International Journal of Petroleum and Geoscience Engineering Volume 04, Issue 01, Pages 58-65, 2016

Transcription

1 International Journal of Petroleum and Geoscience Engineering Volume 04, Issue 01, Pages 58-65, ISSN: Pore Pressure Gradient Prediction Using Well Logs; A Case Study on Malcolm Field, Offshore Niger Delta, Nigeria Abiola O. a, *., Eyinla S. D. b Adeduyite E. T. b a Department of Applied Geophysics, Federal University of Technology, Akure, Nigeria. b Department of Earth Sciences, Adekunle Ajasin University, Akungba Akoko, Nigeria. *Corresponding author. Tel.: address: abiola1703@gmail.com A b s t r a c t Keywords: Pore pressure, Normal compaction trendline, Abnormal formation pressure, Wireline log, Pressure gradient. Accepted: 15 Mar Pore pressure evaluation and prediction was undertaken in Malcolm field, offshore Niger Delta, Nigeria using wireline log data. The pore pressure gradient indicator used is the sonic log from three wells namely; A, B and C which were drilled in the field. Ben Eaton s method was used to estimate pore pressure gradient in this research work. Overpressured intervals were also delineated on the sonic log data with the use of a Normal Compaction Trendline (NCT). In well A, normal pressure at depth 6605 ft and 9265 ft with its pore pressure delineated to be psi/ft. Abnormal formation pressure at depth 6682 ft, 8914 ft, 9547 ft was delineated to be psi/ft, psi/ft, and psi/ft respectively. In well B, at depths 7573 ft and 9014 ft abnormally low pressure of psi/ft and psi/ft respectively were delineated, while at depth 11106ft, abnormally high pressure was delineated to be psi/ft. In well C, at depths 7447 ft and 8299 ft abnormally high pressure was delineated to be psi/ft and psi/ft respectively, while at depths 9676 ft and ft, abnormally low pressure were delineated to be psi/ft and psi/ft respectively. There was no normal pressure in well B and C. As a result of these, accurate pore pressures prediction in overpressured regions is essential to ensure safe drilling operations and reduce the cost of drilling. Academic Research Online Publisher. All rights reserved. 1. Introduction Geophysical well-logging entails the measurement of the physical properties of the surrounding rocks with a sensor located down hole, the record of the measurement as a function of depth is known as a log. With the drilling of most deep wells, formations are penetrated that will flow naturally at a significant rate. In drilling these wells, safety dictates that the wellbore pressure (at any depth) be maintained between the naturally occurring pressure of the formation fluids and the maximum wellbore pressure that the formation can withstand without fracture. Knowledge of formation fluid pressure and fracture pressure, and how these two parameters vary with depth is extremely important in planning and drilling a deep well [1]. According to [2], the place of petroleum in the economic survival of several nations of the world 58 P a g e

2 cannot be overemphasized. The time and the enormous cost of exploration for this all important mineral make it imperative for the attainment of perfection in methods of its detection. The different formation pressure encountered in an area play a vital role both during exploration and exploitation of hydrocarbon resources reservoir. The different kinds of reservoir pressure which are usually encountered during the course of drilling are broadly divided into three main components: Hydrostatic pressure, Overburden pressure and Formation pressure. The vertical pressure at any point in the earth is known as the overburden pressure or geostatic pressure [1]. The overburden pressure at any point is a function of the mass of rock and fluid above the point of interest. In order to calculate the overburden pressure at any point, the average density of the material (rock and fluids) above the point of interest must be determined. The pore pressure of a formation refers to that portion of the overburden pressure which is not supported by the rock matrix, but rather by the liquids which exist in the spaces of the formation. The main overpressure causes are under compaction, fluid expansion, fluid migration and tectonics [3]. If the pore pressure is less than normal hydrostatic pressure, the formation is said to be subnormally pressured but if exceeds the expected hydrostatic pressure for that depth, the zone is termed abnormally pressured. Identifying the over pressured zones in drilling is crucial as it narrows the available drilling mud window [4]. Drilling under tight mud windows increases the possibility of either fracturing of the formation or inviting blow outs. Proper understanding of the pore pressure can reduce the well cost and can help to plan a safe drilling well Location of the study area The study area, Malcolm field is in the offshore Niger Delta lies within latitudes 3 0 and 6 0 N and longitudes 5 0 and 8 0 E, is made up of fresh water swamps and mangrove swamps with relief that increases towards north. Figure 1 is the base map of the study area showing the location of the three wells used for the work Geology of the study area Stratigraphically, Niger Delta is divided into three diachronous units (Fig. 2) of Eocene to Recent age that form a major regressive cycle that is broken up into a series of offlap cycles named the Akata, Agbada and Benin Formations [5-9]. The Benin Formation is the upper alluvial coastal plain depositional environment of the Niger Delta Complex. It extends from the west Niger Delta across the entire Niger Delta area and to the south beyond the present coastline. The formation was deposited in a continental fluviatile environment and composed almost entirely of non-marine sandstone. It consists of coarse-grained sandstones, gravel lignite streaks and wood fragments with minor intercalation of shales. Benin Formation is of Miocene to younger age and has a variable thickness that exceeds 1820 m. In the subsurface, it is of Oligocene age in the north becoming progressively younger southwards but ranges from Miocene to Recent as generally accepted. Very little hydrocarbon accumulation has been associated with this formation. The Agbada Formation underlies the Benin Formation. It was laid down in paralic brackish to marine fluviatile, coastal environments. It is made up mainly of alternating sandstone, silt and shale. The sandstones are poorly sorted, rounded to subrounded, slightly consolidated but majority are unconsolidated. The sandstones grade into shale in the lower part of the formation. Agbada Formation ranges in age from Eocene in the north to Pliocene in the south. The sandy parts of the formation are known to constitute the main hydrocarbon reservoirs of the delta oil fields and the shales constitute seals to the reservoirs. The thickness of the formation reaches a maximum of about 4500 m. 59 P a g e

