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Abstract Full field development of the Upper Jurassic carbonates, offshore Abu Dhabi is exceedingly challenging. The heterogeneous texture, complicated pore systems and intensive lithology changes all mark the regressive cycles of sedimentation. Such complicated characteristics obscure formation evaluation of these formations. Advanced well logging tools and interpretation methodologies are implemented to minimize the petrophysical uncertainties to qualify the products as field development critical elements. This case study highlights a newly applied NMR log interpretation approach. The results help to understand the complex pore system in a tight carbonate layer, along a horizontal drain drilled close to the oil-water contact. NMR log data was acquired in real-time while drilling simultaneously with Gamma Ray, Resistivity and Image Logs. Earlier field studies recommended swapping standard T2 free fluid relaxation cutoff values by actual laboratory NMR measurements for a higher precision suitable for the reservoir texture heterogeneity, the study itself supported the application of higher cutoff values to better discriminate the free fluid in well-connected macro pores from the irreducible which will have a direct impact on the computed permeability. In this case study, a variable free-fluid T2 cutoff was firstly implemented based on arbitrary estimations to match the computed Coates permeability to the offset core values. Free-fluid, irreducible fluids were sequentially computed. A unique NMR-Gamma Inversion (NMR-GI) workflow is further utilized as a mathematically defined approach to process the raw data using probabilistic functions. The result is a more precise pore size distribution, coherent with the geological variations. NMR Capillary pressure was computed. The complex formation texture could be accurately tracked for thousands of feet drilled along the horizontal drain. After validation with offset core, the NMR-GI interpretation was combined with, Archie saturation and Image log analysis for a conclusive assessment. Hydraulic flow units were combined. Successful completion design and production zone selection articulated on the defined open hole log interpretation. NMR while drilling logging and the applied (NMR-GI) methodology prove to be leading tools to assist in resolving carbonate reservoir complexities. Not only that they help to understand the pore system characteristics, but they effectively support well placement, completion and production.
Abstract Nuclear Magnetic Resonance logging measurements (NMR) provide detailed information about rock texture and pore distribution. The main objective of this study is to highlight a carbonate reservoir characterization example in a mature field, offshore Abu Dhabi; providing qualitative porosity, permeability and pore type classification in real time (while drilling), to support efficient field development decision making. Different logging while drilling vendors tools (NMR-WD) operate at different concepts; some use the longitudinal relaxation time (T1) measurements, others apply the transverse relaxation time (T2). In this case, a low magnetic field gradient (T2) tool type was deployed in a tight formation horizontal oil producer. The well objective is to expose the maximum reservoir contact (MRC). Primarily, the acquired (NMR) spectrum was used to deliver accurate total porosity, to compute Archie's water saturation. However, delivering a quantitatively reliable permeability become very challenging in the complex carbonate environment subject to study as it was well linked to (NMR) pore size distribution. At first, a standard (T2) cutoff value was applied. The computed (bulk irreducible water – BVI) was too low and hence the permeability was too high, resulting in inaccurate NMR interpretation. Next, a varying T2 cutoff – per zone was applied based on the changing spectrum profile itself. Finally, a Gamma Inversion technique by the service company was introduced to better quantify the different pore types and the corresponding permeability. The (NMR) log analysis was validated with well core data in addition to production logging results. The data was applied to design the well stimulation and completion programs resulting in a healthy oil producer drain added to the asset. Integration of Gamma Ray-resistivity-NMR and borehole image logs helped to consolidate the interpretation findings hence supporting decision making for mature field development.
Nuclear Magnetic Resonance Logs While Drilling (LWD NMR were recently acquired in the Eldfisk Field in the Norwegian North Sea sector. The quality of this data, especially of the T1 log recorded under drilling conditions, meets and even exceeds the level of wireline logs. In addition to the T1 log recorded while drilling the well, some stationary T2 data and a moving T2 logs while wiping, were recorded. The spectral and vertical resolution of the T1 distributions surpass the resolution of the wiped T2 distributions, and are on par with the best wireline T2 distributions recorded at nearby Ekofisk Field. The vertical 2/7 A-29B well cuts through the Ekofisk, Tor, and Hod formations. The T1 distributions exhibit clear and distinct trends in different reservoir sections. For example, the faster component in the bimodal T1 spectra in the Ekofisk formation represents the water volume, and the slower component the remaining hydrocarbon. The T1 distributions are uni-modal in the water-flooded Tor formation, in response to the lower hydrocarbon saturations. In addition to hydrocarbon typing, the NMR logs can greatly reduce the uncertainty in total porosity. Apparent density and NMR porosities differ by as much as 8 p.u. The need for hydrocarbon and invasion corrections is evident from the T1 distributions that indicate a relatively high proportion of water-filled pores; both in the initial state and water-flooded intervals. Resistivity MAD passes also suggest mud-filtrate invasion, resulting in high Sxo values. Accurate porosity readings are obtained with proper accounting for the fluid density and hydrogen index of the fluids in the flushed zone. The results from this well show that While-Drilling T1 logs are viable alternatives to wireline NMR logs, enabling the accurate assessment of porosity and fluid types. Information obtained from these logs improves the confidence in the evaluation of reservoir properties of a complex chalk reservoir.
