Patnaik, Golak (Vega Petroleum Limited 9) | Abhishek, Karmesh (Vega Petroleum Limited 9) | Naser Khafagy, Abdel (Vega Petroleum Limited 9) | Attia, Wael (Vega Petroleum Limited 9) | Walia, Samir K. (Roxar Software Solutions) | Abd-Allah, Mohammed (Roxar Software Solutions)
The Gulf of Suez is characterized by intensive normal faulting related to rifting of the Red Sea. The Ras El Ush (REU) field displays the same pattern. The field itself is bound on its eastern margin by large NW-SE oriented normal faults dipping NE (towards the Red Sea) and has dip closure to the west. However the nature of the northern and southern boundaries is unclear and they could be limited either by fault or dip terminations.
The REU field has been producing since 1996 from the Nubia and Matulla formations and it currently has around a 26% recovery factor. Nubia and Matulla are isolated from the bottom water aquifer by tarmat layer that acts as a barrier. The field therefore exhibits depletion drive with a sharp pressure decline initially then a secondary gas cab expansion drive.
The objective is to optimize pressure maintenance in the developed blocks either by using Gas Injection or Water Interjection and to evaluate the upside potential using an integrated reservoir management study approach and leveraging advance tools, technologies and best practices.
Since the structure contains dipping beds (up to ~50 deg) and faults, the conventional seismic does not help in interpreting reservoir units and dipping faults. Some of the vertical faults can be picked from seismic. In this case, it's critical to quantify the structural uncertainty due to lateral movement of faults and vertical movement of horizons, particularly near the oil water contact.
An increasing number of deviated wells are being drilled to maximize production and hydrocarbon recovery in the mature reservoirs of the Gulf of Suez (GoS). Successfully drilling a high-angle well in a tectonically disturbed and structurally complex area like the GoS is very challenging, especially in depleted reservoirs. Selecting the optimal mud weight is absolutely essential. Stress orientation and magnitude also have a major impact on wellbore stability.
The region poses significant drilling challenges that vary widely from reactive shale and salt creep to stress-related instability. From the findings of multiple wellbore stability projects we conducted in the GoS, we review the dominant mechanisms of wellbore instability in the GoS. We provide a summary of the failure mitigation measures and an overview of stress magnitude and orientation in the region, demonstrating how it impacts the knowledge of the most stable drilling direction.
Understanding the main causes of rock failure in the GoS resulted in improved drilling efficiency and reduced drilling costs. We show an example, where a new, nearly horizontal (86º) well was successfully drilled through the Asl formation with less than half a day of non productive time during the entire drilling process.
We conclude that acquisition of new, high-quality data would considerably reduce the uncertainty surrounding drilling complex wells in the area and reduce their cost.
As oil fields mature, progressively more effort is being devoted to diagnosing production and reservoir problems and to finding cost-effective solutions for those problems. One of the key inputs for any reservoir management scheme is the timely monitoring of behind-pipe fluid saturation profiles across the field. In the past, this was conventionally accomplished using tools and methods requiring known, non-varying and sufficiently high formation water salinities. In the cases of water flooding with fresh water, mixed water, and/or water of unknown salinity, these methods become useless. In such cases, cased-hole saturation monitoring may only be achieved using tools and methods independent of water salinity such as the carbon-oxygen (C/O) method.
Using the C/O techniques to compute water saturation offers many advantages over conventional techniques that depend on formation water salinity. These techniques relate directly to the volumes of oil and water in the formation, and the conversion of C/O ratio to oil saturation is based on a large database acquired in a wide range of formations and well bore environments. Recent enhancements to spectral processing and job execution have resulted in dramatic improvements in the effectiveness of C/O logs. These enhancements include improved elemental standards and the development of a full spectrum calibration to provide better tool-to-tool accuracy and precision in a wider range of porosities even in the presence of gas. The enhancements also include operational improvements such as combination with production logging tools and appropriate "time-stepping" of shut-in and flowing surveys.