3 The Akata Formation is the lowest unit of the Niger Delta complex. It is composed of mainly shale with sandstones and siltstones locally interbedded. The Formation becomes shalier with depth. It was deposited in a marine environment and has a thickness, which may reach 7000 m in the central part of the delta. The Akata Formation outcrops offshore in diapirs along the continental slope, and onshore in the north east, where they are called Imo Shale. The age of the Akata Formation ranges from Eocene to Recent. One of the most conspicuous geological features of the Niger Delta is its growth fault pattern (Fig. 3). The delta sequence is deformed by syn-sedimentary faulting and folding. [10] described the main structural features of the Niger Delta as growth faults and rollover anticlines. 2. Materials and methods of study The materials used for this research work are well logs from three wells; A, B and C. Methods of evaluating abnormal pore pressures are separated in two categories, prediction and detection methods. The prediction methods normally use data obtained from seismic surveys, offset well logs and well history. Detection methods traditionally utilize drilling parameters and well log information obtained during the actual drilling of a well. The Eaton s method was used in this research work to estimate the pore pressure gradient from well logs. The pore pressure prediction strategy requires petrophysical data: specifically formation resistivity or conductivity, to predict pore pressures. Eaton developed a simple relationship that will predict the pore pressure knowing the normal compaction trendline (NCT) and Overburden Gradient (OBG). The Eaton s equation used is: PP = OBG ((OBG Pn) * (ΔTn/ΔTo)^3.0 (1) Where: PP = Predicted pore pressure (psi/ft) at depth Z, OBG = Overburden Gradient (psi/ft) at depth Z, Pn = the normal pressure at depth Z, ΔTn = the assumed normal sonic slowness (μsec/ft) at depth Z (calculated from the NCT), ΔTo = the observed (measured) sonic slowness (μsec/ft) at depth Z. Fig. 1: Base map of the study area. 60 P a g e

4 Fig. 2: Stratigraphic column showing the three Formations of the Niger Delta. Modified from [9, 11]. Fig. 3: Typical Growth Faults Found Within the Study Area Modified from [9, 11]. 3. Results and discussion A set of curve was picked through smoothed sonic log data from the 3 wells A, B and C, in order to establish Normal Compaction Trend (NCT) from sonic log. These curves were plotted with respect to depth and the normal compaction trend established (Figures 4, 5 and 6). The deviation from this trend is an indication of abnormal pressure [12, 13]. Quantitative pressure analysis using the sonic log is based on calibrating the observed sonic log value and an expected or normal sonic value with known pressure measurements. The depth of the normal compaction trend (NCT) was picked in the three well from 61 P a g e

5 smoothed sonic data and the depth was corresponded to each other in the three wells. This method is based on the principle of flow direction of the pore pressure gradient. Measured Depth P-Sonic Fig. 4: Fitting Normal Compaction Trend to the sonic data of well A. Measured Depth P-Sonic Fig. 5: Fitting Normal Compaction Trend to the sonic data of well B. Measured Depth P-Sonic Fig. 6: Fitting Normal Compaction Trend to the sonic data of well C. Tables 1, 2 and 3 below shows the occurrence of abnormally low pressure, abnormaly high pressure, and normal pore pressure as observed from the normal compaction (NCT) trendline within the three wells. Pore pressure above psi/ft is said to be abnormally high pressure 62 P a g e

