Hydrocarbon detection and reservoir imaging during enhanced oil recovery using an inductive EM multi-frequency method

Transcription

1 Hyrocarbon etection an reservoir imaging uring enhance oil recovery using an inuctive EM multi-frequency metho Carlos. A. Dias * (LENEP/UENF), Olivar. A. L. e Lima (CPGG/UFBa), Héison. K. Sato (CPGG/UFBa) & José. A. C. Moraes (PETROBRAS/UN-BA) Copyright 007, SBGf - Socieae Brasileira e Geofísica This paper was prepare for presentation at the 10 th International Congress of The Brazilian Geophysical Society hel in Rio e Janeiro, Brazil, 19- November 007. Contents of this paper were reviewe by the Technical Committee of the 10 th International Congress of the Brazilian Geophysical Society an o not necessarily represent any position of the SBGf, its officers or members. Electronic reprouction, or storage of any part of this paper for commercial purposes without the written consent of the Brazilian Geophysical Society is prohibite. Abstract We iscuss the results of an experimental stuy performe with a multi-frequency electromagnetic metho over a mature oil fiel in Recôncavo basin, Bahia-Brazil. Five 1.8 km transects 00 m apart an extening over a block of the oil reservoir were surveye. The processe EM ata are represente as cross-sections an maps of apparent resistivity an inuce polarization parameter, using a consistent plotting proceure. All the sections, controlle by well logging ata, allow to recognize the following geological features: (i) the oil sanstone horizons an their trapping shales; (ii) the oil-water interface an some zones of water invasion; an (iii) lateral electric contrasts representing fault zones. These results suggest the real possibility of the use of the spectral EM metho in the irect etection of hyrocarbons, as well as for monitoring the efficiency of the artificial flui injection use for seconary recovery. Also, this experiment brings about a further evelopment in the inuctive measurement of IP an introuces, for the first time, the use of this property in oil reservoir exploration an characterization. Introuction The electronic an computing evelopments are bringing notable progresses in the performance an resolution of EM methos. We have available toay: (i) a high precision an versatile equipment with capacity, couple to a powerful computer, to register an process a larger amount of fiel ata; an (ii) goo techniques for inverting an representing the EM ata. The limitations associate to the resolution of EM methos an environmental an cultural noise effects, normally present in oil fiel, have been overwhelme by the use of a ense an multiple ata acquisition, a careful ata processing an an effective integration of the available geophysical an geological informations. In a structural geological block, having imensions of 800 m by 1,800 m by 1,000 m epth, a frequency omain EM experiment has been mae (Figure 1). T Direct transmitter positions l = 400m T1 l = 00m E-6 E-14 E- -50 E-15 E-30 E-31 E-3 E E E E-7 E-16 E-5 E-34 E E E-17 E-6 E-35 E-3 E E-9 E E-18 E-7 E-19 E-8 E-36 E E-5 E-11 E-0 E-9 E-1 E-1 E Reverse transmitter positions T1' T' Figure 1 Transverse lines, an transmitter positions

2 Hyrocarbon etection an reservoir imaging using an EM metho Methoology The experiment was performe over the selecte structural block, along the lines shown in Figure 1. Five transects 00 m space were surveye along the ip irection of the geological strata. The receiver stations were istribute in such a way as to cover the whole extension of the oil reservoir, as well as a representative portion of its water zone. The transmitter stations were efine in such a way that the T-R istances woul assure a epth of exploration between 300 an 1,000 m. Two positions of the transmitter separate by 500 m were use at each sie of the target block, both to increase epth of exploration an to correct istortions cause by lateral heterogeneities an anisotropy. Receiver System Three high sensitivity coils of 0.8 m length are lai on the groun, with one of them oriente raially to the transmitter loop center. Simultaneous measurements of the absolute value of in-phase an quarature components of the magnetic fiel along r, φ an z, are obtaine for each frequency. In this work, only the raial component has been processe. Transmitter System The spectral EM transmitter is compose of: a square horizontal loop having 00 m or 400 m of sie, having a maximum ipole moment of.5 x 10 6 A.m ; an electronic control unit whose function is to impose to a loop, successively AC current at cycles in 54 frequency values; an a motor-generator to energize the loop. The frequency range is from 1.15 Hz to 10,473 Hz, in values given as f = 1.15x n-1 ; n = 1,, 54. ρ σ / σ Data Plotting - The calculate pair ( a, ) is i a plotte at a subsurface point, as shown in Figure, given by the following cartesian coorinates: where δ x = 1+ r ( ηδ / r) ηδ y = 1+ ; ( / ), ηδ r is the vertical skin epth of the ipole fiel, an η a calibration coefficient, taken to be here equal to 0.4. Data Resolution - Resolution in this metho can be efine as half the variation in the coorinates x an y (see ata plotting), by a change of frequency from a value to the next in the sequence of the measurements taken by the equipment for a given r (T-R separation). By varying r in the range 0.8 Km to.5 Km, in a geological environment having resistivity values from 1Ωm to 3,500 Ωm, an the frequency ranging approximately from 1Hz until 10kHz, the resolution is given by x 4 to 5 m an y 3 to 45 m, except at a very narrow interval of the parameter θ = ( πµ fσ / ) 1/ r aroun 6, where it coul be o two times as larger (Figure 3) T x r P(x,y) Figure : Propose scheme to plot the EM results in a epth profile. Figure 3: Horizontal an vertical resolution versus inuction number. Results In Figures 4 an 5 we show the average resistivity an IP parameter sections for the two irect transmitter positions (see Figure 1) obtaine in line 100. Our geological interpretation is superpose on these images to emphasize the complementarity of the geoelectrical information. In Figure 6, this interpretation is checke against the geological section constructe from well logging ata obtaine along the same line. In general, there is a goo corresponence between the interprete EM sections an maps with the subsurface geology, except by some istortions cause by the proceure use to compute the coorinates of the main attribution points. Effects of cultural noises, mainly at high frequencies, were effectively remove uring the processing stage. y 1,337 1,15 R 1,588 1,887 1,15,4 1,337,664 1,588,4 1,887

