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PORE PRESSURE ANALYSIS IN THE CORRIDOR BLOCK, SOUTH SUMATRA BASIN: DISTRIBUTION, MECHANISM, AND PREDICTION

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Pore pressure distribution in South Sumatra has received little attention in literature. Until this time, the pore pressure in this region has only been determined for new wells before drilling activity. This paper discusses an integrated pore
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  * ConocoPhillips (Grissik) Ltd. ** ConocoPhillips, Houston *** Institut Teknologi Bandung   IPA19-G-279 PROCEEDINGS, INDONESIAN PETROLEUM ASSOCIATION Forty-Third Annual Convention & Exhibition, September 2019 PORE PRESSURE ANALYSIS IN THE CORRIDOR BLOCK, SOUTH SUMATRA BASIN: DISTRIBUTION, MECHANISM, AND PREDICTION Irfan Yuliandri Syukri* Budi R Permana* Phil D. Heppard** Agus M. Ramdhan*** Lambok M. Hutasoit***  ABSTRACT Pore pressure distribution in South Sumatra has received little attention in literature. Until this time, the pore pressure in this region has only been determined for new wells before drilling activity. This paper discusses an integrated pore pressure study with the input data comes from several different sources: wireline logs (gamma ray, density, sonic, and resistivity) from eighty-one wells, seismic velocities, drilling data and clay mineralogical analysis. The objectives of the study were to understand the pore pressure regime, distribution and mechanisms so the overpressure zone(s) in future wells in the Corridor Block, South Sumatra could be  predicted. The analysis revealed that both hydrostatic and overpressure regimes are present in the study area. In accordance with the regional pore pressure regime from Ramdhan et al., 2018, if the depth to basement is greater than 6500 feet, overpressure in the Tertiary section will be present. From the study, the observed overpressure regime was divided into four regimes, OP-1, OP-2, OP-3, and OP-4. These regimes are distinguished from one another by geological formation and magnitude of overpressure. The mechanisms that generate overpressure in this area have been classified into disequilibrium compaction due to rapid burial during the last 20 Ma and hydrocarbon generation. In addition, clay mineral transformation can also contribute to overpressure,  but this is regarded as a minor phenomenon in the study area. To estimate the overpressure magnitude, a single normal compaction trend (NCT) in the study area has  been generated. It was constructed using sonic logs and it was corrected with the amount of erosion. The application of the single NCT together with Eaton’s equation gives a reasonably good pore pressure estimation. In general, the overpressure distribution may be used to estimate pore pressure over the study area. This integrated analysis could predict pore pressure in the area without well control and give more comprehensive analysis of pore pressure distribution. INTRODUCTION Understanding subsurface pressure regimes is crucial for drilling design and activity, analyzing pressure- preserved porosity, identifying hydrodynamic traps, and in-situ stress analysis. In-situ stress regime and magnitude play an important role in development of conductive fractures in basement plays (Hennings et al, 2012). This is a key factor in potentially finding fractured basement reservoirs with prospective deliverability. Pressure regression that has been detected in several basins in Indonesia (Ramdhan et al., 2018) may also occur in the study area, and this may lead to the presence of hydrodynamic traps. Pore pressure is an important input in determining in-situ stress regime, and the inability to correctly determine pore pressure may lead to incorrect in-situ stress regime determination. A poor understanding of  pressure regimes can lead to drilling problems. Several examples are available in the South Sumatra Basin where wells have experienced severe drilling  problems such as loss circulation, kicks, and hole collapse. The results of this study show the occurrence and distribution of overpressure is still in accordance with regional overpressure in the area (Ramdhan et al., 2018) and some local variation still could be observed. A normal compaction trend can be applied in the entire study area for overpressure prediction.    One important finding from this study is that several wells in the study area were drilled underbalanced without any increased background gas, connection gas or kicks despite the presence of porous sandstone, possibly due to normally pressured sandstones which have experienced lateral reservoir drainage. REGIONAL GEOLOGY The Corridor Block is in Sumatra, Indonesia and is situated within the South Sumatra Basin (Figure 1). This is a prolific hydrocarbon basin with a proven  petroleum system which was discovered by the drilling of the of Kampung Minyak Field in 1896 (Riva, Jr., 1983; Ginger and Fielding, 2005). Geologically, the South Sumatra Basin is a back-arc  basin bounded by Barisan Mountains to the southwest and Pre-Tertiary of the Sunda Shelf to the northeast (de Coster, 1974). To the north, the South Sumatra Basin is separated from the Central Sumatra Basin by the crystalline and metasedimentary Tigapuluh Mountains. The Lampung high to the southwest separates South Sumatra Basin from the Sunda Basin and Java Sea (Yarmanto, et al., 2011; Setiadi, et al., 2017). The Tertiary Period was a time of complex structural movement and associated stratigraphic deposition. The resulting tectonostratigraphy has been extensively studied and divided into six stages (Figure 2): 1.   