Several Gulf of Suez fields have undergone water flooding with non-native waters and have thus presented formidable challenges to traditional cased hole saturation monitoring techniques. More recently, operators in the area have started employing enhanced C/O techniques with great success. Enhanced C/O methods are now routinely used to effectively detect unswept hydrocarbons and to track fluid contact movements in water-flooded reservoirs. This paper discusses several case studies from the Gulf of Suez that clearly demonstrate the efficacy and economic benefits of the enhanced C/O methods.
Cased Hole Saturation Monitoring Methods
Saturation monitoring through casing is generally carried out in one or both of two ways: pulsed neutron capture (PNC), which measures the decay of thermal neutron populations, and gamma ray spectroscopy, which determines the relative amounts of carbon and oxygen in the formation. PNC tools first became available to the petroleum industry in 1968 to measure the thermal decay time of neutrons bombarded into the formation. Fast neutrons (exiting the minitron with an energy level of around 14 Mev) are slowed down to thermal energy (0.025 eV) by multiple collisions with formation nuclei. Thermal neutrons are susceptible to capture by formation nuclei, and the resulting nucleus becomes excited and emits a characteristic gamma ray. The thermal neutron population around the tool can therefore be analyzed to yield measurements of formation and borehole capture cross-sections. The capture cross section of the formation, often called Sigma, is determined by analyzing the approximately exponential decline of the gamma ray count rate with time as the neutrons are captured by the surrounding materials (neutron capture) and as they diffuse farther away (neutron diffusion). Because chlorine has a large neutron capture cross section, the PNC technique provides good results in areas with highly saline formation waters.
El-Banbi, Ahmed H. (Schlumberger Data & Consulting Services) | Fathy, Khaled (Schlumberger Data & Consulting Services) | Morsy, Samir Y. (Schlumberger Data & Consulting Services) | Aly, Ahmed M. (Schlumberger Data & Consulting Services) | Alaa, Alam (GPC) | Fattah, Mohamed A. (GPC)
A major integrated study was performed to optimize the development of a complex field and to find opportunities to increase oil production. The field is composed of eight stacked reservoirs. The field was developed with more than 160 wells and has been producing since 1960. A pilot gas injection project was installed in the early 1990's. Despite the large number of wells and long production history, only 10% of the OOIP was recovered to date. An integrated approach was used to construct a dynamic model for the field and couple it to economical calculations. The model was used to optimize the performance of the field, increase recovery factor, and accelerate production. Unique to this paper is the procedure of history matching large amounts of data and long history. A relatively large model was constructed and then was broken down to seven models. The history match was attempted for each model separately. The seven parts of the field were then combined and the history match was refined. This history matching procedure resulted in significant time savings in the study and allowed the study team to dedicate most of their time to field optimization and to understanding the reservoir behavior.
The Bakr-Amer field is one of the General Petroleum Company's (GPC) producing fields in the Gulf of Suez (Fig. 1). It is located in the west-central part of the Gulf of Suez. It is about 10 km to the north of the Ras Gharib oil field and about 40 km to the north of the Ras Shukheir oil field. The Bakr field was discovered in 1958 based on seismic, magnetic, and gravity data. Oil production from the field began in 1960. Amer field was discovered in 1965 and production began shortly after. The Bakr-Amer field is a composite field initially considered as three separate accumulations; namely the South Bakr, North Bakr and Amer oil accumulations. Subsequent work reached a conclusion that the three parts of the field were in fact one continuous reserve. The Bakr-Amer field is 14 km long and represents the central segment of the large NE tilted fault block. It produces oil from several reservoirs made up of reefal limestone, fractured limestone as well as quartzose sandstone (Belayim Nullipore, Lower Miocene limestone, Thebes Formation, Matulla Formation, Wata Formation, Raha Formation, and Nubia Sandstones). Multiple water-oil contacts were identified. The Carboniferous shales of the Nubia-B unit represent the effective seal, which separates the Nubia-CD from shallower reservoirs. The upper horizons (above the -1050 m oil/water contact) are producing from the Lower Miocene, Thebes, Matulla, Wata, Raha, and Nubia-A Formations. On the other hand, the lowest oil pool is producing from the Nubia-CD unit. Fig. 2 shows the historical field oil production. The production of the field during the period 1965 to 1975 came mainly from the Eocene rocks and the recent peak in production from the new developments of the shallowest reservoir (Belayim). Fig. 3 shows the development of water cut over time.
