the microfractures response may at times be mistakenly attributed to matrix.
Summary A prediction method based on the use of performance history of a waterflood proposed in 1978 by Ershaghi and Omoregie is scrutinized here. Using a reservoir simulation approach, performance data for some hypothetical waterfloods are generated to test the application of the proposed technique to various floodpatterns, reservoir properties, and field operating conditions. Recently published results on the behavior of relative permeability curves for immiscible processes areused to substantiate the assumptions inherent in the proposed technique. The limitations of the technique are discussed and applications to some actual case studies are presented. Introduction Conventional waterfloods and modified waterfloodsusing various additives still constitute the bulk of the fluid injection projects active in the U.S. and elsewhere. During the history of a water injection project, reservoir engineers are expected to predict performance using the past response data. A literature review shows that over the last 40 years, there have been many techniques proposed for such prediction purposes. These techniques range from empirical correlations to various analytical models. In addition to these techniques, the advent of reservoir simulation has resulted in the availability of a very powerful tool for performance prediction. Many operators are still reluctant to use reservoir simulators because of inadequate reservoir data or insufficiently trained personnel to conduct simulation studies. Simple models often fail because of the inherentassumption as to the nature of the displacement mechanism or the misrepresentation of the real reservoir conditions. Many years of field and laboratory research by the petroleum industry and the academia has resulted in abetter understanding of the multitude of parameters influencing the efficiency of fluid injection projects. It is well established that for immiscible displacements, reservoir heterogeneity, relative permeability characteristics, fluid viscosities, and flood pattern are the most important factors. No prediction method can be used successfully in afield where the real reservoir is represented by laboratory-derived data and inadequately defined reservoir heterogeneity. A successful prediction technique requires input from the real reservoir performance. Alumped-parameter model that would embody all properties of the reservoir and the operating conditions can lead us to a realistic prediction of performance. In 1978, Ershaghi and Omoregie presented a technique for extrapolation of water-cut vs. recovery curvesin waterflood operations. The technique allows one togenerate a field composite relative permeability ratiocurve that includes reservoir properties as well as operational problems. The main assumptions were (1) the plot of log (krw/kro) vs. Sw is a straight line and (2) the leaky-piston displacement concept of Buckley and Leverett is applicable. Since Ref. 1 was published, many operators have contacted the authors with questions and comments about applying the technique to their specific cases-ranging from natural bottomwater drive to modified waterflood. Two additional papers about the technique haveappeared in the literature by others. This paper is aimed at clarifying ambiguities about the technique and providing guidelines for its application. Review of the Technique Assuming that log (krw/kro) vs. Sw is a straight line, the concepts of fractional flow and the frontal advance theory proposed by Buckley and Leverett may be used toderive the following relationship between the recovery and the fractional water cut: ER = (m . X) + n, where ER = recovery, X = ln[(1/fw) - 1] - (1/fw)fw = fractional water cut, m = 1/[b(1 - Swi)], n = -1/(1 - Swi)[Swi + 1/b ln(A)], A = a(mu w/mu o), and a and b from kro/krw = a ebSw. JPT P. 664^
Studies relative to some formation evaluation aspects of geothermal reservoirs are reported. The particular reservoirs considered were the liquid dominated type with a lithology of the sedimentary nature. Specific problems of interest included the resistivity behavior of brines and rocks at elevated temperatures and studies on the feasibility of using the well log resistivity data to obtain estimates of reservoir permeability. Several papers summarizing the results of these studies were presented at various technical meetings for rapid dissemination of the results to potential users. These papers together with a summary of data most recently generated are included. A brief review of the research findings precedes the technical papers. Separate abstracts were prepared for four papers. Five papers were abstracted previously for EDB.
