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Present-day in situ Pore Pressure Distribution in the Tertiary and Cretaceous Sediments of Zubair Oil Field, Iraq



Hussein Saeed Almalikee and Souvik Sen
 
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ABSTRACT

Background and Objective: Supergiant Zubair oilfield is one of the major hydrocarbon producing assets in southern Iraq. This study presents the assessment of pore pressure distribution across the tertiary and cretaceous sedimentary sequences in five producing wells. Materials and Methods: Indirect pore pressure estimation employing compressional sonic slowness responses as well as direct pressure measurements have been combined to establish regional pore pressure profiles. Results: Study reveals hydrostatic pressure regime in tertiary sediments. Late cretaceous Tanuma shales are under-compacted and therefore reveals mild overpressure, while the primary reservoir middle Cretaceous Mishrif limestones are in sub-hydrostatic pore pressure due to production related depletion. Conclusion: This study will be directly beneficial for determining minimum mud weight for drilling, since too high or too low mud weight can result into mud losses and wellbore collapses respectively leading to expensive non-productive time (NPT) in terms of drilling and loss of put on production (POP) time.

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  How to cite this article:

Hussein Saeed Almalikee and Souvik Sen, 2020. Present-day in situ Pore Pressure Distribution in the Tertiary and Cretaceous Sediments of Zubair Oil Field, Iraq. Asian Journal of Earth Sciences, 13: 1-11.

DOI: 10.3923/ajes.2020.1.11

URL: https://scialert.net/abstract/?doi=ajes.2020.1.11
 

INTRODUCTION

A comprehensive understanding of present day in situ pore pressure distribution is critical for safe and sustainable execution of infill drilling in producing giant oil fields to enhance hydrocarbon production as well as optimization of steam injection for increased recovery1-3. Mud loss and wellbore collapsing are the two major concerns contributing to significant non-productive time (NPT) while drilling through the tertiary and cretaceous sediments of supergiant Zubair oil field, southern Iraq. A proper mud weight design via accurate pore pressure analyses is the prime requirement to address this challenge3.

There has been no previous published literature on the present day pore pressure behavior across the tertiary stratigraphy in Zubair oil field. The only related study was performed by Almalikee and Al-Najim4 from North Rumaila oil field, Iraq, situated in the northwest of the Zubair field. Almalikee and Al-Najim4 established vertical stress and pore pressure gradient from well log dataset. We took this opportunity and investigated the high resolution geophysical logs and downhole direct pressure measurements available from the recently drilled wells.

The main purpose of this study was to determine vertical stress (i.e., overburden stress) and ascertain formation pore pressure. Regional pressure profiles have been proposed for the oilfield. The objective of this study was to model the regional vertical stress and characterize the present day in situ pore pressure distribution across stratigraphy in Zubair oil field to recommend necessary mud weight for safe and stable well bore drilling.

MATERIALS AND METHODS

Study area: The supergiant Zubair oil field is one of the largest oil fields in the world, discovered in 1949 by the Basrah Petroleum Company (BPC), it is located in the southern Iraq, about 20Km west of Basrah city, the field is a semi symmetrical NNW-SSE longitudinal anticline 60 km length and 15 km width as presented in Fig. 1. The duration of this study was from March, 2019 to November, 2019.

Stratigraphy column for Zubair oil field as presented in Fig. 2 is mainly comprised of carbonate rocks with few Shale and Sandstone formations, the main oil reservoir in the field is Mishrif (upper cretaceous carbonate). Four Technostratigraphic mega sequences (start with AP8 to AP11) had shaped the stratigraphic succession for South of Iraq, these mega sequences were controlled by tectonic evolution of the Arabian plate and separated by regional outstanding unconformity surfaces5-6.

Data set: Five onshore vertical wells drilled till middle cretaceous Rumaila Formation were studied to model the vertical stress and pore pressure in the Zubair oil field. These wells had a primary reservoir target of Mishrif limestones and average true vertical depth (TVD) is around 2500 m. High resolution geophysical log suite consisting of gamma ray, resistivity, formation bulk density, compressional sonic slowness and caliper logs were the key inputs for the analyses. Here in this section, methods applied in this study have been elaborated.

This study was conducted from March-November, 2019.

Estimation of vertical stress: Vertical stress is the cumulative pressure by the overburden litho column. This is also known as Overburden stress. Plumb et al.7 expressed the vertical stress (Sv) at a depth (Z) as:

(1)

where, ρ(Z) represents the depth vs. density profile of the entire overburden starting from surface to the depth of estimation (Z). In practical scenario, this is the density log recorded in the hydrocarbon wells8. In Eq. 1, g is the gravitational constant with a value of 9.8 m sec2. Since the wireline logs are not recorded from the surface level, synthetic density was determined by the following power law curve2,8 against the interval not logged:

(2)

where, RHOBsyn is the synthetic density for shallow section, RHOBsur is the surface sediment density, TVD is true vertical depth, AG is air gap (distance between drill floor and ground level), A and α are fitting parameters. The parameters of the power law curve were determined by adjusting the three reference points to match the power law curve to the density log over the depth interval for which density data was available.

