Estimation of overpressures from porosity based method : a theoretical approach applied to the central/coastal swamp depo-belts of the Niger delta basin (original) (raw)
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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.
Overpressured zones and porosity values in shale beds have been estimated for selected fields of the central/coastal swamp depo-belts of the Niger Delta. Pore pressure prediction (PPP), when done accurately can be used to avert disaster and helps in safe drilling. Eaton " s sonic model, Bowers " model and Zhang " s porosity-based model have been applied to predict overpressures using petrophysical log data from six 6 wells, each representing a field of the studied area. Predicted zones of hard overpressures (HOVP > 0.70 psi/ft) are generally below the depth of 10000ft. Top of geopressures (TOG) for the case study wells; WELL_A, WELL_B, WELL_D, WELL_G, WELL_H and WELL_K are 7000ft, 10500ft, 10000ft, 12500ft, 9000ft and 10400ft respectively. Eaton exponent " 5.5 " and Bowers model plotted with robust matches to measured pore pressures (MPP) everywhere except in one well. The porosity model equally yielded better matches to MPPs with higher values of fluid-transit-times typical of hydrocarbon fluid type (oil). These suggest, in combination with Vp-density analyses, a strong evidence of secondary mechanism causing overpressure in the basin. Very hard overpressures characterizes the deeper section of the basin (lower part of Agbada formation) at most of the well locations suggesting a strong evidence of fluid expansion mechanism which is also related to sediments unloading. The robust concordance between PPP and MPP profiles validates the results here and is a better guide for future drilling.
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.
International Journal of Petrochemistry and Research, 2019
The pore pressure in most sedimentary formations such as it is in the Niger Delta, Nigeria are rarely normal. They predominantly occur as over-pressured zones, when these abnormal pressures are not predicted accurately prior to drilling, disastrous occurrences, such as kicks, and blowouts may occur. The existing models used for pore pressure predictions were developed using data outside Niger Delta environment as such a number of them yield inaccurate results. This paper provides a new approach for predicting pore pressures particularly for Niger Delta using offset well logs acquired in the Niger Delta. A number of industries utilized pore pressure prediction models were appraised using offset well data from the Niger Delta. Density and sonic velocity logs were used in generating the overburden pressure and Normal Compaction Trend. Shale trend and overburden pressure were used as inputs in the models for predicting pore pressure using RokDoc. In the development of an appropriate model for pore pressure prediction, Eaton model was modified for Niger Delta environment. Results of the model's sensitivity analysis revealed sonic velocity as the most sensitive parameter while findings from the Goal Seek analysis showed that increasing the exponent in the original Eaton's model from 3 to 3.9 yielded the most concordant result with the measured pressure data. The statistical error analysis conducted revealed that the modified Eaton's model had the least absolute mean percentage error value of 2%. Given the above results, the modified Eaton's model showed more accurate predictions when compared with existing models and will be effective in predicting pore pressure in the study area.
Porosity as an overpressure zone indicator in an X-field of TheNiger Delta Basin, Nigeria
Archives of Applied Science Research, 2011
Over the past decades, overpressure zones have been routinely identified in the Niger Delta by means of resistivity logs, dc component logs, direct measurements (using pressure level detectors attached to BHA) and penetration rate of the drilling bit. The present research, however, applies porosity as an alternative method for overpressure zone detection. The application of porosity as a tool for overpressure detection is demonstrated using plots of porosity versus depth for two wells in an X-field in the Niger Delta. Results show that between 5000ft and 7000ft depth, (for well-1) there is continuous decrease of porosity values with depth. At 7000ft, however, there is an abrupt shift to the right side of the plot, indicating abnormally high porosities (at least for the delta), with values ranging from 25% to 35%. Such abnormally high porosities suggest that the pore fluids support a disproportionately large part of the overburden (from the Benin Formation mainly), leading to an over...
The knowledge of pore pressure plays an essential role in the drilling, planning, and production operations in the oil and gas industry. The aim of this research is to estimate pore pressure from well log data of the Unag Field situated Offshore in the Niger Delta Nigeria. The overpressured layers were categorised into three overpressure zones (A, B and C) using velocity and effective stress methods, respectively. The identified overpressure zones vary in thickness across the wells. Results reveal that overpressures were generated by disequilibrium compaction and pore pressures. The results further show that undercompaction (loading) mechanism of overpressure, which is characterised by gradual, and increasing slight overpressure with depth, may be the main cause of overpressure especially in zones A and B across the three wells. The research compares the Eaton's and Bower's pore pressure prediction methods. The Bower's method predicted pore pressure values better than the Eaton's method, which is in close agreement with the actual RFT data for three different zones in the studied wells. Hence, the Bower's method is proposed as useful and a better method for predicting pore pressures in other fields in the Niger Delta Basin using well logs.
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.
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.