The pre-rift stage occurred in the Eocene Epoch. Gafoer and Purbo-Hadiwidjoyo (1986) and De Coster (1974) suggested Kikim tuffs were deposited in this stage. 2.   In the Eocene to Oligocene, movement of both the Australian tectonic plate to the East and the India plate to the West, and rotation of Borneo, resulted in rifting in the South Sumatra area. This resulted in the formation of north-northwest/south-southeast trending normal faults and horsts and grabens along much of the southern margin of the Sunda Shelf plate. 3.   From the late Oligocene through early Miocene, the South Sumatra area experienced marine transgression. This resulted in the deposition of terrestrial fluvial to deltaic rocks of the Talang Akar Formation (TAF) which onlapped the  basement highs, followed by the Pendopo clastics and Baturaja Formation (BRF) platform and reef carbonates deposited around and over these highs (Kusnama, et al., 1993). 4.   The maximum transgression occurred in the mid Miocene which resulted in the deposition of the marine Gumai/Telisa shale Formations. These intervals form a regional seal for the TAF, BRF and basement. 5.   Marine regression started in the late mid-Miocene and continued to the present day. In this stage Palembang/Air Benakat, Muaraenim and Kasai Formation were deposited. 6.    Northeast directed shortening in the Pliocene resulted in basin inversion forming NW-SE trending structures and thrust faults which are the traps observed today. In addition, this shortening resulted in the inversion of the normal fault in the  basin (Bishop et al., 2001). DETERMINATION OF PORE PRESSURE Drilling Data To determine the amount of overpressure in the Corridor Block of South Sumatra, eighty-one wells were analyzed. The data collected from these wells consists of direct pressure data (RFT, MDT, and DST), drilling data (mud weight, gas content, kicks, and losses), and wireline logs. The pressure data collected in these wells can be used to determine if the stratigraphic section sampled is at hydrostatic  pressure or is over/under pressured by comparing the  pressure value at that depth to the pressure assuming a hydrostatic gradient (0.433 psi/ft used as hydrostatic gradient). Mud weight data, although not a direct measurement of pore pressure, can be used to determine relative  pore pressure in the formations if sufficient  permeability rocks are present. Assuming these conditions, if while drilling, there are no influxes into the well, the formation pore pressure must be no more than the mud pressure. An increase in gas content during drilling is generally interpreted as near balanced drilling conditions indicating that the reservoir pore pressure is the same or nearly the same as the mud pressure. Drilling kicks, (i.e. fluid influx into borehole), indicates that the mud pressure is lower than the formation pore pressure. Finally, mud losses while drilling indicate that the mud pressure is greater than the formation pore pressure. Mud losses can be divided into two categories natural and drilling-induced. The natural losses generally occur in high permeability reservoirs such as poorly    consolidated sandstones and fractured-reservoirs. The drilling-induced losses occur when mud  pressures reach fracture pressure, creating hydro-fracturing, and thus causing drilling mud to penetrate the formation. Wireline Log Data Another tool used to determine the presence of overpressure is wireline log data from shale sections. In these sections, overpressure is expressed by an increase in the resistivity value, an increase in the sonic travel time and a decreasing density. To estimate overpressure in the shale sections, we modified Eaton’s equation for sonic logs (Eaton, 1975) as shown below. ( )  xnnvv t t PP      −−=       (Eq. 1) where P = pore pressure (psi) v     = vertical stress measured from the acoustic transit time (psi) n P  = hydrostatic pressure at the measured point (using 0.433 psi/ft) t    = measured acoustic transit time at measured  point (µs/ft) n t    = normal compaction trend of acoustic transit time at the measured point (µs/ft)  x  = empirical Eaton’s exponent, used to adjust the estimated pressures with measured  pressures The vertical stress is determined by integrating the density log and the sonic log values based on the normal compaction curve (usually called the normal compaction trend (NCT)). Three parameters of NCT equation (Chapman, 1983) were used as follows: ( )  mbzmn  t et t t   +−=  − 0  (Eq. 2) where n t    = normal compaction trend of acoustic transit time at the measured point (µs/ft) 0 t    = matrix transit time at sea-bed or surface prior to erosion (µs/ft) m t    = compressional transit time in the shale matrix, with zero porosity (µs/ft) b  = empirical constant obtained from fitting the hydrostatically-pressured data  z  = depth below the mudline (ft) Since the study area has experienced uplift and erosion, the analysis needs to take into account the amount of erosion in constructing a single NCT for the entire study area. PORE PRESSURE REGIME CLASSIFICATION Using wireline log data, the presence of overpressure was determined for eighty-one wells inside the Corridor Block. Based on the observed pore pressure magnitude, predicted top of the overpressure and geological formation, five pore pressure regimes were identified. (Figure 3 & Table 1): 1.   