Estimation of pore pressure: Fluids trapped in rock’s pore spaces exert pore pressure (PP). Accurate understanding of formation pressure has critical implications in successful well delivery3,8-10. Formation pressure measurement data were available from repeated formation tester (RFT) tool against Mishrif Formation, the primary reservoir. These RFT points are the direct in-situ pore pressure measurements11,12. Since RFT measurements were not taken in other formations, indirect method involving geophysical logs have been employed to estimate PP across the stratigraphy encountered in the studied Zubair field.

Fig. 1:
Location of the study area

Eaton13 had developed a method of predicting PP from resistivity and sonic logs and this is the most extensively used empirical relationship in hydrocarbon industry. As per this method, the magnitude of overpressure caused by disequilibrium compaction can be predicted from the response of resistivity and sonic slowness logs with respect to a normally compacted sediment section. Thus it employs a normal compaction trend line (NCTL) and identifies the pore pressure behavior with respect to the NCTL. High resolution compressional sonic slowness log (DT) was available from wireline suite in the studied wells and following equation had been utilized for pore pressure prediction12:

Fig. 2:
Stratigraphic column for Zubair oil field

(3)

where, Phyd is the hydrostatic pressure, DTn is sonic travel time against normally compacted shale sections as characterized from NCTL13. Shale sections are identified from gamma ray log, based on interpreted gamma ray log responses against clean shale zones14-17. The DT is the observed acoustic travel time, available from wireline compressional sonic slowness logs. A ratio of (DTn/DT) and its variation with depth with respect to the NCTL calculates the pore pressure profile13, 18-20.

As a result of pressure exerted by overburden sediments, fluid in the pore spaces of rocks are expelled, thus rock density increases and it gets compacted. The NCTL represents this compaction equilibrium behavior and hence pore pressure gradient19-21. In a geological situation of rapid sedimentation rate, underlying rocks will not get enough time to expel its pore water following the normal trend of depth vs porosity reduction. As a result, these litho units will retain the pore fluid in the excess porosity and exert higher pore pressure19-21. From petrophysical perspective, with continued compaction along NCTL, compression sonic slowness value shall normally decrease in deeper rocks. But any abnormally porous litho unit will reveal a considerably higher sonic slowness value, deviating from the regional NCTL. The degree of separation of DT log response from NCTL will characterize the magnitude of pore pressure.

RESULTS

Vertical stress: Extrapolated density and wireline density logs have been used to model Sv in Zubair field. The outputs from the five wells have been presented in Fig. 3. Extrapolated density profile has been very effective since there were multiple intervals where well bore was affected by wash outs, as seen in Caliper logs. These are the zones with erroneous density values, since wireline density (RHOB) is measured by a padded tool which touches the borehole wall while logging up. Against the extensively washed out segments, pads cannot reach the enlarged well bore radius and hence produce unreliable data. In this study, a careful investigation of enlarged well bore portions and necessary density correction with the extrapolated values helped us minimized error in vertical stress estimation. Well wise estimated Sv has been plotted against depth to ascertain a depth vs. overburden trend across the stratigraphy as shown in Fig. 4. It provided the following correlation:

Fig. 3:
Extrapolated density log (blue curve) to the surface, wireline density (red curve) and estimated vertical stress (brown curve) for the studied wells in Zubair field

Fig. 4:
Regional vertical stress (Sv) model in Zubair oil field

Table 1:
Regional vertical stress (Sv) magnitude and gradient with depth in the studied formations of Zubair oil field
TVD: True vertical depth

(4)

where, Sv is the vertical stress magnitude in PSI and TVD is the true vertical depth in Meter.

The above relationship has a very high correlation coefficient of 99%. Therefore, we propose this empirical relationship to model regional Sv trend with great confidence. Regional vertical stress magnitude and gradient have been presented in Table 1, it reveals a maximum of 0.99 PSI/ft (equivalent to 3.25 PSI/m) Sv gradient at deeper depths.

Pore pressure: In any oil and gas field, it is mostly the reservoir section in a well where direct formation pressure measurements (i.e., RFT etc.) are conducted. Operator had taken RFT readings in Mishrif Formation, the primary hydrocarbon producer in Zubair field. Data reveals a sub-hydrostatic pore pressure condition in Mishrif limestones resulting from prolonged production induced depletion. Indirect method by using Eaton’s sonic equation yielded pore pressure magnitudes in other formations. The established NCTL on the sonic slowness log data to ascertain the variation of porosity with depth, as presented in Fig. 5.

