We performed laboratory measurements to determine whether NMR logging would improve log evaluation of the Mubarraz Field. The Mubarraz Field is a carbonate field in offshore Abu Dhabi, that produces from the Cretaceous Thamama Group. Existing log evaluation using classical logs has some shortcomings. Some zones that are not highly resistive actually produce water-free oil. This is because Mubarraz samples often have a large amount of microporosity. The microporosity occurs because samples are highly micritized, and grain-sized particles are generally peloidal with internal microporosity. The microporosity holds a large volume of capillary-bound water, which results in low resistivities in zones that still have producible oil in large pores. An additional evaluation problem is that permeability varies widely while porosity remains almost constant. As a result, permeability vs. porosity correlations are less valuable than normal. In laboratory measurements on Mubarraz samples, we found that NMR relaxation measurements can provide:–good estimation of the capillary-bound water volume at an air/water capillary pressure of 25 psi, using a cutoff on NMR T2 relaxation time of 190 ms.–good identification of the grainstones as opposed to the lower-permeability textures (packstone… mudstone) with a T2 threshold of about 225 milliseconds.–estimates of permeability that are significantly better than can be obtained from porosity alone. NMR relaxation measurements contain information about the pore size distribution, and it is this information that permits estimation of production-governing parameters. The estimators are based on parameters that can be obtained from continuous NMR logging. P. 477
TX 75083-3836, U.S.A., fax 01-972-952-9435. AbstractInterval Pressure Transient Testing (IPTT) with an advanced formation tester has been widely applied to measure vertical and horizontal permeabilities. The principles behind IPTT may appear straightforward. But the interpretation of IPTT in complex reservoirs, such as carbonates presents many challenges, most of which are related to uncertainties arising from the heterogeneous nature of such reservoirs.A novel, generally applicable approach is proposed for interpretation of IPPT tests. The approach is integrated and efficient, and ensures that the most probably valid and/or valuable information is used and revealed by IPTT. The approach standardizes the complete interpretation procedure of IPTT in a heterogeneous reservoir, using if available, modern wireline logs (such as NMR and Imaging), dynamic data from wireline formation testers and any other relevant information (such as geological description, core data and local knowledge) as constraints on the interpretation. IPTT is also a multi-layer testing technique. An iterative method to define layering is incorporated in the approach to determine the most probable layering. Many types of possible information that can be used to constrain layering definition are classified. Principles and guidelines on using different sources of information to define and refine the layering are recommended. An advanced regression technology is used to get optimized horizontal and vertical permeabilities of reservoir layers. The key is to find as much useful information as possible to guide the regression. To this end, different sources of permeability are classified and used as initial estimates, for input to the regression. Practical skills to perform IPTT regression are also summarized. In many cases, the operating sequences of the formation tester are primarily designed for downhole sampling rather than IPTT. The complicated downhole sampling sequences can make the pressure responses of gauges complicated and noisy. Hence, a Quality Check (QC) procedure is also included into this integrated approach.Examples from a carbonate reservoir are presented * Trademark of Schlumberger
CopyTight 1995, Socbty of Petroleum Engineers, Inc. This -r was prepared for presentation at the SPE Middle East Oil Show held In Bahrain, 11-14 March 1995. Thm papr was 8elected for prasentaiion by an SPE Program Cemmiltee tofbwing revfew of infofmaticm contained in an abstr.scf submitted by the autlmr(s), Contents of the paper, as presented, have nof been reviewed by the Society of Petrolem Engineam and are subject to correction by the author(s). The material, as prewtted, does not neca85WUy reftect any position of tfw .%cfety of petrokwm Engineem, na offimws, or members. Papers presented at SpE tilm are subkf to publica~~~MV by Edl~i~~mmitt-s of the~~fy of P64roleumEngineers. permission m COPYb remrfctedm an abstnwt of not mare than SW words, Illu$tmtkm may not be copisd. Tha abstracl shculd contain c0nspicucu8 8cfmwk@mnt of where and by whom the paper is prasenfed. Wtfte Librarian, SPE, P.O. Box m, Rkh~d~n.~7~1 U.S.A., Tebx, tS3Z45 SPEUT.
Summary Geochemical logs run through a complex carbonate formation yield accurate elemental concentration logs. These are instrumental to the computation of detailed formation mineralogy and derivative properties, such as matrix density and sigma. The quantitative mineralogy logs also are used to estimate permeability empirically with a correlation observed from core data. Introduction The Utility of geochemical logging was evaluated in a Middle East reservoir described as extremely complex and heterogeneous in terms of its degree of dolomization, argillaceous content, secondary porosity, and porosity distribution. Geologists, reservoir engineers, and petrophysicists working in this reservoir identified well-to-well correlation as a problem, and for log interpretation, they specifically cited problems with the accuracy of (1) porosity determination, (2) degree of dolomitization, (3) characterization and quantification of clay minerals and clastics, and (4) permeability estimations. It appeared that these problems were interrelated, and it was anticipated that, by providing a detailed interpretation of the rock composition, geochemical logging might aid in geologic characterization, petrophysical interpretation, and ultimately, interwell correlation. The objectives of the study were to determine the quality of the primary geochemical interpretation products (elements and minerals) and to investigate the effects these answers had on geological and petrophysical interpretation. The study was composed of a combined logging and core analysis program. The pertinent core analysis was divided into two parts, with standard petrophysical measurements of grain density, porosity, and permeability made on plugs spaced approximately at 1-ft intervals throughout the reservoir, and a subset of samples analyzed for chemistry and mineralogy. The logging program included the Geochemical Logging Tool (GLTSM), which measures elemental concentrations of aluminum, silicon, calcium, iron, sulfur, gadolinium, titanium, thorium, uranium, potassium, and magnesium.1 In addition to a full suite of resistivity and nuclear logs. Results show that the geochemical elemental concentration and mineral concentration logs describe the formation with an accuracy that could not be obtained previously without extensive core analysis. The new logs clearly delineate the zones of dolomitization and quantitatively interpret the amount of noncarbonated minerals. The mineral logs were used to compute a continuous matrix density, which was in turn used to determine porosity. The results of the mineralogy and porosity interpretations are compared directly with core measurements and also with the results of a volumetric log interpretation that does not include geochemical data. The elemental concentration logs and mineral concentration logs were used together to derive matrix sigma and apparent fluid signs, which were in turn used to estimate near-wellbore saturation. Finally, the core and log data also were used to illustrate a relationship between mineralogy and permeability. The result is an accurate formation description in terms of composition and petrophysics, which should enhance correlation and reservoir evaluation. The interpretation is a scientifically based analysis that does not require numerous log-analyst picks or a large degree of regional modification. Instead, the elemental concentrations are transformed through a fixed composition matrix into mineral concentrations. Many of the petrophysical components, such as grain density and matrix sigma, are natural consequences of the mineralogy, and therefore, can be directly computed from the mineralogy. Other properties, such as permeability, can be more affected by diagenetic events and fracturing. Some diagenetic events, like dolomitization, are implicit in the mineralogy, and in this case, the degree of dolomitization can be used to estimate the permeability.
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