OP-1 :  Moderately overpressured  . The top of overpressure is located at the Palembang or Telisa Formations, and is 3 - 5 ppg overbalanced from hydrostatic. 2.   OP-2 : Slightly overpressured  . The top of overpressure is also located in the Palembang or Telisa Formations like OP-1, but has a lower overpressure, only 1 - 3 ppg overbalanced from hydrostatic. 3.   OP-3 : Slightly overpressured  . The top of overpressure is also located in the Telisa Formation like OP-1 and OP-2 but the maximum overpressure occurs in the older Pendopo or Talang Akar Formations, and has an overpressure range the same as OP-2. 4.   OP-4 : Slightly overpressured  . The top of overpressure occurs in the Pendopo or Talang Akar Formations, with range of 1 - 3 ppg overbalanced from hydrostatic. No overpressure is observed in the Palembang or Telisa Formations. 5.   HS :  Hydrostatic pressure.  The sonic log shows a normal behavior, i.e. increasing velocity with increasing depth which is consistent with hydrostatic pressure regime.    OVERPRESSURE GENERATING MECHANISMS One of the most commonly observed overpressure generating mechanisms is the transformation of smectite clay to illite clay. This process results in the expulsion of water from the smectite clay. If this water cannot escape via a permeable layer, the result in an increase in pore pressure. This phenomenon is commonly observed in shale sections and is marked  by an increase in the sonic travel time, decrease in density and decrease in resistivity. To investigate the clay mineralogy of the samples in this study, XRD, XRF, SEM, and IDS were conducted. However, due to sample availability, the analysis was only conducted on the following wells: -   Three wells in S-area -   Two wells in D-area -   Two wells in SM-area Using the pore pressure regime classification above, a cross-plot of sonic and density for all the shale from the top of the bottom of the well was created to determine the mechanisms of overpressure generation for the various areas in this study. This assumes that the all three logs curves, sonic, density and resistivity record a change at the top of the overpressure zone. The phenomenon where this doesn’t occur was studied by Bowers (2002). This  paper concluded that when the density reversal is deeper than the sonic and resistivity, this may indicate the presence of high overpressure due to fluid expansion, smectite-illite transformation,  pressure transfer or tectonic loading. In this paper, we divide the overpressure generating mechanisms into two categories, disequilibrium compaction and stratigraphic unloading as described  below: Disequilibrium Compaction S-8 is a representative well for this pressure type. The well was chosen because the wireline log quality in the shale section is good, and the density is a true log measurement (not constructed from sonic log). From the sonic-density cross-plot of S-8 (Figure 4), the hydrostatic data point is located in the mechanical compaction area, while overpressure data point is in the chemically-enhanced mechanical compaction area. The transition zone from mechanical to chemical compaction can also be observed in the cross-plot, which coincides with top of sonic reversal. This is interpreted to be the top of overpressure (1200 m ~ 4000 ft). In addition, the unloading trend is absent in this cross-plot. Therefore, it is interpreted that the cause of overpressure in this well is loading mechanism leading to disequilibrium compaction. To ensure the cause of overpressure in this area, a clay mineralogical investigation has been conducted. The XRD analysis in S-3 well illustrates the fractions of different clay types with depth (Figure 5). The Smectite (S), mixed layer Smectite-Illite (I/S), and chlorite are relatively constant with depth while, the fraction of Illite (I) starts to increase at the depth around  1000 m which is associated with decrease in Kaolinite (K). This decreasing Kaolinite and increasing Illite is located in the overpressure zone which may indicate Kaolinite to Illite transformation. For this to occur, a temperature of greater than 120°C is required which is not observed in the data and therefore cannot be the mechanism to describe the change in Kaolinite and Illite with depth. Stratigraphic Unloading Three wells exhibit the OP-3 pressure regime (BA-1,  NS-1, and TA-1). The NS-1 pressure-depth plot is shown in Figure 