Table 2:
Regional vertical stress, hydrostatic pressure and pore pressure distribution in various formations of Zubair oil field
TVD: True vertical depth, Sv: Regional vertical stress

The NCTL reveals higher porosity in Tanuma shale. This may be the result of compaction disequilibrium and this behavior can be persistently correlated in all the studied wells in Zubair field. A mild over pressure (200 PSI more than hydrostatic pressure) has been interpreted in Tanuma shale. Figure 6 represents the interpreted vertical stress and pore pressure profiles in the five wells.

The daily drilling reports (DDR) have been studied to look for influx events. Formation fluid influx indicates that the drilling mud weight is not sufficiently overbalanced, thus relates to in situ pore pressure calibration. However, such influx events were not reported and the mud weight was higher than the formation pore pressure. Regional vertical stress, hydrostatic pressure and pore pressure distribution in various formations of Zubair oil field have been graphically presented in Fig. 7 and the interpreted magnitudes have been documented in Table 2.

Fig. 5:
NCTL on compressional sonic log in one of the studied wells form Zubair field, indicating Tanuma shale formation (2144-2172 m TVD) has higher porosity with respect to NCTL

Fig. 6(a-e):
Vertical stress, hydrostatic pressure and estimated pore pressure in (a) Well A, (b) Well B and (c) Well C (d) Well D and (e) Well E along with available mud weight and RFT data set

Fig. 7:
Regional vertical stress, hydrostatic pressure and pore pressure profile for Zubair field along with formation tops, drilling mud weight and RFT measurements from five studied wells

DISCUSSION

This study integrated the wireline geophysical logs and downhole measurements to establish the vertical stress (Sv), hydrostatic pressure and regional pore pressure profile in the tertiary and upper cretaceous sediments of Zubair oil field, southern Iraq. Major findings are as below.

The vertical stress has a gradient of 0.99 PSI/ft. A regional empirical relationship of vertical stress with TVD has been proposed, which will be very effective in case of poor density data or unavailability of the density logs. Pore pressure estimated from sonic slowness log by Eaton’s method reveals hydrostatic pore pressure conditions from Dibdibba to Sadi Formation. Mild overpressure of approximately 200 PSI has been interpreted in Tanuma shale from the sonic log response with respect to the NCTL. Disequilibrium compaction due to rapid sedimentation is responsible for excess porosity in Tanuma shale, which resulted in deviation of DT log from NCTL. Extensively available RFT measurements have been used to interpret present day pore pressure condition in Mishrif formation, which indicates dissimilar depletion in pore pressure in the upper and lower member of Mishrif Formation. This might be a result of permeability variation within the formation. The mud weight has been suggested to be slightly overbalanced with respect to the in situ formation pressure of Tanuma shale. However mud pressure should not be high enough against depleted Mishrif limestones, otherwise it may result in mud losses.

The observations and results of this study were compared with the finding of Almalikee and Al-Najim4 from North Rumaila oil field, Iraq, which encompasses the similar stratigraphic succession as in Zubair oil field. Almalikee and Al-Najim4 established an average 1.02 PSI/ft vertical stress gradient, which is a very close estimate when compared to the studied Zubair field (0.99 PSI/ft). Tanuma shale in Rumaila field has been deciphered as mildly over pressured (150 PSI higher than hydrostatic)4, which indicates a similar pressure distribution pattern and hence geological condition prevailing in the late Cretaceous across the region. Middle Cretaceous Mishrif formation serves as the primary reservoir unit in both the oilfields (Rumaila and this study) and the depletion induced pore pressure reduction has been seen to be consistent in both the cases.

CONCLUSION

This study captures the overburden pressure and present-day pore pressure distribution formation wise. This research establishes that the primary reservoir Mishrif formation is presently in sub-hydrostatic pressure regime due to depletion. The presented interpretation will guide the geoscientists and engineers to better plan and design drilling weight for any new infill well in Zubair oil field.

SIGNIFICANCE STATEMENT

This study establishes the regional vertical stress in the Tertiary formations and deciphers the present day in situ pore pressure regime across the stratigraphy from a normal compaction trend. This study will help the engineers and geoscientists to confidently design drilling mud weight to avoid wellbore influxes and fluid loss related near wellbore damages in the producing horizons.

ACKNOWLEDGMENTS

Authors are sincerely grateful to Basrah Oil Company for the dataset. The SS thanks Geologix Limited for giving the access to Pore Pressure and Geomechanics module of GEO suite of software, which has been instrumental for various analyses presented here. Interpretations and conclusions drawn in this study are solely of authors and do not necessarily represent those of their employers.

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