6 (supressure) while pressures below psi/ft is said to be abnormally low pressure (subpressure). Well A covers the depth interval ranging from 6505 to ft (Table 1). Normal pore pressure of psi/ft could be observed only at depths 6605 and 9265 ft but abnormal high pressure of psi/ft occurred at depth 7113 ft. Abnormal low formation pore pressure also occurred at depths 6682 ft, 8914 ft, 9547 ft with values psi/ft, psi/ft, and psi/ft respectively. All the remaining depths of investigation are moderately pressured. Also, well B, covers the depth intervals of 6235 ft ft (Table 2). The case of abnormally low pressure of psi/ft and psi/ft could be observed at depths 7573 ft and 9014 ft respectively. But abnormally high pressure of psi/ft occurred at depth 11106ft. There were moderately pressure zones in the formation within the other parts of the well. And well C, the depth intervals ranges between 6860 and ft respectively (Table 3). At depths 7447 ft and 8299 ft, abnormally high pore pressure of psi/ft and psi/ft was observed, while at depths 9676 ft and ft, abnormally low pore pressure of psi/ft and psi/ft was observed. While record of slight normal pressure occurred at the remaining depths of investigation within the well. Table 1: The pore pressure values at various intervals for well A. S/N DEPTH (ft) TO TN PP (psi/ft) (msec) (msec) P a g e

7 Table 2: The pore pressure values at various intervals for well B. S/N DEPTH TO TN PP (psi/ft) (ft) (msec) (msec) Table 3: The pore pressure values at various intervals for well C. S/N DEPTH TO TN PP (psi/ft) (ft) (msec) (msec) P a g e

8 4. Conclusion Analysis of sonic and density log data shows that overpressure in the field could be inferred to have been generated by disequilibrium compaction of the lower Agbada Formation. Since the pore pressure g r a d i e n t values estimated represent the pressure in the wellbore at which the formation will crack, for each of the abnormal formation pressure encountered. With the use of accurate pore pressure gradient prediction, well control events such as formation fluid kicks, lost circulation, surface blowouts and underground blowout can be avoided. The pore pressure gradient prediction will be useful when designing future drilling and completion operations to ensure safe drilling operations and reduce the cost of drilling in the study area. Therefore, accurate pore-pressure gradient prediction in overpressured regions like Niger Delta is essential. References [1] Eyinla D. S. and Oladunjoye M. A., Estimating Geo-mechanical Strength of Reservoir Rocks from Well logs for Safety Limits in Sand-free Production, [2] Eyinla D. S, Reservoir evaluation of success oil field, offshore Niger Delta, Nigeria. Unpublished project work submitted to the Department of Geology, Adekunle Ajasin University Akungba Akoko, [3] Swarbrick, R. E. and Osborne, M. J., Mechanisms that generate abnormal pressure: an overview, in Law, B. E., Ulmishek, G. F. and Slavin V.I. eds,. Abnormal pressures in hydrocarbon environments. AAPG Memoir, (70): 13-34, [4] Eaton, B. A., Graphical method to predict geopressures worldwide. World Oil, (182): 51-56, [5] Short, K. C., and Stäuble, A. J., Outline of Geology of Niger Delta. American Association of Petroleum Geologists Bulletin, (51): , [6]Avbovbo, A. A.Tertiary lithostratigraphy of Niger Delta. American Association of Petroleum Geologists Bulletin, (62): , [7] Whiteman, A., Nigeria: Its Petroleum Geology, Resources and Potential, London, Graham and Trotman, 394, [8] Knox, G. J. and Omatsola, E. M. Development of the Cenozoic Niger Delta in terms of the Escalator Regression Model and Impact on Hydrocarbon Distribution. Proceedings of the KNGMG Symposium: Coastal Lowland Geology and Geotechnology Kluwer Acad Publ. Netherlands, , [9] Doust, H., and Omatsola, E., Niger Delta, in, Edwards, J. D., and Santogrossi, P.A., eds., Divergent/passive Margin Basins, American Association of Petroleum Geologists Memoir 48: , [10] Evamy, B.D., Haremboure, J., Kamerling, P., Knaap, W.A., Molloy, F.A., and Rowlands, P.H., Hydrocarbon habitat of Tertiary Niger Delta, American Association of Petroleum Geologists Bulletin, (62): , [11] Shannon, P. M. and Naylor, N., Petroleun Basin Studies, London, Graham and Trotman Limited, , [12] Dutta, N. C., Geopressure prediction using seismic data: Current status and road ahead: Geophysics, (67): , [13] Huffman, A.R., The future of pore pressure prediction using geophysical methods, The Leading Edge, (21): , P a g e