6. The top of overpressure in this well, as observed from resistivity reversal, is located at the depth  1250 m. This estimated overpressure zone, though, does not correspond with an increase in density log as expected. The combination of these factors is interpreted to indicate that the overpressure is generated by an unloading mechanism. From the sonic-density cross-plot of NS-1 (Figure 7), the hydrostatic data points are in the chemically-enhanced mechanical compaction area, while overpressure data points are located off the compaction line. This is an indication that overpressure in this well is generated by an unloading mechanism. This conclusion does not rule out the possibility that hydrocarbon generation could also contribute to the overpressure but this hypothesis needs further study through hydrocarbon maturation indicators such as vitrinite reflectance data to be confirmed. ESTIMATION OF PORE PRESSURE IN STUDY Normal Compaction Trend Sediment deposited at the surface generally has a high initial porosity. As sedimentation continues, the increasing overburden causes the sediment to lose  porosity in a predictable manner. This trend of  porosity loss with depth is called the normal    compaction trend and the rocks which define this trend have a pore pressure which is hydrostatic. Rocks that are penetrated with a porosity which does not fall on this normal compaction line are deemed abnormal compaction intervals. In this paper, a single Normal Compaction Trend (NCT) was generated instead of creating a NCT for each well. This was possible because the clay mineralogy distribution in the wells was fairly constant. Since a single NCT was created for all the wells, it was possible to estimate pore pressure for future wells drilled in the study area. To create a realistic estimate of pore pressure, amount of uplift and erosion must be determined, as it could reduce subsurface temperature significantly. This is because of the effect of temperature on  pressure is quite significant in a formation with a closed fluid system. Furthermore, expansion of the rock framework during and after uplift could also further reduce the pressure. In the study area, there is abundant evidence of recent uplift and erosion. To account for this, a NCT for the eroded section was also required. To describe the eroded section in the wells studied, the following alternative version of equation 2 was used. 55 +=  − bzn  aet   (Eq. 3) where n t    = normal compaction trend of acoustic transit time at the measured point (µs/ft) “a” =  a free parameter resulted from regressing the data in hydrostatic section. Here, “a” and “b” are referred to compaction parameters. z = depth in feet Equation 3 implies that the plot of 55log  −  n t   vs z (depth) will result in a straight line (Figure 8). This assumes that the elastic rebound of travel time due to erosion is negligible. Erosion will only change the “a” (intercept) and not the slope (“b”) . Then, the present day NCT (NCT with some erosion) was constructed in the study area. The compaction  parameters for the present day NCT are shown in Figure 9. It can be seen that the value of “b” for the study area is -0.000168904. Meanwhile, the compaction equation for the study area at maximum burial by assuming that 0 t   = 200 µs/ft (transit time of sediment at sea bed) and m t   = 55 µs/ft is: ( )  mbzmn  t et t t   +−=  − 0  (Eq. 4) ( )  5555200  000168904.0 +−=  −  zn  et   (Eq. 5) 55145  000168904.0 +=  −  zn  et   (Eq. 6) where n t   is in µs/ft, z is in feet. Modifying Eq (5) for the eroded section results in: ( ) 55145  000168904.0 +=  +− es  z zn  et   (Eq. 7) where es  z is the thickness of eroded section in feet. Eq. (7) is the general (unified) NCT for the study area. By applying that equation, the final NCT used in the study effectively describes various amounts of estimated erosion (Figure 9). Application of the general NCT to well data in the study area is shown in Figure 10. It can be seen that the general NCT can reasonably match the data in the hydrostatic section and the trend line can be used. An example of overpressure estimation using the single NCT on S-3 is shown in Figure 11. The estimated pore pressure can reasonably match the measured pore pressure from DST data . The Eaton’s exponent used to match the measured pore pressure is 3 for the sonic log. SEISMIC PORE PRESSURE After generating the 1D pore pressure calculation, the next step was to generate the 3D model. The calculated 1D pore pressure was upscaled to the 3D model as the primary variable. Seismic velocity was used as a spatial control for kriging to interpolate the well-log velocity data into stratigraphic 3D layers, which honored the geological structure (Goovaerts, 1997). The kriging was done within a stratigraphic framework to ensure consistency between the interpreted horizons and geology. The process of model building began with pre-conditioning of the seismic interval velocity data by calibrating it to the check-shot data. The next step

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Sep 22, 2019
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