PORE-PRESSURE PREDICTION FROM SEISMICS: A Case Study of an X-Field in the Niger Delta

Research Article

PORE-PRESSURE PREDICTION FROM SEISMICS: A Case Study of an X-Field in the Niger Delta

Corresponding authorNfor Bruno Ndicho, Department of Geology, Chukwuemeka Odumegwu Ojukwu University, Uli, Anambra State, Nigeria.Email: nforbng@yahoo.com

Abstract

In this work Pore pressure prediction of an X- oil field in Niger Delta Basin was evaluated from seismic reflection data, using the Bower’s and Gardner’s technique of velocity- to-pore pressure gradient. Results show that, the top of overpressure zone is lies at depths varying from 1500m in well 01, through 2048m in well 03 to 1722m in well 05. Sonic log deflections at these intervals all showed significant deflection common with pore pressure zones. These overpressure zones are interpreted to correspond to the Comparison of the overpressure zone with available geophysical logs shows that the interval is under-compacted and has lower bulk density, low interval velocity and high porosity. Considering the young age of the basin, these overpressure zones are attributed to low mechanical compaction, chemical compaction and hydrocarbon generation zones, associated with shale-rich region of the Agbada Formation of the Niger Delta basin. The advantage of pore- pressure prediction from seismic is that, it provides alternative method of determining the expected pressure in a location where there are no well data, or even in areas where there are logs; it complements the prediction that is based solely in offset well logs. The seismic velocity intervals were derived from the seismic check-shot data. The computed pore pressure results derived from seismic were later compared with the available sonic and resistivity logs of the boreholes. Parameters in the velocity-to-pore pressure transform were then estimated using the seismic interval velocity.

Keywords: Pore pressure prediction; seismics; undercompaction; Agbada Formation

Introduction

AThe importance of reservoir pressure in hydrocarbon exploration and exploitation has drawn attention of petroleum geoscientists and reservoir engineer scholars due to the obvious need to make the petroleum Industry environmentally friendly. Opara and Onuoha (2009), opinioned that health hazards due to kicks and mud loss during drilling pose industrial challenges. Opara and Onuoha (2009) suggested that accurate overpressure zone prediction is vital for successful and safe drilling of wells. Before now, the practice has been to monitor overpressure zones during drilling, with attendant consequences of unpreparedness to combat extremely high pressure zones. The potential high impact of drilling through overpressure zone and its implication on cost, health and ecological system thus necessitates an alternative method of pre-drilling pressure zone detection from seismic data. The advantage of pore- pressure prediction from seismic is that, it provides alternative method of determining the expected pressure in a location where there are no well data. Even in areas where there are logs, it complement  the prediction that is based solely in offset well logs. This study thus attempts to predict pore pressure belt of X  Oil field from seismic data. This information on pore pressure condition of a field, even before drilling, enhances better safety preparedness by the drilling crew prior to actual drilling.

Previous Studies

Pore pressure evaluation has caught the attention of many petroleum geoscientists and reservoir engineers due toits place in hydrocarbon exploration and exploitation. The Nigerian sedimentary Basin is believed to be one of the most prolific and economic sedimentary basins in Africa by the virtue of its size of petroleum accumulation discovered and produced as well as the spatial distribution of the petroleum resources, Opara and Onuoha (2009). The understanding of overpressure zones in the subsurface is very important during drilling, exploration risk and reservoir depletion studies, Opara and Onuoha (2009) suggested that accurate pore pressure prediction is vital for successful and safe drilling of wells. In the Nigerian basins, kicks have been observed in permeable zones within the reservoirs, suggesting that mud-weights have been set too low as a result of inaccurate predrilling pressure prediction, Connor et al, (2011). In order to prevent drilling mud losses or kicks, there is need for detailed pre-drilling risk assessment in connection with expected formation pore pressure.

