Induced polarization in hydrocarbon-saturated sands and sandstones: experimental study and general effective medium modeling

Transcription

1 Induced poarization in hydrocarbon-saturated sands and sandstones: experimenta study and genera effective medium modeing Summary We have studied the induced poarization (IP) response of mutiphase porous systems by conducting compex resistivity (CR) frequency-domain IP measurements for sands and sandstones sampes containing sat water in pores and those whose unsaturated pores were fied with synthetic oi. The resuts of our study show that the oi - saturated sands and sandstone sampes are characterized by a significant IP response. We used a generaized effectivemedium theory of induced poarization (GEMTIP) mode to anayze the IP parameters of the measured responses. A conceptua GEMTIP mode of poarizing custers is proposed to expain the observed IP phenomena. Our studies confirm earier geophysica experiments with the appication of the IP method for hydrocarbon (HC) exporation. Introduction The measurement of the eectrica induced poarization (IP) effect has proven to be one of a few geophysica methods providing in situ information about rock mineraogy, especiay in the search for disseminated mineras with eectronic conductivity. At the same time, the method has been appied to study the earth materias that do not contain conductive mineras, ike sedimentary rocks. The previous IP studies of nonmetaic earth materias were focused on cay minera sois, sandy and shay sediments containing cay mineras (Kein and Si, 1982). Laboratory studies of the eectrica characteristics of such rocks show diagnostic signatures of what they consist of and thus, can ead to a proper cassification of rocks in terms of the presence of cay and other materias. At the same time, the study of reservoir rocks was imited and did not incude a quantitative anaysis of the reationships between the petrophysica parameters of the rock sampes and the IP responses. The poarization phenomenon was previousy studied in detai by Wait (1959) and its modern deveopment stems argey from the work done by Bei (1953). The theoretica foundations of the IP effect in compex mutiphase heterogeneous rocks were deveoped by Zhdanov (26; 28a, b). In this paper we study the IP response of mutiphase porous rocks by conducting compex resistivity (CR) frequencydomain IP measurements. This study is based on aboratory anaysis and modeing of induced poarization and resistivity measurements on sandstone sampes from southern Utah. These resuts are compared with those obtained for synthetic rock sampes, prepared using chemicay pure sand, de-ionized (DI) water, and synthetic oi. Preparation of the sampes We prepared two types of sampes for a measurement. The first type is represented by the sandstone sampes coected in southern Utah. They were cut to 3 3 mm cross section and 4 mm ength size and poished to produce a rectanguar shape. The second type of sampes was formed by artificiay prepared water and oi saturated sands and oied sands. Figure 1 shows the photograph of these sampes. The foowing steps describe the sampe preparation. 1. Sieved commercia EMD Co. chemicay purified quartz sand (.1- to.25-mm grain size) was ceaned from cay and dust partices by repeated washing. The quartz minera density was about 2.62 g cm -3. After saturation in satwater at the first stage and in synthetic oi at the second stage, as described beow, the sampes of the second type were paced in a sampe hoder, a poyviny choride (PVC) tube (a cartridge). The PVC tube had 3 mm in externa diameter, 27 mm in interna diameter, and was 7 mm ong. It was covered at both ends by 4 mm-thick brass stoppers and seaed with epoxy gue. 2. We conducted the measurements of the IP responses of the sampes at two states. At the first state, the sandstone sampes and the sand cartridges had been saturated in.1 mm of KC water soutions for 48 hours. We conducted the measurements on haf of the sampes at this state. A sand sampe saturated with the satwater was transferred to a cartridge. We kept the sandstones saturated with the satwater and the sand cartridges isoated from the atmosphere in Zipoc bags to reach and maintain equiibrium between the iquid and soid phases. 3. The other haf of the sampes was saturated with synthetic ight oi containing ess than.5% of vitamin E, which was added to prevent oi oxidation. This process is described beow. The sand sampe saturated in satwater was dried over a Pyrex Buchner funne on a 9 mm perforated pate using 42.5 mm fiter-paper support discs and Whatman quaitative fiter paper, grade 3, at a vacuum pressure differentia in excess of.5 atm. Immediatey after removing the satwater excess by soaking the sand with the residua satwater on quartz grains, it was exposed to synthetic oi and soaked using the same Buchner setups three times. The product was oaded into a cartridge and stored in air isoated bags as described above. 4. For oi saturation of sandstone sampes we prepared a 3/7 voume ratio soution of heptane and synthetic oi. SEG Houston 29 Internationa Exposition and Annua Meeting 774

