Borehole NMR: Different Problems Different Solutions

Transcription

1 Borehole NMR: Different Problems Different Solutions Stefan Menger Mgr. Applications Development Halliburton/NUMAR Abstract During the last decade, NMR has significantly improved the understanding of hydrocarbon reservoirs. NMR logs provide reliable formation porosity and allow for the identification and quantification of reservoir fluids. Initially, NMR logging tools were deployed only on wireline. With the growing success of such systems, the demand developed for an LWD-based approach. LWD systems are preferably used in high-cost offshore wells, where rig-time for wireline logging comes at a premium, and in environments where deployment of wireline systems is difficult or even impossible (e.g., horizontal wells). Another application of NMR in formation evaluation is its use in a downhole fluid laboratory. Deployed as part of a wireline formation testing tool, the NMR module measures and deduces hydrogen index, viscosity, and gas-oil-ratio of formation fluids under reservoir conditions. This paper briefly reviews the principle of NMR measurements. It explains the different approaches to downhole NMR systems and presents field examples. Introduction Formation evaluation strives to answer key questions about a potential hydrocarbon reservoir: How porous is the rock, what are the fluids contained in the rock, and are the fluids producible? Answers to those questions are inferred from downhole measurements (electric, nuclear, sonic, and others logs), or determined by carrying out downhole pump-out tests ( formation testing ), and bringing the fluid samples to surface for further analysis. The successful commercialization of pulse-echo NMR logging (Miller et al., 1990) improved on the understanding of hydrocarbon reservoirs by adding direct measurement of porosity, bound and free fluid fractions, and by allowing reliable fluid typing to the spectrum of downhole measurements. Deployed on wireline, these systems deliver NMR data and results after the well is drilled. In high-cost environments, such as in deep water offshore exploration where information rig time comes at a premium, the preferred method is logging-whiledrilling (LWD). Here the sensors are deployed as part of the drill-string. Located close to the drill bit, they deliver data and results in real-time while the well is being drilled. In 2000 the first LWD-deployed NMR tool (MRIL-WD 1 ) was commercially introduced (Prammer et al., 2000). A third system brings NMR technology to formation testing. A module in Halliburton s Reservoir Description tool (RDT 1 ), the MRILab 1 takes measurements on fluid pumped from the reservoir formation and determines fluid characteristics such as relaxation times, hydrogen index, viscosity, and gasoil-ratio in real-time (Bouton et al., 2001). MRILab measures the fluids under in-situ condition. This virtually eliminates the risk of irreversible change which frequently occurs due to changes in temperature or pressure when the fluid samples are transported from the reservoir downhole to an offsite laboratory. All three systems are now used commercially for different applications. The Principle of NMR Measurements For both the LWD and the NMR fluid laboratory system (Figure 1), the effect of motion on NMR measurements had to be overcome. In the case of LWD, the drilling induced lateral motion causes the NMR signal to dephase if no precautions are taken. To avoid having to stop the sampling process in the case of RDT, the NMR fluid laboratory needs to be capable of performing NMR experiments while the fluid is moving through the instrument. Figure 2a shows the basic principle of pulse-echo NMR: The reservoir fluid is exposed to a magnet field. As the protons are polarized along the magnetic field, the magnetization builds up over time (red dashed curve, polarization time). The polarization behavior is described by the relaxation time T1. This relaxation 1 MRIL-WD, MRILab, MRIL-Prime and RDT are marks of Halliburton

