EFFECT OF WETTABILITI’ ON THE ELECTRICAL RESISTIVITY OF CARBOXATE ROCK FRO31 A PETROLEU3I RESERVOIR BY S. A. SWEENEY AND H. Y. JENNINGS, JR. Cahfornza Research Corporation, La Hnhrn, (‘nlifnrnia Rereaued ,Vovemher 1 1 , 7969
Resistivity measurements were made on porous carbonate rock which contained varying amounts of electrolyte arid aliphatic hydrocarbon. The porous rock was treated so as to make the carbonate surface preferentially water-wet and then preferentially oil-wet. Resistivity data on preferentially water-wet samples were considerably different from the data on preferentially oil-wet samples. The results are discussed in terms of the effevt of changes in wettability on porosity and water saturation values derived from electric well logs.
Introduction ,Iccurate information concerning the porosity and water saturation of subsurface porous rock formations is of importance to those engaged in the exploration and production of gas and oil. Porosity and water saturation of oil-bearing formations are estimated from the interpretation of electric well logs. To a.d in the interpretation of electric logs resistivity measurements are made in the laboratory on rock samples taken from the formation of interest. From the measurements the ratio of the resistance of a water-saturated rock to the resistance of the water with which it is filled is obtained. The ratio, commonly called Formation Factor, can be expressed as : F = Ro/R,. The ratio of the resistance of a rork containing m-ater and oil or gas, Rt, to the resistance of the same rock 100% saturation with water is also obtained. This ratio, commonly called Resis-ivitg Iiidex, I , can be expressed as I = Rt/Ro. Early advances in the quantitative interpretation of electric logs were made by Archie.’ Archie concluded that a study of the resistivity of formations when all the pore.; are filled with mater was of basic importance in the detection of oil and gas by the use of an electric log. Archie, and Wyckofi and Botset,2among others, also studied the variation in the resistivity of sands due to the percentage of water contained in the pores. The study was done by displacing n r y i n g amounts of conducting water from the pores with non-conducting fluids such as gas or oil. ‘These early investigators considered the effect of distribution of water and oil on the resistivity values 1 ut their results did not show distribution to bc a significant factor. More recent investigators including Iieller3 and Moore4 have shown tivity measurements made 011 sandstones weie n function of the manner in which the water was distributed. The changes in distribution were caused by rhanging the ~ e t t a b i l i t yof the widstone surface. The purpose of the study reported in this paper was to determine how changes in wettability affect the resistivity of porous carbonate rocks. Although carbonate rock reservoirs account for more than half of the world’s oil production the study was delayed bemuse of the difficulty with which (1) G . E. Archie, .4m Inst. iMininy -Wet. Engrs., Petroleum Dzuiszon, Trans., 146, 54 (1942). (2) R. D. Wychoff and H. G. Botset, J . Applied Ptus.. 7 , 325 (1936). (3) G.B. Relh r, 0 2 1 and Gas J . , 61,63, Jan 5 (1953). (4) J. C. Moo-e, P,udiirFrs M o n t h l y 22, 30, M a r c h (1958).
carbonate surfaces are made preferentially oil-wet in the laboratory. We recently developed a treatment for niaking carbonate surfaces strongly oil-wet based upon coiitact angle work published by Benner and Barte1l.j With this treatment it was possible to carry out the proposed study.
Experimental Materials.-Twenty-five one-inch diameter carbonate core plugs 3 inches long were selected which covered a range of porosity from 11.8to 31 5%. The samples varied widely in texture but consisted for the most part of clastic carbonate fragments of sand size or larger. Cementation varied from poor to strong and dolomitization ranged from 10 to 78%. The electrolyte solution was the same composition as that associated with the carbonate rock in the reservoir. I t contained 182 g. of NaCl, 30 g. of CaCL and 14 g. of XgC12 per liter and had an electrical resistivity of 0.053 ohmmeters a t 23.5’. The oil was kerosene, boiling point range 179-252”, which had been purified by the method described by Craig, et aL,6 for toluene. Measurement of Electrical Resistivity. Apparatus.The electrical resistance of the cores was measured bv a two electrode In the past the two electrode kiethod had one major disadvantage, polarization. Polarization at current carrying metal electrodes produces both “contact resistance” and “contact capacitance.” The amount of polarization a t the current carrying electrodes depends upon the electrolyte in contact with electrode, the electrode metal and the current frequency. It was found that by using gold-plated or hlonel-metal electrodes and a 1000-cycle current the contact resistance became negligible. The contact capacitance was compensated for in the measurement of resistivity. The power source imposes a 1000-cycle signal directly on the horizontal deflection plates of the oscilloscope. The power source also imposes, through an isolating transformer, a 1000-cycle signal on the Wheatstone bridge. The legs of the bridge comprise two 1000-ohm resistors, the core electrode assembly and a combination variable resistor and capacitor. When the variable resistance and capacitancr match the resistance and capacitance of the core-electrode assembly, no signal is impressed on the vertical deflection plates of the oscilloscope. When the magnitude of the admittance of the variable capacitor is small compared to that of the variable resistance, no significant error in resistance is involved in balancing a series capacitor-resistor combination in the core against a parallel arrangement in the reference. Procedure.-The core plugs were refluxed in toluene for 24 hours or until fresh toluene no longer became discolored. After refluxing, the cores were dried in a vacuum oven and then were leached with distilled water to remove residual salt. The leached samples were saturated 100% with 0.053 ohm-meter water and placed brtween the electrodes for resistance measurement. The remainder of the resistivity data were obtained a t ( 5 ) F. C. Benner and F. E. Bartell, “The Effect of Polar Impurities upon Capillary and Surface Phenomena in Petroleum Production,’‘ A P I Drilling and Production Practice, 1941, p. 341. (6) R. 0.Craig, J. J. Van Voorhis and F. E. Rartell. Trrrs . J ~ W R N A I . , 60, 1225 (1956). (7) P. Et. Lorenz, &bid., 67, 430 (1953).