2 Induced poarization in hydrocarbon-saturated sands and sandstones: experimenta study and genera effective medium modeing After a drying procedure, the sandstone sampes were saturated for 12 hours in this soution to evaporate heptane from the sandstones. We repeated this procedure three times to maximize the oi content in the pores of the sandstone. We used diuted synthetic oi to decrease its viscosity and, therefore, to faciitate the penetration of oi into the pores of the sandstone. The product was stored in air isoated Zipoc bags as described above. Figure 1: Photograph of sandstone (eft) and sand cartridge (right). Data coection and acquisition We tested the viabiity of the IP in HC-saturated sands with mutifrequency EM measurements at Zonge Engineering and Research Organization, Inc, Tucson, Arizona. In order to measure the compex resistivity (CR) of the rock sampe, we injected a current at a frequency range of Hz and measured the ampitude of the votage and its phase with respect to the current. The frequency domain data were recorded for 36 frequencies in the scanned range. The CR measurement invoved normaizing the ampitude and phase data by the DC resistivity. These two parameters were transformed into the actua rea and imaginary resistivity vaues in Ohm-m. Contro experiments To estabish an unsaturated sandstone matrix resistivity as a reference, we used the DC measurements. We found that both bare (unsaturated) sandstone and oi-saturated sandstone (without preiminary satwater soaking) had an amost infinitey arge DC resistivity. The next contro experiment invoved the measurement of sandstone exposed to DI water (not sated KC water) for 12 hours. This sampe had a measurabe DC resistivity caused by residua sat contained in the sandstone matrix. However, the resistivity of the sampe changed with time due to nonequiibrium eectrochemistry in this sampe. Therefore, in our experiments we found that (i) saturation with satwater is necessary ingredient in sampe preparation for EM measurement of dry and oi-containing sandstone sampes, and (ii) for a systematic EM study of sandstone sampes, the ph of sat water shoud exceed the ph of the residua sats. In this study the sampes saturated by satwater ony (as described in section 2 of the sampe preparation) were considered to be the contro sampes for the measurement of the oi-containing sampes (as described in sections 3 and 4 of the sampe preparation). The sampes which were not saturated by oi were prepared exacty under the same conditions as the hydrocarbon-containing sampes. Therefore the difference between the measured EM responses in the controed and hydrocarbon-containing sampes corresponds to the presence of hydrocarbons and not to geometry and measurement artifacts. On the other hand, the measurement of the IP effect in the oi-saturated sand cartridge rues out the possibiity of an IP effect due to metaic incusions in the sandstone sampes, which might contain traces of graphite, etc. Comparison of SCW and SCO sampes rues out possibe contribution of brass eectrodes into IP effect. In addition, a measurement system itsef was isoated from the high frequency sources and was caibrated by the interna reference to avoid possibe induced charges and noninearity in the system response. Thus, we took a possibe precautions to determine the rea IP effect in the sandstones, which was entirey due to cation-radica poarization on the boundary between the oi, satwater, and the surface of the soid rock matrix. Data processing and interpretation We used the measured frequency domain data to cacuate rea and imaginary parts of the compex resistivity curves for different rock sampes. The quaitative anaysis shows that the cacuated resistivity curves have a characteristic shape simiar to the predicted effective resistivities computed according to the GEMTIP resistivity reaxation mode (Zhdanov, 28a). We aso conducted a quantitative anaysis of the observed data using the GEMTIP mode, which wi be discussed beow. Figures 2 presents the recorded imaginary resistivity vs. frequency curves for sampes SCO (sand-cartridge-oi) and SCW (sand-cartridge-water) respectivey. Figure 3 present the recorded resistivity vs. frequency curves for SSO (sandstone-oi) sampes. It is evident that the resistivity reaxation curves for both the SSO and SCO sampes are characterized by a pronounced IP effect. Note aso an essentia negative IP effect in Figure 2 for the SCO sampe. SEG Houston 29 Internationa Exposition and Annua Meeting 775

