Pore Pressure Prediction in Onshore West Niger Delta Using Inverted Seismic Velocity and Derived Velocity (Vp) - Vertical Effective Stress (VES) Coefficients (original) (raw)
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International Research Journal of Advanced Engineering and Science, 2021
This research was carried out in the Agbada Field in the Central Swamp area of the Niger Delta Petroleum basin. Well-log and seismic data were used to predict pre-drill pore pressure in the subsurface rock matrix. Variables estimated from the offset well included overburden gradient, shale pore pressure and fracture gradient. While those estimated from the seismic data included seismic velocities, pore pressure and fracture gradient. Mild overpressures were encountered in the calibration well from 10,800ftss to 11,702ftss where pressures of between 0.5 and 0.51 psi were observed. Hard overpressures of between 0.77 and 0.81psi were also seen from 13,318ftss to the bottom of the well at 14,502ftss. Vertical effective stress reversal which is also an indicator of overpressure was observed at 11,702ftss. Another indicator of overpressure, overburden pressure, increased from 11,800ftss as well, and a corresponding fracture gradient also increased at 14000ftss. While the total drilled depth of the calibration well was 14,508ftss, the total sampled depth of the seismic velocities was more than 18000ftss (about 5000ms). The seismic derived fracture gradient increased between 2800ms and 3000ms (about 13,800ftss) in agreement with that calculated from the calibration well. Vertical effective stress reversal was also observed between 2500ms and 3,500ms (10,000ftss-14,000ftss), just as was observed in the well. Overburden pressure dropped between 2800ms and 3000ms. This was also confirmed from the well results. Mild overpressures were predicted from 2500ms to 2650ms (10,00ftss to 10,700ftss) and hard overpressure from 2700ms to 3100ms (11,000ftss to 13,200ftss). Seismic velocity and well-log data were in good agreement for the pore pressure profile of the calibration well.
Seismic Velocity Analysis for Improved Geopressure Modelling in Onshore Niger Delta
International Journal of Advanced Geosciences
In this study, an improved evaluation of pore pressure using a model based seismic inversion technique has been carried out. Across six wells in the Onshore Niger Delta Basin, post stack seismic volume, angle stack gathers, seismic horizons, check shot, wireline logs, drilling and pressure data were analysed and interpreted. The model based inversion technique was applied to improve the seismic resolution as well as derive acoustic impedance using well velocities along with stacking velocities from velocity analysis of the 3D seismic data. Bowers’ Vp-VES coefficients of 7.43 and 0.77 were used to transform the derived seismic acoustic impedance velocity into seismic pore pressure cube. The seismic acoustic impedance interval velocity reveals much of the geology and resulted to a high resolution seismic pore pressure cube when compared at well location with direct pressure data. The Derived Seismic Pore Pressure (DSPP) also revealed that pore pressure and overpressure can reach or ex...
Applied Physics Research
The Sojuko field was discovered in 2001 in the eastern shallow offshore area of the Niger Delta, Nigeria. Three (3) exploration wells have so far been drilled in the field, two (2) of which are reasonably vertical and the third highly deviated. Three (3) key reservoirs which are laterally continuous across the wells have been identified with proven oil and gas reserves. Pore pressure data from repeat formation test (RFT) measurements acquired in the deviated well show that the wells are entirely hydrostatic to true depth (TD). This research focuses on investigating how seismic amplitudes change with offset/angle of incidence in relation to varying pore pressure regimes at the shale-hydrocarbon sand and shale-brine sand interfaces using well data. The aim is to aid quantitative interpretation in an on-going field-wide exploration drive to de-risk hydrocarbon exploration in the deeper plays in the area which are below TD, and are expected to be overpressured. The study is hinged on en...