Nuclear magnetic resonance (NMR) has been, and continues to be, widely used in chemistry, physics, and biomedicine and, more recently, in clinical diagnosis for imaging the internal structure of the human body. The same physical principles involved in clinical imaging also apply to imaging any fluid-saturated porous media, including reservoir rocks. The petroleum industry quickly adapted this technology to petrophysical laboratory research and subsequently developed downhole logging tools for in-situ reservoir evaluation (see the next section of this chapter). NMR logging, a subcategory of electromagnetic logging, measures the induced magnet moment of hydrogen nuclei (protons) contained within the fluid-filled pore space of porous media (reservoir rocks). Unlike conventional logging measurements (e.g., acoustic, density, neutron, and resistivity), which respond to both the rock matrix and fluid properties and are strongly dependent on mineralogy, NMR-logging measurements respond to ...
Summary Standard formation evaluation of an exploration well in the U.K. southern North Sea was supported by magnetic resonance while drilling (MRWD). In this paper we show that even in tight gas sands, MRWD provides information about porosity and producible fluid fraction and allows estimation of formation permeability. This successful introduction of MRWD technology in a known hard-rock environment illustrates a powerful addition to the logging-while-drilling (LWD) tool suite. Introduction Magnetic-resonance (MR) logging has developed into a powerful petrophysical tool for reservoir characterization. The application of downhole MR logging tools has become widespread within the oil and gas industry. Several "answer" products (such as total porosity or bound-fluid volume) are now considered to be standard and are reliably provided by the service industry. However, the design and completion strategy of development wells often limits or complicates data acquisition using wireline or pipe-conveyed logging. Typical examples are highly deviated, multilateral wells that are commonly required for an economical field development. Furthermore, concerns about borehole stability frequently prevent any traditional openhole wireline data acquisition. Under these conditions, LWD might be the only method of obtaining cost-effective openhole data for formation evaluation. During the past few years, service companies have made significant efforts to add MR technology to the suite of LWD tools. The development of MRWD is complicated by the fact that any MR measurement is very sensitive to vibrations and motions of both the transmitting and receiving radio frequency (RF) antenna and the fluids in the sensed volume. As a result, the design of MRWD tools has to cope with and overcome drilling-induced noise. Halliburton Energy Services has recently introduced its design of an MRWD tool. This particular tool was first tested successfully in a Gulf of Mexico well in fourth quarter 2001. Encouraged by this success, Halliburton included the MRIL-WD™ tool in the bottomhole assembly (BHA) for the reservoir section of an exploration well in the U.K. southern North Sea. This was the first use of this technology in a known hard-rock environment or in a gas well anywhere in the world. The main objective of this trial was to test the feasibility of MRWD technology in highly consolidated, low-permeability gas-bearing sandstones. Furthermore, we wanted to investigate potential step changes in cost effectiveness and operational efficiency for a forthcoming development-well drilling campaign in late 2003. The MRIL-WD™ tool was run in the 8.5-in. hole section in combination with various other measurement-while-drilling tools to acquire T1 and T2 data in drilling and sliding modes, over cored and noncored intervals. To verify the data, and to enable local calibrations, subsequent formation evaluation also included MR logging, and a full compliment of standard service, on wireline. MR Logging MR logging exploits the effect of nuclear magnetic resonance (NMR). NMR is a consequence of the intrinsic magnetic moment of protons and neutrons. For most atoms, such as C and O, the individual magnetic moments of protons and neutrons offset each other, and the effective magnetic moment of such nuclei vanishes. This makes these nuclei invisible to NMR. Protons (H) provide the strongest NMR response and are typically targeted in NMR logging, but the measured signal also can be affected by the response of other NMR-active nuclei, like the Na found in brine. As with bar magnets, when a magnetic nucleus is placed between the poles of an external magnet, it will try to align itself with respect to this externally applied magnetic field. In the macroscopic world, two magnets can be aligned in an infinite number of orientations. At the atomic level, however, these alignments (also called spin states) are quantized. There are only a finite number of alignments a nucleus can take relative to an external magnetic field. This number depends on the shape of the nucleus' magnetic field. The alignment of magnetic moments is disturbed by RF pulses transmitted from an antenna in the tool into the formation. The return of the magnetic moments toward their equilibrium state is governed by various relaxation mechanisms, each of which can be characterized by a spectrum of relaxation times. In MR logging, two different relaxation mechanisms are typically exploited: the longitudinal (T1) and the transverse (T2) relaxation. T1 relaxation characterizes how fast the RF-induced energy is dispersed to the surrounding molecular lattice. T2 relaxation describes how quickly the precessing spins lose their phase coherence. MR is generally considered to be a measurement of only the formation fluids because the relaxation of any MR-active nuclei in the rock matrix is too fast to be detected by logging tools that are commercially available today. In weak, homogeneous magnetic fields, the T1 and T2 relaxation times of formation fluids are quite similar because both are governed by the pressure/volume/temperature properties of the fluids and the structure and surface chemistry of the surrounding rock matrix. However, if the magnetic field is not homogeneous but features a gradient over length scales that are comparable with the dephasing length of the spins, the T2 relaxation process is sensitive to the displacement of the nuclei. The larger the displacements of molecules within the gradient magnetic field, the shorter the measured T2 relaxation times. A variety of RF pulse sequences have been developed that are optimized for measuring T1 or T2 . By varying the timings of these pulse sequences, the measurement can be optimized to the expected MR response of the formation fluids. This is an important part of the prejob planning of MR logging, which is crucial for acquiring useful data. Fig. 1 illustrates a typical procedure for acquiring and interpreting MR logging data.