As oil fields mature, progressively more effort must be devoted to diagnosing production and reservoir problems and to finding cost-effective solutions for those problems. One of the key inputs to any reservoir management scheme is the timely monitoring of behind-pipe fluid saturation profiles across the field. Using the C/O ratio to compute water saturation offers many advantages over conventional techniques that depend on formation water salinity. The C/O ratio relates directly to the volumes of oil and water in the formation, and conversion of C/O ratio to oil saturation is based on a very large database acquired using laboratory formations with a wide range of wellbore environments. Recent enhancements in spectral processing techniques for pulsed neutron spectroscopy tools have improved accuracy and precision of measured carbon-oxygen (C/O) ratios. These include improved elemental standards and the development of a full spectrum calibration to provide better accuracy and precision in a wider range of porosities and in the presence of gas.
Many of the Gulf of Suez fields are characterized by edge water drive mechanisms or are undergoing water flooding with different salinity water than the formation water. These conditions have traditionally presented formidable challenges to cased hole saturation monitoring, often resulting in highly uncertain results. This paper highlights several case histories from the Gulf of Suez which demonstrate how Enhanced C/O methods were used to effectively detect unswept hydrocarbons and track gas/oil/water contact movements in the reservoirs.
Cased Hole Saturation Monitoring Methods
Saturation monitoring through casing is generally carried out in one of two ways. One way, pulsed neutron capture, measures the decay of thermal neutron populations, and the other, inelastic gamma ray spectroscopy, determines the relative amounts of carbon and oxygen in the formation. Pulsed neutron capture (PNC) tools first became available to the petroleum industry in 1968 to measure the thermal decay time of neutrons bombarded into the formation. Fast neutrons (exiting the minitron with an energy level of around 14 Mev) are slowed down to thermal energy (0.025 eV) by multiple collisions with formation nuclei. Thermal neutrons are susceptible to capture by formation nuclei, and the resulting nucleus becomes excited and emits a characteristic gamma ray. The thermal neutron population around the tool can therefore be analyzed to give formation and borehole sigma measurements. Because chlorine has a large neutron capture cross section, the PNC technique provides good results in areas with highly saline formation waters.
Sigma, the capture cross section of the formation, is determined by analyzing the approximately exponential decline of the gamma ray count rate with time as the neutrons are captured by the surrounding materials (neutron capture) and as they diffuse farther away (neutron diffusion). Sigma is inferred from this observed decline in the gamma ray count rate versus time. In addition to the neutron capture, two key environmental effects, diffusion and borehole contamination, contribute to the observed decline or decay rate and need to be carefully characterized in order to determine the correct Sigma throughout the wide range of operating conditions typically encountered in the oilfield. These effects are controlled by such parameters as borehole size, casing size, casing weight, borehole fluid salinity, porosity, and lithology.
GOS HOOK TYPE WELLS, DIRECTIONAL PLANNING, TECHNIQUES APPLIED AND PROBLEMS ENCOUNTERED.
This paper addresses the various aspects of hook type wells introduced and drilled within GUPCO operations during the last two years, The first well of this category was October-G10, drilled in October 1992 from October "G" platform to a target point in the Nubia formation. Several wells of the same type have been drilled through 1993 and 1994. This group includes October-H1, Ramadan 3-57, July 6269 and SB 374-3. Drilling hook type well profiles has resulted in increased production and more reserve recovery. The driving force behind using this profile was the reservoir requirements where it was required to hit a target within few meters at a certain angle and direction.
