Granitic basement reservoirs have been the focus of increasing attention in the Asia Pacific region in recent years, following several new oil and gas discoveries in this complex reservoir type. Accurate formation evaluation in fractured, crystalline, granitic reservoirs is notoriously difficult. Furthermore relatively little research has gone into understanding logging tool response or pressure transient behavior in these reservoir types, and developing suitable workflows for formation evaluation. In this paper the authors propose an improved methodology for integrating various open-hole logs, production logs and well test data to better evaluate the reservoir potential of a fractured granitic formation. Since the wells are either horizontal or highly deviated, the framework also serves as one of the primary methods to asses the lateral extent of the reservoir. A case study from the region is used to illustrate the workflow. Image log interpretation, advanced acoustic measurements, nuclear logs and production logs with distributed local sensors are combined with well test data to derive the best possible evaluation of the fracture network around the borehole and the degree of connectivity with the reservoir at large. The advantages and limitations of the proposed workflow are also discussed and the stage set for further work in this complex environment. Background Fractured granitic reservoirs differ from other types of fractured reservoirs in that they are generally considered to have no primary porosity. All the pore space in the rock is formed through fracturing and diagenetic processes. The resulting pore structure heterogeneity makes formation evaluation extremely challenging. Another complicating factor is the lithology, where the granite composition can vary and sub-vertical extrusive dykes cause abrupt lateral variations in matrix properties. This paper deals specifically with granitic oil bearing formations from southern offshore Vietnam. The typical basement reservoir consists of a faulted basement high. The majority of the porosity is thought to be associated with fractures and fault zones. High fracture density enhanced by hydrothermal alteration forms the majority of the effective pore space. Unlike basement plays in other regions the weathered zone at the top of the structure and the dykes are usually not productive, though the interface between dykes and host rock can be (Le Ngoc 2007, Nguyen 2003, Tandom 1999). In this environment the majority of new wells drilled are highly deviated or horizontal, and drilled to intersect sub-vertical fault zones. Most of the fractures are small and form the storage capacity of the reservoir. To be productive, a well should also encounter enough larger, permeable, fractures which are sufficiently connected to the storage capacity of the reservoir. The objective of the following interpretation methodology is partly to quantify as much as possible the total fracture porosity, but mainly to identify the large permeable fractures which are necessary for the well to be productive.

Summary In recent years, energy companies in the Asia Pacific region have focused increasing attention on granitic basement reservoirs, following several new oil and gas discoveries in these complex reservoirs. However, accurate formation evaluation in fractured, crystalline, granitic reservoirs is notoriously difficult. Furthermore, relatively little research has been conducted to understand loggingtool response or pressure-transient behavior, or to develop suitable workflows for formation evaluation in these types of reservoirs. In this paper, we propose a method for integrating various openhole logs, production logs, and well-test data to better evaluate the reservoir potential of fractured granitic formations. Because the wells are either horizontal or highly deviated, this workflow also serves as a primary method of assessing the lateral extent of a reservoir. We include a case study from the region to illustrate the workflow. Image-log interpretation, advanced acoustic measurements, nuclear logs, and production logs with distributed local sensors are combined with well-test data to derive the best possible evaluation of the fracture network around the borehole and the degree of connectivity with the reservoir at large. We also discuss the advantages and limitations of the proposed workflow and set the stage for further work in this complex environment.

