Abstract:
The present disclosure relates to a method to estimate a subsurface formation property. A downhole logging tool is provided and disposed in a wellbore. Multiple measurements of various measurement types are obtained at various depths of investigation using the downhole logging tool. The multiple measurements may include natural gamma ray measurements, density measurements, resistivity measurements, nuclear measurements, and nuclear magnetic resonance measurements. The signal-to-noise ratio of the measured signals is increased using, for example, lateral stacking and multi-shell inversion. The subsurface formation property is estimated using the increased signal-to-noise ratio signals. The subsurface formation property may include porosity, adsorbed gas volume, free gas volume, bound water volume, free water volume, oil volume, and kerogen volume. A fluid analysis may be performed using a multi-dimensional nuclear magnetic resonance technique. Fluids such as water, oil, gas, and oil-based mud in the wellbore may be identified and/or evaluated.

Description:
CROSS-REFERENCE TO OTHER APPLICATIONS 
     This application claims, under 35 U.S.C. 119(e), priority to and the benefit of U.S. Provisional Application No. 61/245,103, filed Sep. 23, 2009. 
    
    
     BACKGROUND 
     1. Technical Field 
     The present disclosure relates generally to the logging of subsurface formations surrounding a wellbore using a downhole logging tool, and particularly to using the logs to estimate one or more formation properties. 
     2. Background Art 
     Logging tools have long been used in wellbores to make, for example, formation evaluation measurements to infer properties of the formations surrounding the borehole and the fluids in the formations. Common logging tools include electromagnetic tools, nuclear tools, and nuclear magnetic resonance (NMR) tools, though various other tool types are also used. 
     Early logging tools were run into a wellbore on a wireline cable, after the wellbore had been drilled. Modern versions of such wireline tools are still used extensively. However, the need for information while drilling the borehole gave rise to measurement-while-drilling (MWD) tools and logging-while-drilling (LWD) tools. MWD tools typically provide drilling parameter information such as weight on the bit, torque, temperature, pressure, direction, and inclination. LWD tools typically provide formation evaluation measurements such as resistivity, porosity, and NMR distributions (e.g., T1 and T2). MWD and LWD tools often have components common to wireline tools (e.g., transmitting and receiving antennas), but MWD and LWD tools must be constructed to not only endure but to operate in the harsh environment of drilling. 
     Determining fluid types from continuous nuclear magnetic resonance multi-dimensional logging can be accomplished in certain situations, such as where the signal-to-noise ratio is relatively high. However, this technique has not been possible to be applied to tight shale gas formations because of the low signal-to-noise ratio associated with such formations. For example, the free fluid volume in shale is typically small and the gas therein may not completely polarize. Furthermore, there is a low hydrogen index. Those factors contribute to a weak measured signal while the noise level is generally high because of high formation temperatures, thereby making the signal-to-noise ratio unfavorable to continuous nuclear magnetic resonance logging. 
     A logging tool such as Schlumberger&#39;s ECOSCOPE tool is capable of measuring multiple depths of investigation ranging from about 6 inches to 12 inches into the formation. There is a need to determine porosity and individual fluid volumes and saturations such as adsorbed and free gas, oil, oil-based mud filtrate, and bound and free water in low porosity shale gas formations from NMR multi-dimensional logging data. There is similarly a need to determine the kerogen volume in the formation. 
     SUMMARY 
     The present disclosure relates to a method to estimate a subsurface formation property. A downhole logging tool is provided and disposed in a wellbore. Multiple measurements of various measurement types are obtained at various depths of investigation using the downhole logging tool. The multiple measurements may include natural gamma ray measurements, density measurements, resistivity measurements, nuclear measurements, and nuclear magnetic resonance measurements. The signal-to-noise ratio of the measured signals is increased using, for example, lateral stacking and multi-shell inversion. The subsurface formation property is estimated using the increased signal-to-noise ratio signals. The subsurface formation property may include porosity, adsorbed gas volume, free gas volume, bound water volume, free water volume, oil volume, and kerogen volume. A fluid analysis may be performed using a multi-dimensional nuclear magnetic resonance technique. Fluids such as water, oil, gas, and oil-based mud in the wellbore may be identified and/or evaluated. 
     Other aspects and advantages will become apparent from the following description and the attached claims. The accompanying drawings, described below, illustrate typical embodiments of the invention and are not to be considered limiting of the scope of the invention, for the invention may admit to other equally effective embodiments. The figures are not necessarily to scale, and certain features and certain view of the figures may be shown exaggerated in scale or in schematic in the interest of clarity and conciseness. 