Summary Computer applications in petroleum engineering education have expanded slowly during the last 2 decades. Among the many factors hindering progress have been budgetary constraints of universities, lack of faculty support, interest, and knowhow, and inadequate professionally designed software. With the advent of personal computers professionally designed software. With the advent of personal computers and the ready accessibility of minicomputers, educational use of computers is now bound to increase rapidly. Microprocessors present the practical possibility of improving significantly the training and productivity of possibility of improving significantly the training and productivity of current and future petroleum engineers. Coordinated effort by academic institutions, along with active cooperation by SPE, could substantially increase the extent to which this potential is realized. Introduction Our knowledge of the evolution of computer use in petroleum engineering education is based on a hazy petroleum engineering education is based on a hazy recollection of incomplete data and should be viewed in that context. Universities have not spearheaded the expanding use of computers to solve complex petroleum engineering problems. In fact, the incorporation of computer training problems. In fact, the incorporation of computer training into petroleum engineering curricula has been pulled along by industry's expanding use of computers. The reason for this is that in the 1960's very few petroleum engineering faculty members were qualified by experience or training to operate in, let alone lead, such a technological development. Furthermore, although a few faculty members may have envisioned the need for computer application, the large sums needed to support the required interdisciplinary effort were simply not available. The mixture of talents needed to push ahead was brought together in the research laboratories of some of the larger oil companies. Applied mathematicians and theoretical physicists were put to work alongside petroleum and chemical engineers, with the goal of developing the methodology and knowledge needed to take advantage of the evolving capability of stored-program computers. Only a few years were required for these efforts to generate practical results. The first full-scale oilfield simulation study was the modeling of Aramco's Safaniya field and was completed in 1961. Developments after that date were rapid. By the end of the decade, the volume of computerized petroleum engineering calculations was already very large. Until the advent of the microcomputer in the 1980's, computer use was overwhelmingly dominated by two applications: reservoir simulation and economic evaluation. In this paper, we review historical trends and attitudes of petroleum engineering schools toward computer applications, discuss the state of the art, and suggest a syllabus to take advantage of the potential benefits of computer-aided instruction (CAI) and computer-aided design (CAD) in petroleum engineering education. Historical Trends The growth of computer use in petroleum engineering education has generally lagged the expanding industrial use of computers. The 1960's through 1973 were years of retrenchment for petroleum engineering schools. Fortunately, some of the growth in industrial computer use did spill over into various schools. Some of this spillage resulted from industry practitioners acting as part-time instructors (e.g., T.D. Mueller at Stanford U. and H.K. van Poollen at Colorado School of Mines). In a few instances, cross-fertilization came from professors working some months in industry (e.g., C. Weinaug at Esso Prod. Research Co.) or experienced engineers moving Prod. Research Co.) or experienced engineers moving from industry into academia (e.g., R.A. Morse to Texas A and M U.). The one outstanding exception to the directionality of flow was the Keith Coats blitzkrieg through Austin, Texas and into the consulting business. The crude-oil pricing revolution in Oct. 1973 ushered in a golden decade for petroleum engineering education. Practically overnight, industry's demands for petroleum Practically overnight, industry's demands for petroleum engineers far exceeded even the most dedicated educators' bountiful dreams. Starting salaries skyrocketed. Classes overflowed. Because academic salaries were lower than in industry, retaining an adequate faculty was difficult and, in some cases, impossible. Most schools filled the breach with greater use of part-time faculty. This action did not ordinarily provide the ability to expand the course content to incorporate computer use to the extent desired. JPT
Summary This paper presents a methodology for the geostatistical estimation of reservoir properties to handle uncertainties in observation and modeling. Given certain known well-log data in a geological region, the Kriging methodology is used to estimate or predict spatial phenomena at nonsampled locations from the estimated random function. The approach assumes that the data are accurate and precise, and the random function is generated from a thorough descriptive analysis of the known data set. Regarding the assumptions considered in classic Kriging, it is realistic to assume that spatial data contain a certain amount of imprecision, mostly because of measurement errors, and information is lacking to properly assess a unique random-function model. A methodology is presented for the geostatistical estimation of reservoir properties to handle uncertainties in observation and modeling. A combination of regular, or classic, Kriging and the fuzzy-logic method is proposed. As such, imprecise input data and variogram parameters are modeled on the basis of fuzzy-logic theory, while the predictions and variances are computed from Kriging analysis characterized by membership functions. Last, an optimization method is included to solve the constrained fuzzy-nonlinear-equation system. The proposed methodology was implemented, and a user-friendly integrated tool was developed, which enables the user to create a grid structure on the basis of the input data, conduct statistical analysis, and run fuzzy Kriging for various problems. We used the tool to run a test case using the SPE 10 (SPE Comparative Solution Project, Model-II 2000) porosity data. With the fuzzy-Kriging methodology, two maps are generated with upper-bound values and lower-bound values. Compared with true data, the upper-bound map trends to include higher values better, while the lower-bound map trends to include lower-value parts better. In addition, a case study has been conducted using measured core-permeability data in a heterogeneous reservoir to demonstrate the viability of the technology.