Overpressure Prediction In The North-West Niger Delta, Using Porosity Data
Overpressure prediction in the North West of Niger Delta, using porosity data was carried out to safeguard hazards associated with drilling accident due to blowout. In the absence of seismic data to predict overpressure, porosity-dependent parameters and acoustic impedance could be used to predict the tops of overpressured zones in the area of study in the Niger Delta. Overpressure prediction is vital for safe and economic drilling. Composite logs were used to obtain the required data by digitizing the logs and deduction using the appropriate relationships. The findings from the study show that porosity decreases with depth, with overpressure zone detected at about 3500m depth due to porosity deviation from normal trend. Pressure gradient in the upper normal pressure of the field is determined to be 0.989 psi/ft, this implies that within the established normal pressure gradient of 0.71 -1.1 psi/ft in the Niger Delta. Formation overpressure gradient is determined to be 1.40 psi/ft. The overpressure zone coincides within the high shale-to-sand ratio of Agbada under compacted Formation. The identification of the tops of overpressure zones in any formation penetrated by a borehole enhances the use of normal drilling techniques of the borehole. This also reduces the cost of drilling the entire well as the special drilling technique will be applied only in the overpressure zones. This finding can aid in the prevention of drilling accident and resource wastage in exploration activities.
Pore Pressure Prediction using Seismic Velocity: Case Study in LN Field Kutai Basin
2017
Pore pressure estimation is important for both exploration and drilling projects. During the exploration phase, a prediction of pore pressure can be used to evaluate exploration risk factors including the migration of formation fluids and seal integrity. To optimize drilling decisions and well planning in abnormal pressured areas, it is essential to carry out pore pressure predictions before drilling. Mud weight and fracture gradient are essential parameters to have wellbore stability, prevent blowout, lost circulation, kick, sand production and reservoir damages. Predrill pore pressure accurate prediction allows the appropriate mud weight to be selected and allows the casing program to be optimized, thus enabling safety by preventing wellbore collapse and economic drilling by reducing the cost. The goal of this study is to estimate pore pressure relation with subsurface velocity in the Sefid-Zakhor gas field. Manufactured sonic logs are modified using the check shot interval velocity of Sefid-Zakhor well No. 1. The final acoustic impedance model is converted to the velocity model by removing density. Finally, the velocity model is converted to pore pressure using Bowers (in: IADC/SPE drilling conference proceedings, 1995) relation. The results of the pore pressure model are validated by pore pressure data obtained by the MDT well test tool. Generally, the results show the normal trend for pore pressure in the area, except in the left side of the anticline in the 2D seismic section, because of tectonic uplifting.
3-D MODEL OF PORE PRESSURE PREDICTION BY SEISMIC
3D model of pore pressure prediction becomes a critical tool as a platform for effective interpretation toward the drilling process. Inversion seismic velocity is crucial for modeling the seismic data into P-wave, S-wave, and density view, which then is transformed into pore pressure model. Generation of density cube is used to measure the overburden gradient; thus, pore pressure can be calculated by overburden gradient and seismic velocity. To lower the drilling cost and compromising the HSE, an accurate prediction of pore pressure gradient and fracture gradient is important to anticipate the mud needed and avoid any major problem while drilling. Thus, the casing and well completion can be performed for hydrocarbon production. Lost circulation will occur if the mud supplied into the wellbore is exceed the fracture gradient and kick occurs if mud supplied is less than pore pressure gradient. Because of effective stress has a relationship with seismic velocities, Eaton's method and Bowers method is used to estimating the pore pressure of the formation before drilling process. The generation of 3-D model is very useful to interpret the factors mechanism of overpressure generation. Sample model data of Malay Basin is cited in this paper as an example of Eaton's method applied. The magnitude overpressure of Malay Basin is trending toward the northwest direction which correlates with the measurement of the Eaton's method applied. The 3-D model of pore pressure prediction of Malay Basin was generated by Central Seismic Imaging (CSI), UTP, and used as a reference for this study purpose. The seismic method somehow need be compared to others method to predict the accurate pore pressure anomalies. Overpressure thin interbedded layer of sand and shale which usually in the deltaic and tidal environment is difficult to detect by the seismic because of low frequency and resolution.
