Iraqi Journal of Chemical and Petroleum Engineering Vol.15 No.2 (June 2014) 9- 14 ISSN: 1997-4884 Using Sonic Log to Predict Abnormal Pressure Zones in Selected Oil Wells (Western of Iraq) Talib A. Saleh Petroleum Engineering Department, College of Engineering, University of Baghdad Abstract Two oil wells were tested to find the abnormal pressure zones using sonic log technique. We found that well Abu-Jir-3 and Abu-Jir-5 had an abnormal pressure zones from depth 4340 to 4520 feet and 4200 to 4600 feet, respectively. The maximum difference between obtained results and the field measured results did not exceed 2.4%. In this paper, the formation pressures were expressed in terms of pressure gradient which sometimes reached up to twice the normal pressure gradient. Drilling and developing such formations were dangerous and expensive. The plotted figures showed a clear derivation from the normal trend which confirmed the existence of abnormal pressure zones. Keywords: Normal pressure, Subnormal pressure, Abnormal pressure, Formation pressure, Formation pressure gradient, Transition zone Introduction Abnormal pressures have been identified in about 180 sedimentary basins and most oil and gas generation in such basins occurs within the over0pressured fluid compartments [1]. Knowledge of the pressure distribution in a given area frequently minimizes the problems associated with all phases of operation: geophysics, geological, drilling and petroleum engineering. Certainly, the most serious problem met by the group of oil well drilling is that of abnormal pressure zones. This problem is related to tertiary geological age and it has, as mentioned before, a direct relation to geophysical prospecting, geological structures and petroleum engineering. Generally, normal pressure is considered with formation pressure which is approximately equal to hydrostatic head of water column if the formation is opened to the atmosphere and this is equal to 0.465 psi/ft which is called pressure gradient (p.g) while abnormal pressure zones are those which have (p.g) more than 0.465 psi/ft [2]. It may reach twice of this value as observed in the North Sea. Working with such formations is hazardous and very expensive [3]. It is interesting to mention that there is an upper transition zone through which there are indications of existence of abnormal pressure zone. Studies in petroleum industry showed that the rate of penetration is reduced by an increase in the mud Iraqi Journal of Chemical and Petroleum Engineering University of Baghdad College of Engineering Using Sonic Log to Predict Abnormal Pressure Zones in Selected Oil Wells (Western of Iraq) 10 IJCPE Vol.15 No.2 (June 2014) -Available online at: www.iasj.net column pressure [4]. As we go deeper and deeper the penetration rate is decreased because we will be met by harder rocks. This trend has been reversed through drilling over pressured zones and the pressure gradient will be largely increased [5]. In other words, in an abnormal pressure zones, the mud weight should be increased to balance the formation pressure. Many studies showed that the shale density and/or resistivity can be used to detect the over pressured zone [6, 7, 8]. The geological section of the studied area is shown in Fig.1 and the location map of the studied area is shown in Fig.2 In this work, we have applied the sonic log techniques at selected wells which gave accurate results compared with field data. Causes of Abnormal Pressure 1- Very rapid deposition of large quantities of sands and shale predominating may result in sand bodies that are completely surrounded by shale and as compaction progressives, water is unable to escape and this give an abnormal pressure zone especially if the rock still has sufficient tensile strength [9]. 2- Abnormal pressure refers to areas of high topographic relief where the outcrop of the pressured sand acts as an elevation high enough to cause artesian head; this is especially true in rapid filling (tertiary) sedimentary basins [10]. 3- Thick gas cap at the interface of water/gas is caused by growth faults [11]. 4- Fluid transfer is due to the alteration of montmorillonite to illite [12]. 5- Routes of communication between beds having different pressures are available [12]. 6- There are other miscellaneous causes such as reverse osmosis which is the movement of ions in water down a water concentration gradient (i.e. from fresh to saline). The ions will continue to move until the salinities balance or pressure prevents further movement. That pressure is postulated to be as much as 4000 psi in the subsurface where shales can act as the semipermeable and mineral decomposition such as the transformation of gypsum to anhydrite (CaSO4.2H2O to CaSO4) and the release of water of crystallization [11, 12]. Results and Discussion The data and the results concerned with the wells Abu – Jir3 and Abu – Jir5 are listed in Table1 and Table2, respectively. Figure1 shows a typical log tsh versus depth of well Abu – Jir3. The normal curve was constructed on the log and deviation of tsh from the normal trend clearly began at about 4340 feet which was the depth at which abnormal pressure