PREFERENTIALCY OIL-WET
Fig. 1.-Summary
of resistivity ratio data.
saturations other than 100%. The cores were placed in capillary pressure cells which have capillary diaphragms attached to Monel plates for electrical contact. The capillary diaphragms were porous plates having pores about 1 micron in diameter. When a preferentially water-wet diaphragm is completely saturated with water, oil cannot enter the pores of the diaphragm unless the applied pressure is great enough to overcome the entry pressure of the porous diaphragm. At pressures less than the entry pressure, water will flow through the diaphragm and preferentially exclude the non-wetting fluid, oil. In the experiments reported here, a 1 0 0 ~water o saturated core was placed in contact with a preferentially water-wet diaphragm. Capillary contact was achieved between the core and the diaphragm nTith a diatomaceoas earth-water mixture. The core was completely surrounded by oil and pressure was applied to the oil. Water displaced from the core passed throngh the diaphragm into a receiver where its volume was measured. Cores contain pores of various sizes. Each of these pores will permit oil to ent,er a t its characteristic capillary entry pressure. This pressure depends upon the pore radius, interfacid tension and the wetting properties of the solid matrix. As oil enters a pore, water is displaced and the water saturation decreases. The cores are allowed t,o stand a t several predetermined capillary pressures until water production has stopped. The electrical resistance then is measured. The maximum capillary pressure reached in these tests was 20 p.8.i. Chanaes of Wettabilit7.-Water-oil imbibition tests were performed by saturating the core 100% with one phase and then completely immersing the core in the other phase. Displaced fluid was measured in a calibrated buret at regnlar time intervals. Wlien neither water or oil was displaced, the core w:is said to have neutral wettability. For preferentially water-wet cores only oil was displaced and for preferentially oil-wet cores only water was displaced. hleasurements on more than a hundred toluene extracted limestone cores of the type used in this study showed them to he of neutral wettability. I t was concluded in an earlier study8 that the surfaces of some cores from oil-bearing formations were preferentially oil-wet because of a highly tenacious, sorbed layer of organic material. It was further ______
(8, H. U J m n i n y s J r , Produmrs M o n t l l y , 21, 20, March (1937)
established that in every case the cores were made preferent,ially water-wet. hy hmting in an oxidizing atmosphere. The first series of resistivity tests were made on the toluene extracted weather cores of neutral wettability. At the completion of these tests the cores were muffled for one hour a t 450'. At 500" the dissociation pressure of calcium carbonate is 0.11 mm.,9 a value less than the partial pressure of carbon dioxide in ordinary air. The aver:tge weight loss was 0.5 g. per core. This treatment gave imbibitions up to 72y0 pore volume water starting with the core 100% saturated with oil. Attempts to make a carbonate rock strongly preferent,ially oil-wet by treatment with siliconlOill were unsuccessful. K e prepare preferentially oil-wet sandstone surfaces by coating the surface with a silicone material. Surh a surface represents an extreme oil-wet condition. A technique was finally developed for carbonate using a solution of high molecii1:tr weight organic acid in benzene. The most successful trentment consisted of saturating the core with 105%solution of Eastman practical grade Naphthenic Arids (P23SS) iii benzene. The core was refluxed in the solution for 30 minutes. Excess solution was gas driven from the core and the rcmaining benzene was removed in a vacuuni oven. A t the present time this treating procedure represents the extreme preferentially oil-wet condition yielding imbibitions of oil up to 56y0 pore volume starting with the core 100% saturated with water. The ideal treatment would cover all the carbonate stirface with a monolayer of acid which woiild alter the wettability of the surface but would cause a negligible change in the physical dimensions of the matrix. The average increase in weight of rock due to the naphthenic acid treatment was 0.13 g. per core. Surface area measmement,s using low temperature krypton adsorption by the BET method on samples of this core material gave values of 0.50 sq. meter per gram. A monolayer of stearic acid on all the surfaces of an average Core would weigh about 0.08 g.