Pressure build-up in a rock-fluid system varies significantly and the variation depends on the basin history, geological structure, thickness, rock composition of the sedimentary basin and the activity of the geodynamic processes of the basin. In drilling operations, it is necessary to determine pore pressure of various formations penetrated by the borehole. The knowledge of pressure gradient enables the driller take extra precautions while penetrating abnormally high pressure zones in order to avoid kicks, which could be fatal due to blow outs. Such abnormal pressures that cause blow out are called g eo-pressure and the formations where they occur are known as overpressure zones, Nfor et al, (2011). Pore pressure can be deduced from seismic response by studying the interval velocities. Generally, high pore pressure intervals have lower velocities compared to the same lithology and geologic structure of a formation with normal pressured intervals of the same depth, Dutta, (2002). Seismic velocities are strongly affected by sediment compaction rate which also affects the pore pressure build up in a formation. Under this assumption, seismic velocities can be used to predict pore pressure regimes in areas that have not been drilled, Nwozor et al (2013). The rock velocity is also affected by several factors which depend on each other. Such factors are density, porosity, pore fluid type, fluid saturation, lithology and clay content. Base on petroleum sector and geological challenges in hydrocarbon recovery from reservoir, special attention demands that, not every velocity anomaly can be caused by pore pressure variations, because coarse grain size with high porosity value underlain by well compacted shale at any depth will cause seismic velocity interval to be low. Therefore the geological knowledge of sonic velocity, density and resistivity sensitivityto pore pressure are used as a guide to the seismic velocity interpretation to avoid ambiguities.

The Field under study- (X oil field) is located in the Niger Delta basin of Nigeria. The prolific Niger Delta basin of Nigeria, is  located in the southern part of Nigeria, and is bounded approximately  y longitudes 5000’E – 8000’E and latitudes 4000’N –  7000’N, covering an area Of about 75,000 sq. km with a sedimentary thickness of between 30,000 to 40,000ft, Burke, (1972).This Tertiary Basin has been extensively discussed by workers such as, Burke, (1972); Merki, (1972); Murat, (1972); Short and Stauble, (1967); etc. The basin consists of three main lithostratigraphic sedimentary units. Pliocene to Recent aquiferous Benin Formation, underlain by a Miocene Pliocene deltaic marine to paralic petroliferous sand/shale Agbada Formation and a basal low density, high pressure marine shale’s belonging to the Oligocene to Miocene Akata Formation. Thetarget of oil exploration/exploitations within the basin is the Agbada Formation, which contains the best reservoirs. The presence of a combination of rollover structures, faulted anticlines, growth faults and some thick shale columns, are favorable traps for petroleum accumulation and also prone to overpressure, especially if very close to the naturally overpressure underlying Akata shale.

O’Connor et al., (2011) has presented regional pore pressure analysis of the Niger Delta, while Olatunbosun. et al (2014); Omolaiye and Ayolabi ; Uko and Tamunobereton (2003 )have studied the Afam section of the Niger Delta, detection of pore pressure from synthetic data and overpressure prediction using porosity data respectively. Considering the large extent of the Niger Delta, there was thus need to focus on other regions of the Niger Delta for similar pore pressure predictions, using seismic checkshots; hence the presence studies.

Materials and Methods

The application of pore pressure prediction technique in this work was based on the combination of Dutta (2002) and Bowers ( 995; 2002) theories developed to transform geologically calibrated seismic interval velocities to pore pressure. The basic data used for this work are seismic sections, check shot data and geophysical logs suites of the X oil field. The two main data processing tools used here include petrel and excel spreadsheet software. The following procedures were adopted in this research work.

Determination of Pore Pressure from Seismic Velocity

In this work, velocities derived from check-shot were developed to pressure gradient using Dutta (2002) and Bowers (1994; 2001) theories that assume that, the elastic wave velocities depend on the pore pressure and the total stress tensor according to their equation

A Brief Geology of  the Niger Delta basin.


Figure 1. A Base map (Top) and subsurface map (Bottom) of study area in Niger Delta sedimentary basin of Nigeria

δij = Sij- αPδj …………………………………. equation (1)

Where

δij = 1, i = j
α = Coefficient
P = Pore pressure
αij = Effective pressure
δij = overburden pressure.

The vertical component of the total stress was computed using Eaton (1975) and Bowers (1994) equation, where the value of α = 1. Denoting the vertical component of the effective stress tensor (effective pressure) αij as σ and the vertical Component of the total stress tensor (overburden pressure) δij as S then the vertical component may be written as:

σ = S−P ………………………………………… equation (2)

Where:
P = Pore pressure
S = Vertical component of the total stress tensor
σ = Vertical component of differential stress tensor

With a known vertical component of the total stress, the seismic velocity was used to determine the differential stress tensor, so that the pore pressure can be predicted using equation (2).

