Masafumi Fukuhara

Nuclear magnetic resonance pulse sequences for use with borehole logging tools

An NMR pulse sequence for use in the borehole environment is provided which combines a modified fast inversion recovery (FIR) pulse sequence with a series of more than ten, and typically hundreds, of CPMG pulses according to [Wi -180-τi -90-(tcp -180-tcp -echo)j ]i where j is the index of the CPMG echoes gathered, i is the index of the wait times in the pulse sequence, Wi are the wait times, i are the recovery times before the CPMG pulses, and tcp is the Carr-Purcell spacing. Measurements are made of the signals induced in the formation as a result of the magnetic fields. Determinations of Mo and/or T1 are then made from the measurements according to relationships which relate Mpo, T1 and T2 to the signal magnitude. Other relationships which provide stretched exponentials or multiple exponentials can also be used. From the Mo and/or T1 determinations, formation parameters such as porosity and permeability may be derived according to equations known in the art. In obtaining the most accurate determinations of formation parameters in the least amount of time, the various pulse sequence parameters (I, J, Wi, and τi) are optimized prior to logging. Additional accuracy is obtained by integrating a gated portion of the echoes rather than by measuring amplitude, and by utilizing a phase alternated CPMG sequence in repetitive measurements in order to eliminate baseline shift error.

Borehole measurement of NMR characteristics of earth formations, and interpretations thereof

Borehole NMR logging apparatus and methods, and methods for the interpretation thereof. A logging tool is provided which produces a strong, static and homogeneous magnetic field B0 in a Volume of an adjacent formation on one side of the tool to measure nuclear magnetic resonance characteristics thereof. In the preferred embodiment, the tool has an RF antenna mounted on the outside of the metal body of the tool, directing focussed oscillating magnetic fields B1 at said Volume to polarize or tip the magnetic moments of hydrogen nuclei of fluids within rock pores. The same antenna can be used to receive signals of proton precession in the Volume of interest immediately after transmission of the RF polarizing field B1. Extremely rapid damping of the antenna between the transmitting and receiving modes of operation is accomplished by a Q-switch disclosed herein. The invention provides for the direct measurement of NMR decay having transverse relaxation time T2 behavior, and further provides for the fast repetition of pulsed measurements from within a borehole. An additional magnet array may be mounted offset from the first magnet configuration to prepolarize a formation before it is measured in order to pre-align a larger number of protons than the single magnet configuration could do by itself. Additional features of the invention are disclosed which increase the Signal/Noise ratio of the measured data, and improve the quality and quantity of borehole NMR measurements, per unit of time spent. Disclosed interpretation methods determine fluid flow permeability and longitudinal relaxation time T1 -type parameters by directly comparing the measured decay signals (such as T2 and T2 * type decay) to a representation which responds to both the decay time tdec and the imposed polarization period prior to such decay, tpol. The parameters of amplitude and T1 are determined and combined with certain preferred methods to generate robust values of formation characteristics such as fluid flow permeability. Other related methods are disclosed.

Temperature Monitoring Results for Methane-hydrate Sediments in the Nankai Trough, Japan

The Japan Oil, Gas and Metals National Corporation (JOGMEC) conducted the Kisoshisui (Tokaioki-Kumanonada) drilling campaign in the Nankai Trough area during 2003–2004 as part of the activities of the MH21 Research Consortium, which is supported by the Ministry of Economy, Trade and Industry (METI). Through this campaign, various subsea hydrate studies were conducted using various data, including logging while drilling, wireline, coring, and temperature measurement. Using recent technology, a precise in-situ temperature measurement system was successfully developed and deployed at the beginning of the drilling campaign. The goals of the study were to determine whether the measurement system and setup can reliably measure the equilibrium in-situ formation temperature, to determine if the measurements adequately delineate the geothermal gradient, and to identify any disturbances in the thermal regime of the hydrate-bearing sediment and the possible causes of the disturbances. The temperature of the hydrate-bearing sediment was monitored in a deep-water well for 1.5 months. Because the measurements were collected over a long period of time, the equilibrated formation temperatures could be readily determined after observing the thermal relaxation from drilling and sensor deployment effects. The temperature profile and the temperature gradient through the hydrate-bearing zone were estimated in a quasi–steady-state condition. A correlation between the tidal cycle and sea-floor temperature change is also discussed.