The first Magnetic Resonance While-Drilling (MRWD) tools have been built and field tested. The hardware was designed to provide data compatible with the Magnetic Resonance Imaging Log (MRIL®) wireline tool in terms of processing and interpretation. The paper discusses the theory of robust MRIL measurements in the drilling environment and reviews the theory of T1 measurements. We report on the results of a field test in the Gulf of Mexico. The MRWD device logged the entire TD run from casing to TD and provided additional hydrocarbon-typing data in wiping mode over the target zones. Wireline MRIL data and conventional logs were used to verify the MRWD measurements. Introduction We previously reported on our initial field experiences with the Magnetic Resonance While-Drilling (MRWD) tool in a recent transaction paper.1 The present paper focuses on the petrophysical deliverables of the MRWD measurement and their robustness in the drilling environment. Furthermore, we report on a field test conducted in the Gulf of Mexico in March 2000. The test was run with an experimental prototype version (EX) and we were able to compare the results with wireline data from the MRIL-Prime tool.2 The comparison indicates satisfactory performance both during drilling and wiping operations. The primary application areas of MRWD will be high-cost offshore exploration and development wells, potentially in deep water. Typical bit sizes for the final TD run are 8–1/2 to 10–5/8 in. High rates of penetration (ROP) and long bit runs are common, aided by poorly cemented sediments, oil-based mud systems and advanced bit technologies. In this context, open-hole logging operations are expensive in terms of added rig time and risky in terms of hole stability. Additional problems are highly laminated reservoirs (e.g. turbidites) that frustrate traditional log interpretation because electrical and nuclear measurements may show little or no contrast identifying reservoir rocks. In these situations, breakthroughs in reservoir characterization have been achieved by computing hydrocarbon volumes directly from MRIL data, circumventing the problems with conductivity/resistivity.3 Real-time MRIL data from an LWD device will result in faster, better and cheaper evaluation and exploitation of these reserves. Logging-while-drilling (LWD) tools are expected to replicate as close as possible the functions of their wireline counterparts. In particular, the expectations for the LWD version of an MRIL device are:To provide a measure of rock porosity that is lithology-independent and that does not require radioactive sources;To collect a spectrum of NMR relaxation times that is suitable for input to lithology models that estimate bound fluid volumes, free fluid volumes and rock permeability;To enable fluid typing by exploiting the inherent differences in T1 and/or T2 (longitudinal and transversal relaxation times) between the water, oil and gas phases;To withstand the shock, vibration and erosion associated with drilling;Not to interfere with the drilling process; andNot to interfere with any other LWD or MWD measurements. Tool Hardware and Sensor Physics Fig. 1 shows the basic tool layout conforming to these requirements. The tool has two main sections: the sensor and the electronics package. The total length of the EX version tool of 42 ft is driven by the inclusion of extra electronics and will be reduced in the forthcoming commercial tool. The tool diameter is under-gauge at about 7–1/4 in. All compressional, torsional and tensional stress ratings exceed those of standard 4½ IF API connections. To meet the sensor-to-sensor non-interference requirement, a distance of 45 ft to the MWD directional package is recommended.
Summary The first Magnetic-Resonance-Image Logging-While-Drilling (MRI-LWD*) tools have been built and field tested. The hardware was designed to provide data that are compatible with the Magnetic Resonance Imaging Log (MRIL®) wireline tool in terms of processing and interpretation. This paper discusses the theory of robust MRIL measurements in the drilling environment and reviews the theory of 7 relaxation time measurements. We report on the results of a field test in the Gulf of Mexico. The MRI-LWD device logged the entire total depth (TD) run from casing to TD and provided additional hydrocarbon-typing data in wiping mode over the target zones. Wireline MRIL data and conventional logs were used to verify the MRI-LWD measurements.