The value of the overburden pressure (S) at any depth H is the combined weight of the fluids and formation above H and it is given as

S= Hρg …………………………………………………… equation (3)

Where:
S = Overburden pressure
H = Depth
ρ (z) = Density as a function of depth (z)
g = Acceleration due to gravity

Gardner et al. (1974) equation was then used to obtain the formation density from seismic velocity. The relationship between the velocity and density is given as

ρ =aVb ………………………………………… equation (4)

Where:
ρ = Density
V = Formation velocity b = Exponent
a= coefficient

Exponent (b) and Coefficient (a) are conversion factors of Gardner’s transform equation used to calculate density from seismic velocity. The coefficient (a) and exponent (b) describe the variation in velocity with increasing effective stress. The value of coefficient and exponent used by Gardner in his work are 0.23 and 0.25 respectively. Gardner also obtained his effective pressure value by using his another transform equation,

V= Vo+ Aơ^B ……………………………………………Equation (5)

Where:
V = Formation velocity
Vo = Velocity of unconsolidated fluid saturated Sediment
(taken to be 1600 m sec−1)
σ = Vertical component of differential stress (Effective pressure)

The coefficient A and exponent B describe the variation in velocity with increasing effective stress.

Rewriting Equation (5) above, he obtained:

σ= ((V- Vo)/ (A)) ^1/B ……………………… Equation (6)

The values of A =0.23 and B =0.25 according to Gardner’s transform equation was obtained by adjusting a known density log value from a Well within his study area. He acknowledges that  his study area is of low density and recommend coefficient and exponent adjustment when applying the transformation equation. Emujakporue O.G (2014) who applied Gardner equation in his work accepted adjusting Gardner’s coefficient and exponent values from density log within his study area in Niger delta basin to obtain coefficient as 4.5641 and exponent as 1.461. These coefficient (A) .26105 and exponent (B) .28375 values applied in this study were obtained by adjusting Gardner’s values with 13.5 percent increment related to WELL 06ST density log values in the study area.

Pore pressure values were calculated from Equation (2) by subtracting computed values of effective pressure from overburden pressure. The overburden pressures were calculated from equation (3), density from equation (4) while effective pressures were calculated from equation (6). To identify the over-pressure zone using effective pressure data, zero (0) value of effective pressure estimated from the X seismic data is a zone of pressure equilibrium between the overburden pressure and pore pressure, while negative values of effective pressure are zone of over-pressure in shale or sandstone with seal preventing fluid from moving upward, also is the positive effective pressure values which are zone of under-pressure because, overburden pressure is greater than pore pressure.

 Figure2. correlation of X-well 01 velocity interval and effective pressure graph against depth

Figure 3. correlation of X-well 03 velocity interval and effective pressure graph against depth

Figure 4. correlation of X-well 05 velocity interval and effective pressure graph against depth

Location and Extend of X-Oil Field Horizontal Pore Pressure Belt

Figure 5. X-oil field horizontal pore pressure belt ( with two proposed wells)l showing overpressure zone.

The result indicates that there is potential for high pore pressure, correlating zones were velocity reversed, it is possible that not all the zones with velocity reversal is over-pressured.The relation of effective pressure and pore pressure is that,  both of them makes up overburden pressure and at the depthwhere Effective pressure starts increasing negatively can be referred to as the top of the overpressure zone while the depth t which the pore pressure starts increasing above overburden  pressure is also the top of overpressure zone. Having consi deredall factors that could cause seismic velocity reversal; I hereby predict the depth below as top overpressure zone and the base of overpressure zone respectively in X-oil field. Below are the graphic presentations of velocity interval versus depthand effective pressure versus depth.

Conclusion

The present research work has presented seismic velocity interval as an alternative indicator for pore pressure belt in the Niger Delta basin. This method is more precise and can be easily applied. It is even more economical than other methods. Seismic velocity interval is a major criterion in determining a good reservoir hence lending credence to its usefulness as pore pressure indicator. The advantage of pore- pressur  belt prediction from seismic is that, it provides alternative mthod of determining the expected pressure in a location where there is no well data; even in areas where there are logs, it complements the prediction that is based solely in offset well logs.

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