TX 75083-3836, U.S.A., fax 01-972-952-9435. AbstractThe identification of condensate banking has always been a challenge. Furthermore, large productivity losses can result from the absence of early detection of a condensate bank in the near well bore area of the well. The traditional means of detecting a condensate bank range from comparison of the dew point to downhole pressure measurements, identification of composite radial models and quantification of skin using pressure transient analysis. One of the methodologies that have been more theoretical than practical has been the detection of a leaner stream of effluent at the well head during production. This type of approach has been quite challenging in the past, as a high resolution measurement of the condensate to gas ratio is essential to a successful diagnostics of condensate banking.The paper presents a case of analysis of the development of a condensate bank during a well test. The multiphase flowmeter identified a gradual reduction of the condensate to gas ratio with increasing choke sizes. The methodology of diagnostics is demonstrated, in particular with the discrimination against liquid loading issues.The PVT compositional analysis provides a verification of the analysis, and the observation of the evolution of the phase diagram leads a further understanding the downhole and near well bore thermodynamic phenomena.The degradation of the productivity of the well is also analyzed, with a significant drop of gas productivity observed even on smaller choke sizes at the end of the test.Finally the paper presents a numerical simulation match of the data and provides a number of recommendations for the utilization of single well -near well bore compositional models to help interpreter to obtain better and simpler matches.This paper provides a new methodology to make full use of the benefits of the dual energy gamma Venturi multiphase flowmeters in the evaluation of gas wells. Operational issues related to gas well testing with traditional test separatorsThe test of gas wells has always been a challenge compared to testing oil wells. The high level of energy contained in the stream in the form of compressible fluids, the higher pressure usually encountered at surface due to the lower hydrostatic head in the tubing and the potential presence of toxic components such as H2S in the effluent contribute to increase the Health and Safety risks inherent in the handling of gas wells.On the operational side, the presence of water in the stream combined with a large temperature drop across restriction or the choke can lead to severe plugging issues with hydrates.Erosion can also be a serious risk encountered with the combination of high fluid velocities (in particular at low pressure) and a bit of sand. Perforation of the walls of the surface piping can present very serious risk to the operational personnel and the facilities.However, the main difficulty of testing gas wells comes from the determination of accurate gas, condensate and water flow rate measurements. The sh...

Interference testing is the oldest, but still the most effective, way of establishing communication between wells and determining the interwell reservoir transmissibility. However, these tests are not frequently run because the results are commonly difficult to analyze due to unforeseen complications. This paper presents several practical methods to design and interpret effective interference tests based on the properties of the line-source solution. In single-well transient tests, early-time features of the exponential integral function occur at times that are too early to be observed. However, the distinctive pressure transient derivative features appear at an observation well much later during an interference test, and can be used to estimate the storativity and transmissibility ratios of the reservoir. The pressure response and the log-derivative of the pressure intersect on the log-log diagnostic plot for the observation well and the pressure response itself exhibits an inflection point. Based on these characteristics, simple geometrical methods are proposed to estimate reservoir parameters. Moreover, we formulate a new expression for the "travel time", or delay in the response. The particular case of fall-off or buildup is studied in detail, as the time lag in reservoir response can bring extra information. A field example is included to demonstrate the application of these methods in a field case and their usefulness to a practicing well test engineer.

TX 75083-3836, U.S.A., fax 01-972-952-9435. AbstractIs it a good idea to drill dual-lateral wells in a differentially depleted carbonate formation? The question is difficult. However, a state-of-the-art production-logging tool brought an answer that sanctioned the re-entry drilling program of this offshore field in the Persian Gulf.The new-technology production services platform equipped with eight electrical e-probes for flowimaging capability was run on coiled tubing for the first time in this offshore field to characterize the production of a new dual-lateral well. The main objective was to evaluate the total contribution of the horizontal drains. However, since the well was perforated in the different layers crossed by the well path, another critical objective was to determine the need for a stimulation program or a new completion design. Reservoir pressure in the different layers was expected to be non-uniform, so an additional objective of the logging operation was to obtain information on the differential depletion.Production logging in highly deviated wells is difficult with standard sensors. One reason is that a conventional tool using differential pressure cannot measure fluid density. Moreover, flow regimes can be extremely complex, giving rise to phenomena such as water re-circulation, which is common in slanted wells and impossible to measure with a simple production logging string.The data showed that both legs are contributing equally to the total production and observed water recirculation on the bottom of the well. The perforations are not contributing to the total flow. Moreover, a crossflow observed during shut-in conditions suggested different pressure levels in the two layers. This paper describes the logging operation and discusses how the results helped optimize the redevelopment plan of this aging field.