A recently developed nuclear magnetic resonance (NMR) downhole fluid analyzer provides in-situ fluid characteristics at reservoir PVT conditions. The device is part of the Reservoir Description Tool (RDT*), a modular formation sampler and tester. This paper presents the theory and the experimental evidence that led to the use of NMR as a means to assess the amount of mud filtrate contained in the pumped formation fluid. In particular, the problem of differentiating oil base mud filtrate from connate hydrocarbons is difficult and has not been addressed by conventional measurements. NMR relaxometry uses the inherent differences in T1 time constant values and distributions to make these distinctions. The T1 data is available in real time and can be used to determine the optimum pump-out rates and durations while sampling reservoir fluids. Introduction Advances in wireline formation fluid sampling allow the recovery of representative hydrocarbon samples. It has become imperative to obtain high-quality samples with minimum mud filtrate content, while simultaneously minimizing the pumpout time required per station. Traditionally, contrasts in resistivity and/or dielectric properties between connate fluids and contaminants have been exploited. These methods fail if neither phase contains water. The problem of differentiating oil base mud (OBM) filtrates from crude hydrocarbons has received considerable attention. So far, only optical analysis methods have been described to address the problem. It has long been known in NMR wireline logging that crude oils and oils introduced from the borehole exhibit different nuclear relaxation time behavior.1 This is readily understood given that the filtrates typically have lower viscosity and therefore slower relaxation rates. The mixture of hydrocarbon chains of different lengths and mobility in crudes gives rise to a spectrum of relaxation times, spanning 1–2 orders of magnitude. Filtrates, on the other hand, are characterized by a single T1 relaxation time. With the advent of a downhole NMR analysis system2 it has become possible to exploit the differences in relaxation behavior in a systematic and quantitative fashion. To this end, we studied the NMR characteristics of a range of crude oils, of typical mud base oils and of their mixtures. We found that with reasonable measurement times, contaminant levels down to 10% can be detected. The downhole NMR fluid analyzer fulfills the signal-to-noise and measurement speed requirements to perform oil-v.-filtrate differentiation as part of an overall NMR fluid analysis. The Downhole NMR Fluid Analyzer The analyzer is part of the RDT (Reservoir Description Tool).3 It continuously analyzes in real time the fluids being pumped from the formation. The main measurement is relaxometry by hydrogen nuclear magnetic resonance. Protons are being brought in magnetic resonance by a combination of a static 1,000-gauss magnetic field and radiofrequency (r.f.) pulses tuned to 4.2 MHz. There are a number of measurement outputs:Hydrogen index. This is the hydrogen density compared to a reference fluid. The reference fluid is water at room temperature and atmospheric conditions. With few exceptions, oils have hydrogen indices between 0.8 and 1.Relaxation time T1. The bulk relaxation time constant reflects the mobility of the fluid's molecular components. The higher the mobility, the higher the macroscopic T1, and vice versa. Molecular mobility entails both in-place reorientation and Brownian motion.
A second generation of LWD Nuclear Magnetic Resonance (NMR) tools has been developed and commercialized. These MRI-LWD* (Magnetic Resonance Image – Logging While Drilling) tools extend the technology by providing key improvements in data quality and operational efficiency. Improvements include: automatic mode switching, better vertical resolution, improved signal-to-noise ratio (SNR), real-time data transfer, built-in log quality control, increased battery life, and extended environmental ratings. The paper also examines the context for the deployment of this system and the expanding role of NMR in formation evaluation. MRI-LWD belongs to a family of NMR tools which includes logging-while-drilling, wireline open-hole logging, formation testing/fluid sampling, and laboratory core analysis. Introduction: NMR in Formation Evaluation The motivation for development of a Nuclear Magnetic Resonance (NMR) LWD device was the understanding that NMR provides critical reservoir information, important for well planning and construction. The earlier this information is available, the more useful it is. NMR logging relies exclusively on hydrogen relaxometry with the unique property that only the hydrogen in the fluid state is responsive. These signals are modulated by bulk fluid properties (hydrogen density, diffusivity, and surface wetting) and by reservoir rock features (porosity, pore sizes, and clay contents). The main limitation of NMR logging has been its shallow depth-of-investigation, which restricts the measurement to the invaded/flushed zone. NMR can help answer questions like:1Is the rock porous? If so, how much porosity is present?Are there movable/producible fluids present?In what fractions (gas/oil/water) will fluids produce?How are the gas/oil/water columns separated? Where are the transition zones? What is the height of each column?Are there any potential obstacles to production, i.e. pore-clogging clays, tar layers, etc.?What are the properties of each producible fluid? What is the gas/oil ratio in the hydrocarbon phase? How viscous is the oil? These issues are addressed by a family of open-hole logging tools: the MRI-LWD logging-while-drilling system, described in this paper, along with the MRIL-Prime* wireline logging tool and downhole NMR fluid analyzer,2 a module within the RDT* formation tester.3 The Principles of NMR Logging-While-Drilling The basic building blocks of the MRI-LWD are: a permanent magnet which projects its field perpendicular to the borehole, into the formation; a radiofrequency (RF) antenna, which acts both as transmitter and receiver; a battery pack and power supply; and electronics for data acquisition, data storage and communication. The first generation of MRI-LWD broke new ground for NMR logging by making the measurement immune to random tool motion.4,5 The motion-insensitive reconnaissance log (RL) mode, which is active during most of the bit run, indicates porosity (not yet corrected for variations in hydrogen density) and the presence of free fluids. The evaluation log (EL) mode, which is activated during sliding and wiping operations, allows fluid typing in the near-borehole zone and the determination of porosity and free-fluid volume corrected for gas effect in a manner identical to MRIL* wireline data processing. The reconnaissance mode is a T1 measurement. It measures the gradual build-up of formation fluid nuclear magnetization to an equilibrium value that corresponds to the externally applied static magnetic field. This build-up is initiated by the transmission of an RF saturation pulse at the nuclear resonance frequency (nominally 500 kHz for this tool). The saturation pulse momentarily randomizes the nuclear magnetism within a "prepared" volume. A unique RF saturation pulse shape is used such that the prepared (randomized) volume is much thicker than the normal measurement volume.
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