2 time depends on the fluid type. At any time during the polarization process, a pulse-echo experiment, employing a series of radio-frequency pulses to excite the protons ( CPMG series) can start. The initial amplitude of the NMR signal is defined by the polarization state of the fluid at the time when the CPMG pulse sequence is started and by the reservoir porosity. The amplitude decay of this so-called echotrain, characterized by the relaxation time T2, is governed by the type of fluid and acquisition parameters (magnetic field gradient, inter-echo spacing TE). A sequence of T2 experiments performed at different polarization times is used to map out the polarization curve. Typically only a few T2 echoes are acquired to obtain the T1 information (Fig. 2b). For the LWD system and the fluid laboratory, the T1 was designed such that it is inherently motion insensitive. At the start of each experiment, the magnetization in a large volume is destroyed. In the LWD system, this saturated volume has the shape of a thick-walled cylinder (Figure 3), while the entire sensor volume in the downhole NMR fluid laboratory is saturated. After a certain polarization time TW during which the tool can move laterally, the T2 experiment reads out the polarization state yielding one point on the T1 curve. This sequence is repeated with different polarization times. The volume excited during the T2 experiment is much smaller than the initially saturated volume. Its shape and location is designed such that it always falls into the previously saturated volume. Since the T2 echo train contains only a few echoes, and thus is very short in time (typically less than 15ms), any motion effect is greatly reduced. In practice, motion induced errors can be ignored. For further information about motion-insensitive T1 experiments, the reader is referred to Prammer et al. (2000). The interpretation of T1 and T2 data is similar: In both relaxation times information about the pore size and the pore fluid is encoded. For instance, clay-bound fluid relaxes very fast (short relaxation times), while hydrocarbons relax slower (Figure 4). For a long time, wireline NMR logging focused on T2 logging. With the advent of multifrequency tools, T1 logging has become possible. Thus the practice of T1 measurements, which is primarily used in laboratory NMR experiments, has found its way back to the borehole (Prammer et al., 2002). NMR Deployment and Applications Downhole NMR is currently deployed in three different ways: Wireline, LWD, and Downhole Fluid Laboratory. Each method solves different problems and has different applications Wireline and LWD: NMR on wireline, implemented in the MRIL-Prime 1 tool series and the MRIL-WD version, primarily delivers nuclear source-free porosity. With environmental and security concerns growing, the aspect of not having to use chemical radioactive sources to determine formation porosity attracts more and more attention. Further, the porosity fractions (porosity associated with clay-bound, capillary-bound, and free fluid) are used as input to existing models to determine the permeability of the reservoir rock. Another important application is the unambiguous identification of the fluid contained in the pores of the reservoir rock. A field example from the Gulf of Mexico is shown in Figure 5. For comparison, the natural gamma log is shown in track 1. Tracks 2-4 present the NMR porosity fractions acquired with the MRIL-WD tool: Track 2 shows the T1-based curves acquired while the well was drilled with blue denoting the free fluid associated porosity, gray the capillary-bound porosity, and green the claybound porosity. The dual-tw data in tracks 3-4 was acquired with the MRIL-WD during an after-drilling wipe pass over the same interval. In all tracks, the sand bodies can easily be recognized by the abundance of free fluid (blue shading). Track 5 shows the interpretation of the dual-tw data: The lower part of the log example shows the oil-water-contact (OWC), where the free water is shown in blue, bound water is shaded gray, and the oil volume is displayed in green. The field example in Figure 6 is culled from a logging campaign offshore Europe. Similar to the previous example, track 1 shows the gamma log, while tracks 2-4 present the MRIL-WD data. For comparison, track 2 also contains the neutron and density porosity. Only in the sands (large blue shaded areas) these two logs indicate the true formation porosity, in the shales they exhibit the well know spread. The T2-based logs were acquired with two different inter-echo spacings thus enhancing the response of the NMR signal to gas. The suppressed amplitude of the TE=4.8ms signal in the upper part of the lower sand (denoted with A ) could be an indication for gas. These two examples demonstrate that T1 measurements as described above are suitable to acquire NMR logs while drilling. There is a good agreement between the while-drilling results and the result acquired during the post-drilling wipe pass. Fluid Laboratory: The downhole NMR fluid laboratory, MRILab, determines different fluid properties: Hydrogen Index, T1, T2, and Diffusivity. Based on those and ancillary data (fluid capacitance), it is possible to unambiguously identify the fluid. This information is used in multiple ways. By determining