3 Induced poarization in hydrocarbon-saturated sands and sandstones: experimenta study and genera effective medium modeing Figure 2: The imaginary parts of the resistivity vs. frequency curves for the water (SCW) and oi-saturated sand (SCO) cartridges. Both Ohoeft (1985) and Titov (24) observed a simiar phase/imaginary part increase at higher frequencies (near 1 3 Hz) that is seen in the data for sampes SSO and SCO. Ohoeft (1985) attributed this increase to the cation exchange capacity (CEC) effect at higher frequencies. He aso reported sizabe negative IP effects in severa sampes. However, the nature of the negative IP effect sti has to be carified (Waker, 28). Athough the sand-cartridge-oi (SCO) sampe exhibited a broad IP peak, it sti coud be compared to the IP peak in the satwater-saturated sandstone sampe (SSO). The difference is possiby due to the principay different configuration of pores in the sandstone sampes and, therefore, to the different geometry of the distributed capacitor in HC-saturated sands. We appied the principes of the generaized effective-medium theory (Zhdanov, 28a) to anayze and interpret the observed data. Basic formuas of the effective-medium theory of induced poarization In the framework of the GEMTIP mode, we represent a compex heterogeneous rock formation as a composite mode formed by a homogeneous host medium of a voume V with a compex conductivity tensor ˆ σ ( r) (where r is an observation point) fied with grains of arbitrary shape and conductivity. Rea and Imag resistivity, Ohm-m img1 Re Frequency, Hz Figure 3: The rea and imaginary resistivities of the oi-saturated sandstone (SSO) sampe. In the genera case, the rock is composed of a set of N different types of grains, the th grain type having a compex tensor conductivity ˆ σ. The grains of the th type have a voume fraction f in the medium and a particuar shape and orientation. Foowing Zhdanov (26, 28a), we can write the foowing expression for the effective conductivity of the poarized inhomogeneous medium: N 1 1 ˆ ˆ p ˆ ˆ e I p = + I r I p + Γ + f = 1 ˆ σ ˆ σ ˆ ˆ σ ( ) ˆ ˆ σ, (1) where σ ˆ e is an effective-medium conductivity tensor; ˆ σ is an anomaous conductivity tensor; p ˆ σ p ˆ = Ι+ σ is the poarized anomaous conductivity; p is a surface poarizabiity tensor; Γ ˆ is a voume depoarization tensor; and index corresponds to the grain of the th type. The ast formua provides a genera soution of the effective conductivity probem for an arbitrary mutiphase composite poarized medium. This formua aows us to find the effective conductivity for incusions with arbitrary shape and eectrica properties. That is why the new composite geoeectrica mode of the IP effect may be used to construct the effective conductivity for reaistic rock formations typica for mineraization zones and/or petroeum reservoirs. ˆ We used a simpest case of a spherica two-phase GEMTIP mode for data modeing. It coud be shown that in this case SEG Houston 29 Internationa Exposition and Annua Meeting 776

4 Induced poarization in hydrocarbon-saturated sands and sandstones: experimenta study and genera effective medium modeing formua (1) coud be simpified (Zhdanov, 26, 28a) and used to determine mineraization or hydrocarbon saturation from the recorded eectrica data: m 1 1 ρe α = ρ 1 + fm 1, C 1 + ( iωτ ) (2) 1/ C ρ ρ a = 3 ; τ = ( 2 ρ + ρ ), 2ρ ρ α + where ρ [Ohm-m] is the resuting effective resistivity, eα ρ [Ohm-m] is the matrix resistivity of the rock being modeed, f is a voume fraction voume of a grain, grain chargeabiity, ω [Hz] is an anguar frequency, m is a τ [second] is a time constant, C is a decay coefficient, [Ohm-m] is a grain resistivity, a [m] is a grain radius, and α is a surface poarizabiity coefficient. Conceptua mode of sand-cartridge-oi sampe We consider a mutiphase mode, which is formed by sand custers (Prince et a, 1995), covered by a ayer of conductive satwater and oi matrix as a structura mode. In this mode, the satwater-fiing space is considered as a conductive grain, which sustains the arge current. The voume fied by oi is considered as a rock matrix having mosty dieectric properties. The areas of contact of the