Torque and drag models have been used to optimize well path planning, resulting in lower torque and drag values, Daily post appraisal of the drilling operations to monitor hole cleaning effectiveness. Combination of advanced steerable systems and PDC bits enabled GUPCO to drill these wells cost effectively.
For the past 25 years the majority of the directional well paths designed and drilled by GUPCO were classic build and hold design. In some cases this profile was modified; to build, hold and drop "s shape" or the catenary profile, where a continuous build is employed through out at least two phases of the hole, How ever these unique trajectories were still in the same plane.
If a correction run was required, calculations and precautions usually took place to do the required correction in a soft drilling section, like South Gharib formation salt or shale in the Miocene Clastics formation. The controlling factor behind this limited application of mud motors, was the limited power output of the old style mud motors. There were no available mud motors capable of rotating the first versions of PDC bits.
As the fields in the GOS have matured, and to improve reserve recovery, Specific reservoir targets like a target point to be reached in a pre-determined angle and directions were recommended. In some cases this may not be achievable from a fixed already existing platform, so the Hook Type well design was introduced.
Applying the new well profile designated as 'Hook Type" is not restricted to a certain GOS field but was applied to essentially all (October, July, Ramadan and Sidki re-drill project).
The new achievements in the drilling industry in the last few years. Such as new versions of PDC bits, powerful mud motors, and advanced computer software.
The Gulf of Suez Petroleum Company(GUPCO) has recently used "slim-hole" drilling technology to enhance the economics of developing a marginal field by significantly reducing drilling and development costs. Drilling costs have been reduced by approximately 20%. Risks have also increased with slim-hole technology, but are within manageable levels. In order to ensure a deep point of injection is maintained, small but difficult-to-fish gas-lift equipment has been installed inside of a 5-inch production liner. To date, no corrosion has been observed in the October field which would weaken the slim-hole bottomhole assembly and prevent future recovery of the packer and tubing. Sidetrack flexibility has also been reduced with the smaller tubulars.
The October Nezzazat field is located offshore in the Gulf of Suez and is penetrated by wells which were drilled to the prolific Nubia formation. Independent development of the Nezzazat field using conventional drilling and completion technology has not been possible due to less favorable economics. Historically, the Nezzazat has been developed by recompleting watered-out downstructure Nubia wells. Nezzazat development timing has therefore been subject to Nubia depletion. Wellbore placement in the Nezzazat has in the past been wholly a function of Nubia development requirements.
The recent application of creative and innovative ideas have helped accelerate the development of the October Nezzazat field. Since most of the October platforms have been fully-drilled and since setting a platform to develop the marginal Nezzazat field was economically prohibitive, additional slots were added to platforms which could support the additional loads in order to drill the marginal Nezzazat wells. Further, since gas-lift compression is limited at the October production complex, any additional drilling would have resulted in a re-allocation of available gas-lift gas. This would take lift-gas from existing wells and defer the full production impact of the new wells. As an alternative, less-expensive shoreline compression was installed and gas-lift gas was delivered offshore by injecting through an existing, under-utilized pipeline. Finally, with less-expensive slim-hole completions, the October Nezzazat field may now be developed independently of the Nubia formation resulting in greater production and reserves from the Nezzazat.
Currently, fourteen wells produce about 35,000 BOPD from the Nezzazat. Cumulative production through June, 1994 was 33.7 MMBO. The Nezzazat is classified into three formations: Matulla, Wata, and Raha.
The October Nezzazat field overlies the Nubia field and has historically been developed via Nubia-to-Nezzazat recompletions because of the limited reserves and relatively low potential rate of the Nezzazat. Reducing the cost of drilling through slim-hole completions and other innovative approaches have enabled the Nezzazat field to be economically developed independently of the Nubia resulting in additional rate and reserves.
Industry definitions of "Slim-Hole vary widely. Slim-hole typically references wells with hole sizes smaller than 4 3/4-inch and in many cases requires specialty drilling equipment.