3 Carlos. A. Dias, Olivar. A. L. e Lima, Héison. K. Sato & José. A. C. Moraes Discussion In most cases, the combination of IP an resistivity ata was enough to elineate the geoelectrical moels for the stuie block. The maps an sections allow to conclue that: i) the lithological interfaces an the faults are pretty well marke by istinct contrast of IP an resistivity; ii) the Sergi formation storing oil is shown as a zone of high resistivity an IP parameter; iii) the istinction of oil/water 3 contacts are also electrically notable at most sections an maps, an in the central zone very sharply. Furthermore, the following inferences can be rawn from the EM ata: a) a certain region at the top of the Sergi reservoir is being better raine in terms of oil prouction. Within it an expressive lowering of apparent resistivity an IP effect may be representing a progressive replacement of oil by water, in the west portion of the central zone of the Figure 4: Reservoir imagin through an apparent resistivity section along the line 100,composing measure ata for two transmitter positions (at stations 400 an 900). Figure 5: Reservoir imagin through an apparent IP section along the line 100,composing measure ata for two transmitter positions (at stations 400 an 900).

4 Hyrocarbon etection an reservoir imaging using an EM metho 4 Figure 6: Comparison between the EM interpretation an the geological section along the line 100 stuie block; b) the original oil/water interface within the Sergi sanstone appears to have been isplace a short istance away, mainly in a sub vertical irection, in consequence of the forceful injection of water; c) in several sections an on the central shallower zones we observe another possible influence of the water/vapor injection pattern use for seconary recovery. This occurs in the shape of a resistive anomaly penetrating the upper shales of Caneias Formation. In this zone we also observe a slightly increase in the inuce polarization parameter, probably ue to injection of oil or fresher water than native formation water, into fractures. Conclusions 1) In general, the consistency of the electromagnetic information obtaine in this experiment is fully satisfactory in confrontation to the available geological an well log ata; ) Small istortions are observe when comparing a geoelectrical section with the corresponing geological section constructe using well log ata: the faults are somehow curve in the irection of the transmitter position, for the lower frequencies; 3) Sato (1979) an we now, in this experiment, are the first ones to make inuctive IP measurements, in the frequency omain, successfully; 4) The high consistency an the reprouctiveness of the measure parameters an their location in subsurface, practically invariant by transmitter an receiver positions, are the warranty of the intrinsic value of this new metho an techniques use. Acknowlegments The authors acknowlege the petroleum engineer Carlos G. S. e Aquino, while Reservoir Manager of PETROBRAS/UN-BA, by his ecisive support for this project, for the geological an geophysical ata isclosure an for many fruitful iscussions to evaluate the EM results contributing to enrich the present work. This work is part of a project jointly supporte by FINEP/CT-PETRO an PETROBRAS within the Brazilian Research Network on Geology an Geophysics of Mature Oil Fiels. We also acknowlege the geophysicists Antonio A. Carrasquilla, Marco A. Ceia an Marcos B. Machao by their participation in the fiel an/or office work, an CNPq by research fellowships to C. A. Dias an O. A. L. e Lima. References Dias, C. A., 1968, A non-groune metho for measuring inuce electrical polarization an conuctivity. Ph.D. Thesis, Univ. of California, Berkeley, USA.

5 Carlos. A. Dias, Olivar. A. L. e Lima, Héison. K. Sato & José. A. C. Moraes 5 Dias, C. A. an Sato, H. K., 1981, A multifrequency electromagnetic metho for interpretation of IP an resistivity: Theory an experimental work using a system operating in the range 1 to 43,008 Hz. 51th Ann. Internat. Meet., Soc. Expl. Geophys., Los Angeles/CA, Exp. Abs., 1, 7-8. Sato, H. K., 1979, Electromagnetic metho for inuce polarization an resistivity interpretation using a multi-frequency system prototype. Master Dissertation in Geophysics, Fe. Univ. of Bahia, Brazil (in portuguese).