    
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         FIG. 1  illustrates an exemplary well site system. 
         FIG. 2  shows a prior art downhole logging tool. 
         FIGS. 3A and 3B  are plots showing diffusion versus relaxation times for water and gas, in accordance with one embodiment in the present disclosure. 
         FIG. 4  illustrates a detailed fluid analysis, in accordance with one embodiment in the present disclosure. 
         FIG. 5  is a porosity model, in accordance with one embodiment in the present disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     Some embodiments will now be described with reference to the figures Like elements in the various figures will be referenced with like numbers for consistency. In the following description, numerous details are set forth to provide an understanding of various embodiments and/or features. However, it will be understood by those skilled in the art that some embodiments may be practiced without many of these details and that numerous variations or modifications from the described embodiments are possible. As used here, the terms “above” and “below”, “up” and “down”, “upper” and “lower”, “upwardly” and “downwardly”, and other like terms indicating relative positions above or below a given point or element are used in this description to more clearly describe certain embodiments. However, when applied to equipment and methods for use in wells that are deviated or horizontal, such terms may refer to a left to right, right to left, or diagonal relationship as appropriate. 
       FIG. 1  illustrates a well site system in which various embodiments can be employed. The well site can be onshore or offshore. In this exemplary system, a borehole  11  is formed in subsurface formations by rotary drilling in a manner that is well known. Some embodiments can also use directional drilling, as will be described hereinafter. 
     A drill string  12  is suspended within the borehole  11  and has a bottom hole assembly  100  which includes a drill bit  105  at its lower end. The surface system includes platform and derrick assembly  10  positioned over the borehole  11 , the assembly  10  including a rotary table  16 , kelly  17 , hook  18  and rotary swivel  19 . The drill string  12  is rotated by the rotary table  16 , energized by means not shown, which engages the kelly  17  at the upper end of the drill string. The drill string  12  is suspended from a hook  18 , attached to a traveling block (also not shown), through the kelly  17  and a rotary swivel  19  which permits rotation of the drill string relative to the hook. As is well known, a top drive system could alternatively be used. 
     In the example of this embodiment, the surface system further includes drilling fluid or mud  26  stored in a pit  27  formed at the well site. A pump  29  delivers the drilling fluid  26  to the interior of the drill string  12  via a port in the swivel  19 , causing the drilling fluid to flow downwardly through the drill string  12  as indicated by the directional arrow  8 . The drilling fluid exits the drill string  12  via ports in the drill bit  105 , and then circulates upwardly through the annulus region between the outside of the drill string and the wall of the borehole, as indicated by the directional arrows  9 . In this well known manner, the drilling fluid lubricates the drill bit  105  and carries formation cuttings up to the surface as it is returned to the pit  27  for recirculation. 
     The bottom hole assembly  100  of the illustrated embodiment includes a logging-while-drilling (LWD) module  120 , a measuring-while-drilling (MWD) module  130 , a roto-steerable system and motor, and drill bit  105 . 
     The LWD module  120  is housed in a special type of drill collar, as is known in the art, and can contain one or a plurality of known types of logging tools. It will also be understood that more than one LWD and/or MWD module can be employed, e.g. as represented at  120 A. (References, throughout, to a module at the position of  120  can alternatively mean a module at the position of  120 A as well.) The LWD module includes capabilities for measuring, processing, and storing information, as well as for communicating with the surface equipment. In the present embodiment, the LWD module includes a resistivity measuring device. 
     The MWD module  130  is also housed in a special type of drill collar, as is known in the art, and can contain one or more devices for measuring characteristics of the drill string and drill bit. The MWD tool further includes an apparatus (not shown) for generating electrical power to the downhole system. This may typically include a mud turbine generator powered by the flow of the drilling fluid, it being understood that other power and/or battery systems may be employed. In the present embodiment, the MWD module includes one or more of the following types of measuring devices: a weight-on-bit measuring device, a torque measuring device, a vibration measuring device, a shock measuring device, a stick/slip measuring device, a direction measuring device, and an inclination measuring device. 
     Electromagnetic logging tools can use one or more tilted or transverse antennas, with or without axial antennas. Those antennas may be transmitters or receivers. A tilted antenna is one whose dipole moment is neither parallel nor perpendicular to the longitudinal axis of the tool. A transverse antenna is one whose dipole moment is perpendicular to the longitudinal axis of the tool, and an axial antenna is one whose dipole moment is parallel to the longitudinal axis of the tool. Two antennas are said to have equal angles if their dipole moment vectors intersect the tool&#39;s longitudinal axis at the same angle. For example, two tilted antennas have the same tilt angle if their dipole moment vectors, having their tails conceptually fixed to a point on the tool&#39;s longitudinal axis, lie on the surface of a right circular cone centered on the tool&#39;s longitudinal axis and having its vertex at that reference point. Transverse antennas obviously have equal angles of 90 degrees, and that is true regardless of their azimuthal orientations relative to the tool. 
