It has recently become possible to make pulsed nuclear magnetic resonance (NMR) measurem¢nts of subsurface geological formations in situ. The longitudinal relaxation time,Tl, has long been the parameter of greatest interest to petrophysicists. However modeling shows that Tl measurements are not repeatable when NMR logging tools move past bed boundaries. Therefore measurements of the transverse relaxation time, T2, must be relied upon. Unfortunately, reservoir rocks have properties that make T2 measurements difficult to interpret. We discuss the relationship between T1 and T2, and the measurement conditions for which T2 gives meanin,gful petrophysical information.'

The Nuclear Magnetic Resonance (NMR) response of gas in gas shale nanopores is different from that of bulk gas, where relaxation is dominated by spin rotation and diffusion is unrestricted. Gas shales are characterized by very low porosity and ultra low permeabilities. Their porosity is dominated by nanometer-scale pores in the organic kerogen that restricts diffusional motion, in addition to having very high surface-to-volume ratios that enhance surface relaxation. At high pressure, the gas exists as an adsorbed phase on the pore surface and as free gas phase in the pore interior. Thus, relaxation and diffusion properties of gas in gas shales are controlled by the combined effects of adsorption, enhanced surface relaxation, restricted diffusion and molecular exchange between the adsorbed and free phases. One of the biggest challenges is the understanding of such effects in order to determine the quantity of free and adsorbed gas from NMR data, and to devise novel techniques to log these unconventional plays. Proper estimation of fluid volumes also requires the knowledge of the hydrogen index for the gas restricted in the gas shale nanopores, which is yet another challenge. The NMR responses of methane gas in Haynesville shale plugs cored from a well in East Texas, USA were studied in laboratory experiments using a 2 MHz NMR spectrometer at elevated pressures up to 5 kpsi. The effects of adsorption, surface relaxation and restricted diffusion have been characterized, and the hydrogen index of the gas has been measured. Mineralogy, elemental analysis and Brunauer-Emmett-Teller (BET) experiments have also been carried out on the same plugs to understand the formation characteristics. In the samples studied, faster relaxation modes (few tens of milliseconds) and slower apparent diffusion coefficients (an order of magnitude less than their bulk values) for the confined gas molecules in comparison to their bulk properties have been observed for the first time with the help of 2D-NMR experiments at high pressure. It has been observed that the relaxation spectra for bound water and the gas in the small pores overlap. Additional information is required to resolve these two fluids. Subsequently, the diffusion dimension is investigated to resolve the various fluids in the nanopores. We formulate new relaxation and diffusion models for the interpretation of the dynamics of gas restricted in gas shale and propose that multi-dimensional NMR logging with pulse sequences optimized for gas shales be further tested in the field, to help quantify the total gas in place.

TX 75083-3836, U.S.A., fax 01-972-952-9435. AbstractOne of the main applications of MRILab, the recently introduced NMR fluid analyzer, is the estimation of mudfiltrate contamination during the pump-out phase, prior to securing high quality fluid samples for PVT analysis. The NMR fluid analyzer utilizes T 1 saturation recovery measurements for contamination analysis. The main objective of this paper is to discuss the principles behind recently developed T 1 -based novel methodologies that yield accurate and robust contamination estimates. The proposed algorithms were used to estimate the contamination levels in an offshore well drilled with OBM. Comparisons with laboratory assays on nine fluid samples show excellent agreement, thus proving the concept of using an NMR fluid analyzer for contamination analysis in sampling applications. A secondary objective of the paper is to illustrate the benefits of integrating fluid analyzer data with open-hole and NMR logs. Such integration leads to an overall improved interpretation, particularly in terms of hydrocarbon fluid properties.

The potential use of NMR as a direct indicator of hydrocarbon saturation via techniques such as the Differential Spectrum Method (DSM) has generated significant interest in the petrophysical community in recent years. Although originally developed for applications involving natural gas, the DSM has also been used successfully in light hydrocarbon environments. However, success has been limited to the low end of the viscosity spectrum because of the T1 separation requirements between the brine and hydrocarbon phases. The T1 separation requirement in higher viscosity applications can be eliminated by using the Enhanced Diffusion Method (EDM), where diffusion is turned into the dominant relaxation mode for the wetting brine phase. Given that brine is more diffusive than the hydrocarbons, the longest apparent T2 from the brine phase can be made short enough to cause separation between the two phases in T2 space, thereby eliminating the need for T1 separation. Wait time manipulation can then be used to quantity hydrocarbon volumes when the two phases are separated in T2 domain. This paper focuses on the determination of residual oil saturation using EDM, while also providing guidelines for job screening and acquisition parameter selection. Several case histories provided are used to illustrate basic concepts and different methodologies available. P. 267

Dielectric logging was introduced in late 70s to mostly measure water filled porosity in the flushed zone independent of water salinity and Archie exponents m and n. Although the technology generated a lot of interest upon its introduction, it eventually disappeared over the years, mostly due to moderate accuracy of the early devices, oversimplified interpretation models and other hardware related complications. Results from extensive field testing of a new generation of dielectric wireline tools indicate that robust and reliable dielectric logging is now feasible in a wide range of environmental conditions and formations. Benchmarking of dielectric measurements against other logs and core data has shown that water-filled porosity can be measured accurately using dielectric logging tools, provided the matrix mineralogy is well defined. Consequently, the technology has gained rapid acceptance in applications involving flushed zone water saturation in environments with variable or unknown salinity, heavy oil identification, and discrimination of non-reservoir rock with high organic content. Dielectric logging is gradually replacing NMR Log-Inject-Log for residual/remaining oil saturation (ROS) measurement, especially in the situations where connate and injected water salinities can be vastly different. Due to the success in ROS applications, most of the recent testing has been focused towards carbonates drilled with water-based-mud. This technology is also being tested in shaly sands and in wells drilled with oil based mud at present. Additional work is underway to accurately characterize dielectric properties of carbonates so as to be able to perform quantitative textural analysis. In shaly sands, high resolution clay volumes and thin bed analysis are challenges that will be addressed in the future.