3 whether the fluid produced during downhole fluid testing using the RDT is borehole fluid or connate fluid, and by further distinguishing between connate water, oil and gas, pump out times can be minimize. Figure 7 shows a screen shot taken during such a pump out sequence. Focusing on the center track, the upper part of the waterfall plot shows a sharp relaxation time peak with long T1 s typical for gas. This signifies gas that remained in the flow line from the previous test. Further down, oil-based mud filtrate enters the MRILab sensor, and after about 20 minutes of pumping, the transition to connate gas can be seen. By following the transitions of the T1 spectrum, one can predict the time required to produce a clean fluid sample, where the contamination with mud is not greater than the threshold required by the customer. Another application of MRILab is shown in Figure 8. Employing a special sequence of T1 and T2 experiments and overlaying the result on a previously published interpretation chart (Lo et al., 2002), the gas-oil ratio (GOR) of the fluid is determined. The T1 and T2 results also yield the fluid viscosity in realtime. Downhole fluid analysis offers the additional advantage over post-logging lab analysis, that all the experiments are performed on the fluid under in-situ conditions. Thus, the results deliver more realistic information about the reservoir. Summary Nowadays, downhole NMR comes in different packages. Each deployment method is optimized to provide answers for a specific range of petrophysical questions. Together, the data acquired with the different NMR systems provides detailed information about the reservoir rock and the fluid contained in it. References Bouton, J., Prammer, M.G., Masak, P., Menger, S.: Assessment of Sample Contamination by Downhole NMR Fluid Analysis, paper presented at the 76 th Annual Technical Conference and Exhibition of the Society of Petroleum Engineers, New Orleans, Sep. 30 Oct. 3, Lo, S.-W., Hirasaki, G., House, W.V., and Kobayashi, R.: Mixing Rules and Correlations of NMR Relaxation Time With Viscosity, Diffusivity, and Gas/Oil Ratio of Methane/Hydrocarbon Mixtures, paper 77264, SPE Journal, 24-33, March Miller, M.N, Paltiel, Z. Gillen, M.E., Granot, and J. Bouton, J.C.: Spin Echo Magnetic Resonance Logging: Porosity and Free Fluid Index Determination, paper SPE presented at the 65 th Annual Technical Conference and Exhibition of the Society of Petroleum Engineers, New Orleans, LO., 1990 Prammer, M. G., Drack, E., Goodman, G., Masak, P., Menger, S., Mows, M., Zannoni, S., Suddarth, B. and Dudley, J.: The Magnetic Resonance While-Drilling Tool: Theory and Operation, paper presented at the 75 th Annual Technical Conference and Exhibition of the Society of Petroleum Engineers, Dallas, TX, Oct. 1-4, Prammer, M.G., Akkurt, R., Cherry, R., Menger, S.: "A New Direction In Wireline And LWD NMR", paper DDD presented at the 43 rd Annual Logging Symposium of the Society of Professional Well Log Analysts, Oiso, June 2-5, ft. 14 Diam. of Investigation Sensitive Region 4-1/2 I.F. Connection Spiral Wear Pad 4-1/2 I.F. Connection Data Access Port (SWRO) Power, Electronics and Data Memory Spiral Wear Pad Antenna and Magnet Figure 1: Schematic drawing of MRIL-WD tool (left) and the MRILab antenna section (right)

4 (A) (B) Figure 2: The principle of NMR measurements: (A) The protons of fluid exposed to a static magnetic field are polarized. At any time during the polarization (red dashed curve), a CPMG NMR experiment can be started (blue curves). (B) Only very few echoes (typically 1-10 echoes) are needed to map out the polarization curve

5 Figure 3: Here, the geometry of the MRIL-WD tool is shown as an example of a T1 logging sequence. During the first step in the T1-logging sequence the magnetization in a large volume (gray shaded area) is destroyed ( saturated ). After a certain polarization time (recovery time), a CPMG pulse-echo experiment, typically acquiring only very few echoes, reads out the magnetization yielding one data point in the polarization curve. This process is repeated with different polarization times to map out the entire polarization curve. Solids.invisible to MRI Fluids rock matrix dry clay irreducible clay bound moveable water hydrocarbon clay bound irreducible movable water hydrocarbon T1 Recovery T2 Relaxation time, sec Figure 4: Petrophysical information (pore size and fluid type) is encoded in both T1 and T2 relaxation time curves

6 Figure 5: MRIL-WD field example from the Gulf of Mexico. Track 1: natural gamma log; tracks 2-4: NMR porosity fractions acquired with the MRIL-WD tool; Track 5 contains the interpreted data. For details refer to the text. (A) Figure 6: MRIL-WD field example from Offshore Europe. Track 1: natural gamma log; tracks 2-4: NMR porosity fractions acquired with the MRIL-WD tool. For comparison, track also shows the neutron and density porosity. For details refer to the text. -6-

7 Figure 7: MRILab example. The center track ( blue waterfall plot ) shows the T1 spectra as they were measured during the pump-out process. Clearly the changes in the characteristic of the T1 spectra can be seen. Figure 8: Interpretation of MRILab measurements. The red dot shows the T1 result and diffusivity D determined by NMR superimposed a model (Lo et al., 2002). This yields a gas-oil ratio (GOR) of about 90 which was confirmed by post-logging laboratory measurements. Further, the fluid viscosity values derived from T1 and diffusivity are in good agreement