started. In this well, a normal pressure gradient of 0.465 psi/ft was measured and was increased in the adjacent shales (from depth 4360 to 4520 feet). The normal tsh curve began at 60sec/ft at depth equal to 4320 feet and decreased logarithmically to 15sec/ft at 4520 feet. In the same concept, Figure4 shows abnormal pressure zone from depth 4200 to 4600 feet. A correlation was made from sonic log data relating the difference tsh observed and tsh in normally pressured zones to the formation pressure gradient, as shown in Figures 5 and 6, respectively. From these figures, an estimation of the formation pressure gradient and as a result formation pressure at any depth may be obtained. Talib A. Saleh -Available online at: www.iasj.net IJCPE Vol.15 No.2 (June 2014) 11 Conclusions 1- The use of the Sonic Log to detect the beginning of abnormal pressure zone was applied with good results in the studied area. 2- The interval transit time recorded by Sonic Log might be thought of as function of lithology and porosity. In other words, since the porosity of the shales decreased with compaction, tsh should decrease with depth. This was shown in Figures 3 and 4. 3- Plotting of pressure gradient with difference in tsh and equivalent mud weight (Figures 5 and 6) could be used to evaluate a given drilling objective through determining mud weight and hence casing requirements at any depth. References 1- Hunt, J. M.: "Generation and Migration of Petroleum from Abnormally Pressured Fluid Compartments", AAPG Bulletin, V.74, No.1, PP. 1-12, January, 1990. 2- Prestel, A., and Memok.: "Estimation of Formation Pressure", Jour.pet.Tech., pp.717-723, June, 1978. 3- Herring. E.; "Estimating Abnormal Pressures from Log Data hi the North Sea", Presented at the Second Annual "J Meeting of the SPE of AIME, London, England, April, 1973. 4- Tomson, J.: "Application of Drilling Data to Over Pressure Detection", Jour. Pet. Tech., pp.1387-1391, Nov., 1979. 5- Myer, L.: "Measuring and Using shale Density to Aid in Drilling Wells of High Pressure", J.P.T, 1423-1425, Nov. 1982. 6- Hottman, C, and Johnson, R.: "Estimation of Formation Pressure from Log-Derived Shale Properties", Jour. Pet. Tech., June, 1965. 7- Morris, R., and Biggs, W.: "Using Log-Derived Values of Water Saturation and Porosity", SPWLA Symposium, 1967. 8- Burke, J., Campbell, R., and Schmidt, A.: "The Litho-Porosity Crossplot", the Log Analyst SPWLA, Nov.-Dec., 1969. 9- Hussien, M. and Kolb, L.: "Geological Significance of Abnormal Pressure Formation", J.P.T, pp.961-963, August, 1994. 10- Jeorge, K., and Selman, N.: "Water Production from Abnormal Pressure Formations", J.P.T., pp. 317-32, August, 1993. 11- Kavach, N., and Talib, A.: "Some Aspects of Abnormal Pressure Zone", J.P.T., pp.212-218, Nov., 1995. 12- Pettijohn, F., "Sedimentary Rocks," Harper and Brothers, New York, 1957, (Second Edition). 13- sissakian, v.k. 2000: Geological map of Iraq. 14- Mahmoud A. Als'adi, PH.D thesis 2010(formation evaluation of abo-jir field) western iraq. 15- Final well reports (abo-jir field). Using Sonic Log to Predict Abnormal Pressure Zones in Selected Oil Wells (Western of Iraq) 12 IJCPE Vol.15 No.2 (June 2014) -Available online at: www.iasj.net Fig. 1: Geolgoical section of the studied area (after Sissakian , 2000) Fig. 2: Location map of the studied area Talib A. Saleh -Available online at: www.iasj.net IJCPE Vol.15 No.2 (June 2014) 13 Table 1: Well Abu – Jir 3 Pressure estimation from sonic log data Depth Ft t nor. µSec/ ft tobs. µsec/ft tobs. - tnor. F.P.g Pressure Psi Equivalent mud weigh P.P.g 4320 60 60 0 0.465 2008.8 8.937 4340 50 50 0 0.465 2081 8.937 4360 40 50 10 0.6 2616 11.533 4380 40 55 15 0.62 2715.6 11.916 4400 30 60 30 0.83 3652 15.952 4420 25 65 40 0.93 4110.6 17.875 4440 23 70 47 0.94 417306 18.067 4460 20 60 40 0.93 4147.8 17.875 4480 17 40 23 0.81 3628.8 15.568 4500 15 45 30 0.88 3735 15.953 4520 15 35 20 0.80 3616 15.568 Table 2: Well Abu – Jir 5 Pressure estimation from sonic log data Depth Ft t nor. µSec/ ft tobs. µsec/ft tobs. - tnor. F.P.g Pressure Psi Equivalent mud weigh P.P.g 3700 300 300 0 0.465 1720.5 8.937 3750 250 250 0 0.465 1743.75 8.937 3800 150 150 0 0.465 1767 8.937 3850 210 210 0 0.465 1790 8.937 3900 190 190 0 0.465 1813.5 8.937 3950 200 200 0 0.465 1836.75 8.937 4000 190 190 0 0.465 1860 8.937 4050 180 180 0 0.465 1883 8.937 4100 150 150 0 0.465 1906.5 8.937 4150 145 145 0 0.465 1929.75 8.937 4200 110 120 10 0.6 2520 11.532 4250 110 150 40 0.93 3922 17.367 4300 105 155 50 0.44 4042 18.067 4350 100 160 60 0.9345 4065 17.961 4400 95 170 75 0.942 4144.8 18.105 4450 85 155 70 0.935 4140.75 17.884 4500 77 130 53 0.941 4234.5 18.086 4550 72 110 38 0.91 4140.5 17.490 4600 68 100 32 0.89 4094 17.106 Fig. 3: Pressure estimation from sonic log data Using Sonic Log to Predict Abnormal Pressure Zones in Selected Oil Wells (Western of Iraq) 14 IJCPE Vol.15 No.2 (June 2014) -Available online at: www.iasj.net Fig. 4: Pressure estimation from sonic log data Fig. 5: Relationship of (pressure gradient, travel time and equivalent mud weight) in Abu – Jir 3 Fig. 6: Relationship of (pressure gradient, travel time and equivalent mud weight) in Abu – Jir 5