     An example of a tool that can be the LWD tool  120 , or can be a part of an LWD tool suite  120 A of the system and method hereof, is Schlumberger&#39;s ECOSCOPE tool, as shown in  FIG. 2 . Such tools may have onboard or surface processors that control or assist in controlling an operation. 
     While applicable to various logging techniques, one embodiment of the present disclosure is applicable to a logging while drilling tool of the type shown in  FIG. 2 . Although the tool itself is shown as a single collar  200 , it can be considered to have multiple distinct measurement devices to measure, for example, resistivity, thermal neutron porosity, hydrogen indices, density, spectroscopy, capture cross section (Sigma), borehole diameter (ultrasonic caliper), azimuthal density/PEF density, azimuthal formation natural gamma ray, and drilling dynamics (e.g., inclination, pressure, etc.). The tool may further comprise a pulsed neutron generator (PNG) with all the sensors required for the various related nuclear measurements, including near and far neutron detectors. The tool may comprise a first gamma ray detector (SSn) that is used for gamma ray spectroscopy, the sigma measurement, and the neutron gamma density measurement; and a second gamma ray detector (LSn) that provides the primary input for the neutron gamma density measurement. 
     In one embodiment, multiple passes of multi-shell, multi-dimensional nuclear magnetic resonance data are acquired. For a particular depth of investigation, various measurements can be made in which the polarization wait times and/or the echo spacings in the pulse sequence are varied. The raw nuclear magnetic resonance echoes of the multiple passes are preferably laterally stacked to improve the signal-to-noise ratio. The laterally-stacked data may be combined using a multi-shell inversion scheme to further improve the signal-to-noise ratio. 
     Multi-dimensional nuclear magnetic resonance techniques such as relaxation versus diffusion (e.g., D-T2 maps) can identify and evaluate various fluids such as water, oil, gas, and oil-based mud in a downhole formation. Similar to the case of increased surface relaxation effects that shift the relaxation time of water in large pores to shorter times for water contained in small pores, adsorbed gas has an added surface relaxation effect compared to free gas and therefore has a shorter relaxation time.  FIG. 3A  illustrates that water in small pores has a shorter relaxation time than water in large pores due to the increased surface relaxation effect. Similarly,  FIG. 3B  illustrates that adsorbed gas has a shorter relaxation time than free gas because of the increased surface relaxation effect, analogous to the well-understood water case. That effect or trait allows one to separate the adsorbed gas from the free gas, manifested as separate peaks in the D-T2 map, for example. 
       FIG. 4  shows a detailed analysis of fluid volumes including bound water, free water, oil-based mud filtrate, heavy oil, free gas, adsorbed gas, and kerogen. Three particular portions corresponding to three different wellbore depths are shown in the three D-T2 maps. The D-T2 map labeled “Adsorbed gas” ( FIG. 4E ) shows a single peak above the gas line corresponding to adsorbed gas. The D-T2 map labeled “Adsorbed and free gas” ( FIG. 4D ) shows two peaks on the gas line: the left peak corresponds to adsorbed gas and the right peak corresponds to free gas. The difference in their relaxation times separates the peaks. The D-T2 map labeled “Free gas” ( FIG. 4C ) shows a single peak just above the gas line that corresponds to free gas. 
     Referring to the porosity model shown in  FIG. 5 , porosity may be determined from a nuclear magnetic resonance measurement as the sum of all the individual fluid volumes, excluding kerogen, corrected for their respective hydrogen indices. The kerogen volume is equal to the difference between total porosity determined from a source such as density, density-neutron, or sonic and total porosity determined from a nuclear magnetic resonance measurement. 
     If the kerogen density is input, the porosity model shows that the volume of kerogen can be computed as shown in the 1 st  track of  FIG. 4 . If the average fluid density is input, the porosity model shows that the kerogen density can be computed as shown in the 2 nd  track of  FIG. 4 . 
     Examples of formations in which the disclosed method may be used include gas-bearing or gas-adsorbed shales and coal beds. 
     It should be appreciated that while the invention has been described with respect to a limited number of embodiments, those skilled in the art, having benefit of this disclosure, will appreciate that other embodiments can be devised which do not depart from the scope of the invention as disclosed herein. Accordingly, the scope of the invention should be limited only by the attached claims.