OTC 18192Normal Resistivity Trends for Geopressure Analysis in Mexican Offshore Wells Víctor López-Solís/Petroleos Mexicanos (PEMEX); David Velazquez-Cruz/Instituto Mexicano del Petroleo (IMP); Agustín Jardinez-Tena/Petroleos Mexicanos (PEMEX); Gustavo Espinosa Castañeda/Instituto Mexicano del Petroleo (IMP) Copyright 2006, Offshore Technology Conference This paper was prepared for presentation at the 2006 Offshore Technology Conference held in Houston, Texas, U.S.A., 1–4 May 2006. This paper was selected for presentation by an OTC Program Committee following review of information contained in an abstract submitted by the author(s). Contents of the paper, as presented, have not been reviewed by the Offshore Technology Conference and are subject to correction by the author(s). The material, as presented, does not necessarily reflect any position of the Offshore Technology Conference, its officers, or members. Papers presented at OTC are subject to publication review by Sponsor Society Committees of the Offshore Technology Conference. Electronic reproduction, distribution, or storage of any part of this paper for commercial purposes without the written consent of the Offshore Technology Conference is prohibited. Permission to reproduce in print is restricted to an abstract of not more than 300 words; illustrations may not be copied. The abstract must contain conspicuous acknowledgment of where and by whom the paper was presented. Write Librarian, OTC, P.O. Box 833836, Richardson, TX 75083-3836, U.S.A., fax 01-972-952-9435. Abstract The normal compaction trend and the pore pressure gradient play a very important role in oil wells design for drilling operations. It is essential understand the physical principles originating these pressures and evaluate the models of quantification for a particular geographical area. In this paper, we show the results from our analysis of geopressures in 11 (eleven) offshore wells drilled in the Offshore Area in Gulf of Mexico from PEMEX. Our work focuses in the normal compaction trends using curves of Resistivity Logs. Moreover, the Eaton´s model was regionalized in order to defining the magnitude of the pore pressure with high precision. From those results we inferred that the accuracy of the predictions depends of slope from normal compactions trend. Finally, using the profiles of real pressure data, we built a “Pore Pressure Cube” for the area of study, which simplifies the quantifications of pressures during the process of planning and design of the wells. Introduction There are several mechanisms originating abnormal pressures. This phenomenon is related to physical, geological, geochemist and mechanical processes. Frequently is difficult to establish what phenomena is the most important for a particular geological frame due to their dependences in the abnormal pressures origins. Between the causes of abnormal pressures, referred in the literature 1, we picked up the following: 1. Compaction disequilibrium 2. Tectonic stress 3. Aquathermal expansion 4. Mineral transformation 5. Hydrocarbon Generation 6. Thermodynamic effects 7. Osmosis 8. Hydraulic head From our point of view the main cause cited of abnormal pressure behaviour is sediment compaction disequilibrium. Present work focuses in the identification of compaction changes. Our approach considers Terzaghi 2 theory, and Hottman & Johnson3 Log Analysis method in order to define changes in the shale compaction trend and we exploit Eaton´s 4 Pore Pressure Equation for resistivity logs. We describe pore pressure results using Eaton equation considering resistivity compaction trends from Mexican offshore wells. Further, we analyze resistivity compaction trends and defined a series of trends that we call “overlay graph” of resistivity trends. Finally, we define a new alpha exponent for Eaton’s equation; such changes in the exponent allow us to define pore pressure behaviour with more accuracy, this fact was successfully validated using resistivity logs from several offshore wells. Pore pressure results with original Eaton’s exponent From previous works in pore pressure prediction to offshore Mexican wells5,6. We found that original Eaton equation for resistivity and transit time “overestimate” pore pressure than results obtained from well real measures. In figure 1, the red line at right, show pore pressure with 1.2 Eaton exponent; green lines; in same figure, show ECD and mud weight used to drill well. We can see that pore pressure is higher than real mud weight. From this analysis we already verified that when we used original Eaton exponents, predicted pore pressure is higher than real. We considered that those coefficients in the Eaton equation must be adjusted to obtain more accurate results for Mexican Offshore wells. Resistivity compaction trends analysis In 1965 Hottman y Johnson3 developed a relationship between logs response and abnormal pressures in shale. In brief, they were reasoning that well compacted shale rock with less quantity of water (less porosity) is more resistive than a less compacted shale rock. Then they concluded that a sequence of normally compacted sediments should have a normally increasing shale resistivity trend. Then, any shale resistivity decrease from the established normal trend indicates the presence of abnormal pressure zone. We analyzed several wells drilled in Mexican Offshore fields with Eaton’s equation. D. From several wells. 