individua grains are considered as narrow conductive passageways through the thin water fims on the grain surfaces. These areas of contact form arge sand custers, covered by a ayer of water, which are treated as arge conductive passageways. The poarization of the satwatersaturated custers in this condition shoud be arger than that of the saturated oi, which occupies the rest of the porous space. In this mode the IP effect is caused mosty by the eectrica doube ayers formed on the boundaries between the sand custers and oi matrix. The conceptua mode is capabe of expaining the observed frequency dependence of the IP effect in structura terms. Our mode suggests that at the critica water content, pore water becomes predominanty adsorbed on the soid surface. Therefore, a generic cation exchange capacity (CEC) effect and the eectrica doube ayers on the surface of the sand custers form a spatiay distributed capacitor (see right pane 2 in Figure 4). The shape and composition of this spatiay distributed capacitor define the nature of the IP effects in hydrocarbon-saturated sands. We cacuate the resistivity reaxation curve for this mode using the GEMTIP approach. The GEMTIP curve fits we the ρ eectrica imaginary resistivity response of the sandcartridge-oi (SCO) sampe, as shown at Figure 4. Resistivity, Ohm-m -1-2 R, -3 Ω-m Frequency, 1 2 Hz Figure 4: The eectrica imaginary resistivity response of the sandcartridge-oi (SCO) sampe. The ines formed by stars represent experimenta data, whie the ines formed by circes corresponds to the theoretica GEMTIP mode data. The conceptua mode of oied sandstone sampes is shown in the right pane. Concusions Imaginary In this paper we have examined the possibiity to detect the IP effect in HC saturated rock sampes. We observed a pronounced IP effect in sandstone sampes, which were artificiay saturated with synthetic oi. The observed IP effect was attributed to membrane poarization phenomena. We have deveoped a conceptua mode of sand-cartridgeoi sampe. The GEMTIP method was used to anayze the IP effect in the HC-saturated sands and sandstones. In an agreement with the earier geophysica experiments our study confirms the feasibiity of empoying the IP method for HC exporation. Acknowedgments Imaginary SSO sampe Frequency, Hz Observed observe predicte Predicted Mode Oi matrix (high R) Sand custer with sat water ayer (ow R) The authors acknowedge the support of the University of Utah Consortium for Eectromagnetic Modeing and Inversion (CEMI). A specia thanks to Scott Urquhart, Emmett Van Reed, and Anna Szidarovszky from Engineering and Research Organization Inc. for their hep with EM measurements of rock sampes. SEG Houston 29 Internationa Exposition and Annua Meeting 777

5 EDITED REFERENCES Note: This reference ist is a copy-edited version of the reference ist submitted by the author. Reference ists for the 29 SEG Technica Program Expanded Abstracts have been copy edited so that references provided with the onine metadata for each paper wi achieve a high degree of inking to cited sources that appear on the Web. REFERENCES Bei, D. F., 1953, Induced poarization, a method of geophysica prospecting: Geophysics, 18, Kein, J. D., and W. R. Si, 1982, Eectrica properties of artificia cay-bearing sandstone: Geophysics, 47, Keer, G. V., and F. C. Frischknecht, 1966, Eectrica methods in geophysica prospecting, vo. 1: Pergamon Press, Inc. Ohoeft, G. R., 1985, Low frequency eectrica properties: Geophysics, 5, Prince, C. M., R. Ehrich, and Y. Anguy, 1995, Anaysis of spatia order in sandstones 2: Grain custers, packing faws, and the sma-scae structure of sandstones: Journa of Sedimentary Research, A65, Titov, K., A. Kemna, A. Tarasov, and H. Vereecken, 24, Induced poarization of unsaturated sands determined through time domain measurements: Vadose Zone Journa, 3, Wait, J. R., 1959, The variabe-frequency method, in J. R. Wait, ed., Overvotage research and geophysica appications: Pergamon Press, Inc. Waker, S. E., 28, Shoud we care about negative transients in heicopter TEM (HTEM) data?: 78th Annua Internationa Meeting, Expanded Abstracts, Zhdanov, M. S., 26, Generaized effective medium theory of induced poarization: 76th Annua Internationa Meeting, SEG, Expanded Abstracts, , 28a, Generaized effective-medium theory of induced poarization: 78th Annua Internationa Meeting, SEG, Expanded Abstracts, F197 F211., 28b, Geophysica technique for minera exporation and discrimination based on eectromagnetic methods and associated systems: U. S. Patent 7,324,899 B2. SEG Houston 29 Internationa Exposition and Annua Meeting 778