Experimental Results The results of the resistivit'y meastiremeiit s on the cores in three conditions of wettabilit'y are sumniarized in Fig. 1. The st,raight lines drawn through these point's were obtained by the inethod of least squares. The equatioii for these straight lines on log-log coordinates may be \vrit)ten I = Sw-"
where n is defined as the saturation exponent and S, is the fractional water saturation of the core. The equations relating resistivity index to water saturation for neutral and preferentially water-met respectively. cores are 1 = Sw-1.9s and I = Sw-1.61, The data for the preferentially oil-wet condition separated into two distinct curves. The equations for t'he lower water saturation port,ion of these ciirves are I = 0.000027 Sw-12.27 and I = 0.37 S, respectively. The equation of the straight h i e relating t,lie formation factor of n rock to its porosity may be written F =
,$-"
where 4 is the porosity and the esponent, m is tlrfined as the cenientation factor. The .m T-nliies for the ciirves relating formation fnctor wit,h porosity are as follows: neutml, 1.92; preferentinlly waterwet, 1.57; and preferentially oil-n-ct 2.01, ~
Discussion The dnt,a obtained in this stiidy +on. the striking (9) 1%.€1. Wi1l:ird ana A. W . Boldyreff, J . Am. Chem. SOC.,62, 1888 (1930). (10) W. P. Gillism. H. A. Liebhafsky and A. F.Winelon. ibid., 63, 801 (1941). 111) C . G . Dodd, J . W . Davisand F. D. Pidgeon, THISJOURNAL, 86, li8-1 (1951).
influence that the wettability of the rock surfaces has on thc resistivity of the rock. l h e data show that a t a %ivenfractional water saturation the resistivity is greater xhen the surface is preferentially oil-wet than when the surface is preferentially water-wet. I t can be concluded that in the oil-wet system a rbortiori of the water exists in discrete nonconnected globulcs which are unable to assist with the conduction of current. This concept has been developed in some detail by Ileller3 and by ?tloore4 t o explain their results on sandstone cores. Signific: n t differences are apparent, however, in the result: obtained for carbonate rocks and those published lor sandstone. Iieller3 i h o w a break in the water saturationresistivity index curve for preferentially waterwet sanditones in an air-water system occurring a t a wate- saturation of 30%. It was concluded that below the critical saturation of 30% more and more of the water becomes separated in discrete drops which are not available for current conduction resulting i i a more rapid increase in resistance. KO such breax was observed in the carbonate results though datn were obtained a t saturations as low as 87;. Apparently the critical saturation does not evist in t i e water-oil-carbonate system or, if it does, it exists at a water saturation lower than that which can be attained in the laboratory by normal capillary pressure displacement. The preferentially oil-\vet resistivity index-saturation curre for carbonate shows a break as does Iieller's curve for sandstone. However, the sanditone cur1 e slopes less steeply after a break a t 55% whereas the carbonate slopes more steeply after the break. Ileller concludes that the water saturation in the interconnecting pores is reduced first tiecause the water content is forced out into the larger pores bv capillary pressure. On further water red,ic tion the water becomes discontinuous but is prefent in stringers through the center of the larger porcs. The resistance does not increase as rapidly af:er the break because the water that remains is present mainly in the large pores whose water content does not so greatly affect the total resistivity The difference in behavior between preferentially oil-n et carbonates and sandstones may be due to a difference in pore size distribution. Studies of formation rock through interpretation of mercury injection lvrves show that carbonate rocks have complex i ut characteriytic pore size distributions. .lpparent lv the c o w iised ill this qtudy had large
pores and large interconnecting pores which reniained saturated a t high water saturation. IIowever, after a critical saturation was reached the water in these pores became disconnected and the resistivity increased more rapidly. Additional support to the pore size distribution interpretation is found in the separation of the oil-wet curves. An interesting result of this study was the separation of the preferentially oil-wet data into two distinctly separate curves. Inspection of the saniples which fell in the two groups showed that they possessed other distinguishing properties. For example, a separation into the same two groups could be made by analysis of the pore size distribution and by petrographic examination. The separation of curves suggests the use of resistivity measurements on preferentially oil-wet samples as a classification method for carbonate material. Further study of oil-wet carbonate resistivity curves and pore size distribution information will undoubtedly shed more light on the capillary pressure clesaturation mechanism. The formation factor of a porous medium is the simplest resistivity quantity that can be measured. Archie' found empirically that m in the equation F = 4 - m was related to the degree of consolidation of the rock. A soft, friable sandstone had an m value of 1.3 while a highly cemented sandstone had an m value of 2.0. The treatment used in this study also changed the value of m. The decreased resistivity observed after muffling the cores could be attributed to the resulting increase in water saturation. But the subsequent decrease in water saturation as a result of depositing naphthenic acid in the core is not sufficient to account for the m = 2.01 curve. Apparently the uniform coating of naphthenic acid is more effective in partially sealing off the necks of pores containing mater 1% hich had previously been contributing to current flow. It is apparent from the data obtained in this study that a knowledge of the Tvettability of the formation being penetrated by the well is essential to accurate determination of porosity and water saturation from electric logs. Laboratory resistivity measurements should be made on samples that have the same wettability as the formation being logged. Acknowledgment.-The authors are grateful to nh. D. Brazier for carrying out the resistivity measurements. They also wish to acknowledge the valuable suggestions made by Professors R. S. Hanqen and L. S.Bartell of Iowa State College