1948. The graph allows us to define normal trend even in first stages of wells with high accuracy. V.2 is higher than ECD and mud weight used to drill well.0 ohms-m.. we have pore pressure prognosis more accurate to real measures. Pore pressure models published in oil industry. S. B. Regionalized pore pressure model In his paper. it can be represented by an equation as such we show in figure 3. 6. Figure 7 shows Lankahuasa area and a view of pore pressure cube. The resistivity normal trend analysis showed that trends have an average slope of 5x10 -4 and a mode of 4x10-4. Eaton admitted that validation process of his model was by trial-error tests. In figure 4. References 1. Proyecto F. 1975. and Johnson. The pore pressure calculated with alpha=1. he recognized that correct value from alpha coefficient from his equations was a great question mark until was evaluated with much data. Eaton. Conclusions We define a resistivity normal compaction trend in order to decrease uncertainty of pore pressure prognosis.8 that in Mexican offshore fields the normal pressure zone is since mud line to up 2. We can see quantitative differences in pore pressure calculated with alpha=1. must be adapted to behavioral pattern of resistivity normal trend from Mexican Offshore Fields. D.. From all wells drilled in Lankahuasa center cube.42461. June 1965.: “Abnormal Pressures in Hydrocarbon Environments”. We observed. his equations are the most used for drilling engineers worldwide due to predicted pore pressure with good accurate. R.000 m. we show overlay graph that describe normal resistivity trends from our study area. we built a pore pressure volume that we used to design casing sets. However. K. In table 1 we show our results. Proyecto F. J. we show our results from wells drilled in the Mexican Offshore fields (North Area). Overlay graph of normal trends The overlay graphs were product from normal trend analysis from Mexican Offshore wells. 8. E. Velazquez-Cruz. Terzaghi. Banuet-Sanchez. Ben E.. In figure 5. In figure 6 we show pore pressure analysis from Mexican offshore well. . 7. D. In figure 2. Tab. L. Mancilla-Castillo. et al “Estudio de Geopresiones y Estabilidad de Pozos en el Área Marina de la División Norte”. R.. Instituto Mexicano del Petróleo. 1994. In addition. Villahermosa.2 and with alpha=0. 4. B. D. paper SPE 39903 presented at the SPE International Petroleum Conference and Exhibition of Mexico.2 We define from a gamma-ray log the shale points to translate on a resistivity curves for all wells in analysis. 2004. Behavioral differences of resistivity normal trends slopes from Mexican Offshore fields. With regionalized Eaton’s equation main component (alpha). AAPG Memoir 70.40. Velazquez-Cruz.: “Sistema de Computo para la Detección de Presiones Anormales a partir de Información Sísmica (SISMIC 1. 1996.: “Soil Mechanics in Engineering Practice. John Wiley and Sons”.0)”. “The Equation for Geopressure Prediction from Well Logs” SPE 5544. We calculated pore pressure with several values of alpha coefficient until we found correct alpha values for pore pressure measures. we can obtain a set of equations that represent normal pressure trends for any study area. 3.Velazquez-Cruz. March 1998. Julio del 2005. from previous experiences7. This resistivity normal compaction trend was used with LWD to determine pore pressure while drilling in exploratory wells with good results. López-Solís. 5. C..8 to 1.:“Estimation of Formation Pressures from Log-Derived Shale Properties”. Instituto Mexicano del Petróleo. Proyecto CDC-0302. The origin ordinate varies from 0. The pore pressure predicted when we used overlay graphs is more precise. Hottman. This graph is used to track pore pressure while drilling exploratory wells. We identified the normal trend from any well. et al: “Detection of Abnormal Pressures System from Seismic Data and Geophysical Well logs”.Velazquez-Cruz. et al. Instituto Mexicano del Petróleo. K. we show typical behavior of normal resistivity trend from offshore wells. et al: “Análisis de Geopresiones de los campos Ku-Maloob-Zaap”. affect pore 18192 pressure magnitude when we used equations originally published.30545. JPT. 2. and Peck. 4 KOSNI-101 0.Typical behavior of normal resistivity trend from Mexican Wells.Eaton’s alpha exponent for wells drilled in Mexican Gulf Coast. Figure 2.Normal trend equation from resistivity logs. Figure 1-Pore pressure analysis from offshore Mexican well.4 LACAZTZU-1 0. .4 LANKAHUASA NTE-1 0.3 CHIHUIX-1 0.4 AVERAGE 0. Figure 3.18192 3 WELL ALPHA KOSNI-1 0.39 Table 1.4 LANKAHUASA-1 0.4 SIHINI-1 0.4 LANKAHUASA DL2 0. Results from regionalized pore pressure model.Pore pressure volume from Lankahuasa area.4 18192 Figure 6. Figure 4-Normal resistivity trends analysis from Mexican offshore wells) Figure 7.) Figure 5.Overlay graph of normal trends .