This study investigated the cause of identified zones of overpressure in some selected wells in a field in the Niger Delta sedimentary basin. Two models were used each for predicting pore pressure and the corresponding fracture pressure using well log and drilling data. Shale lithology in Niger Delta is massive and characterized by high pore pressure; hence shale compaction theory is utilized in this study. The petrophysical data were evaluated using Ikon's Science Rokdoc software. The two major pore pressure prediction techniques employed are the Eaton's and Bowers' models while the Eaton's fracture pressure model and the Hubbert and Willis fracture pressure prediction models were utilized for fracture prediction. The density and sonic logs were used respectively to generate the shale trend and the shale normal compaction trend used for the prediction. The wells studied showed disequilibrium compaction of sediment to be the major mechanism that gave rise to overpressure in the Niger Delta. Clay diagenesis and fluid expansion were also observed as the secondary over-pressure generation mechanism in well X-1. This secondary overpressure mechanism was observed to start approximately at depths of 10,000 ft (TVD). The top of over-pressure and the pressure range in the wells studied varied from 6000 to 11,017 ft (TVD) and 1796.70 to 5297.00 psi respectively. The Eaton's model under-predicts pore pressure at the depth interval where unloading mechanism is witnessed. Since the study revealed presence of secondary overpressure generation mechanism, Bow-ers model was observed to be the most reliable pore pressure prediction model in the area.
Springer, 2021
Few wells targeting high temperature, high pressure intervals in most tertiary sedimentary basins have achieved their objective in terms of technicalities and cost. Since most shallow targets have been drilled, exploration focus is drifting into deeper plays both onshore and in deep offshore areas. To ensure safe and economic drilling campaigns, pore pressure prediction methodologies used in the region needs to be improved. The research aims at generating and testing a modification of Eaton's equation fit for high temperature, high pressure intervals on a field. The evolution of pore pressure in the field was established from offset well data by making several crossplots, and fracture gradient was computed using Mathew and Kelly's equation. Eaton's equation parameters were then calibrated using several wells until a desired field scale result was achieved when compared with information from already drilled intervals i.e., kicks and RFT data. Seismic velocity data resulting from high density, high resolution velocity analysis done to target deep overpressured intervals were then used to predict 1D pore pressure models at six selected prospect locations. Analyses reveal depths shallower than 3800 m TVD/MSL with geothermal gradient 3.0 °C/100 m and pressure gradient less than 1.50sg EMW are affected mainly by undercompaction; depths greater than 3800 m TVD/MSL with geothermal gradient of 4.1 °C/10 m and pressure gradients reaching 1.82-2.12sg EMW are affected by unloading with a narrow drilling margin for the deep highly pressured prospect intervals. Eaton's n-exponent was modified to 6, and it proved accurate in predicting high overpressure in the first prospect wells drilled.
Pore Pressure Prediction in Offshore Niger Delta: Implications on Drilling and Reservoir Quality
Despite exploration and production success in Niger Delta, several failed wells have been encountered due to overpressures. Hence, it is very essential to understand the spatial distribution of pore pressure and the generating mechanism in order to mitigate the pitfalls that might arise during drilling. This research provides estimates of pore pressure along three offshore wells using the Eaton's transit time method. An accurate normal compaction trend was estimated using the Eaton's exponent (m=3). Our results show that there are three pressure magnitude regimes: normal pressure zone (hydrostatic pressure), Transition pressure zone (slightly above hydrostatic pressure), and over pressured zone (significantly above hydrostatic pressure). The top of the geopressured zone (2873 mbRT or 9425.853 ft) averagely marks the onset of overpressurization with the excess pore pressure ratios above hydrostatic pressure varying averagely along the three wells between P * = 1.06-24.75 MPa and the lithostatic load range is λ = 0.46-0.97 and λ * = 0.2-0.9. The parametric study shows that the value of Eaton's exponent (m = 3-6) need to be applied with caution based on the dominant pore pressure generating mechanism in the Niger Delta. The generating mechanisms responsible for high pore pressure in the Offshore Niger Delta are disequilibrium compaction, unloading (fluid expansion) and shale diagenesis.