Pattnaik, Chinmaya (FDGS, Kuwait Oil Company) | Rao, Narhari Srinivasa (FDGS, Kuwait Oil Company) | Al-Ashwak, Samar (FDGS, Kuwait Oil Company) | Al-Ajmi, Neema Hussain (FDGS, Kuwait Oil Company) | Kidambi, Vijaya Kumar (FDGS, Kuwait Oil Company) | Al Anzi, Abdullah Motar (FDGS, Kuwait Oil Company) | Dashti, Qasem (FDGS, Kuwait Oil Company) | Staffelbach, C. (Corias) | Barbé, Jean-françois (Corias)
Fracture characterization is vital for efficient field development of naturally fractured Carbonate reservoirs. Successful development of fracture reservoir in the study area was possible due to early recognition of fracture play right from the exploration phase and through careful execution of relevant data acquisition campaign in the initial stages of field development. Comprehensive and integrated studies have been carried out over the past few years to arrive at an understanding of the conceptual model in deciphering structural evolution of North Kuwait Jurassic.
Extensive core and image log data was acquired in the initial stages of field appraisal, which helped in comprehensive forward planning in design of deviated and horizontal wells. The data analysis steps included accurate and reliable reorientation of the cores. These data were calibrated with the image logs, along with available seismic attributes, which resulted in better understanding of structural evolution and sweet spotting of horizontal wells. This reorientation of the core data also helped in establishing a number of quantitative fractures attributes such as frequency, spacing, dip-azimuth and aperture along with mapping of in-situ stress directions. The detailed integration of these data also helped in accurately mapping the local and regional present day stress and its variations spatially across the fields. Stress direction across the field was helpful for deciding the azimuth of wells during well planning along with selection of completion strategy for current set of horizontal drilling Campaign.
Drilling and testing results have been encouraging through enhanced reservoir performance in these tight carbonate reservoirs, based on these integrated studies.
Pradhan, San Prasad (Kuwait Oil Company) | Acharya, Mihira Narayan (Kuwait Oil Company) | Fidan, Erkan (Kuwait Oil Company) | Rao, Narhari Srinivasa (Kuwait Oil Company) | Al-Awadhi, Mansour (Kuwait Oil Company) | Singh, J.R. (Kuwait Oil Company) | Dashti, Qasem M. (Kuwait Oil Company)
Deep HP-HT sour carbonate reservoirs in Northern Kuwait have varied matrix properties and fracture intensities. The wells are drilled with barite laden OBM with 1,000-2,000 psi overbalance. The intervals suffer substantial formation damage during drilling as is evident from the fact that the wells normally do not get activated, in spite of creating an underbalance of 5,000-6,000 psi by displacing mud with a lighter fluid.
During the early exploration phase of these reservoirs, long and/or multiple intervals were perforated and treated with conventional matrix stimulation using 28% retarded/ emulsified acid in stages with chemical diverter (gel based and visco-elastic surfactant based). Post stimulation PLT survey in these wells indicated, that only about 5-10% of the total perforated interval contributed to the production; concluding that the diverters were found to be ineffective leading to sub-optimal reservoir management due to poor zonal contribution.
As part of strategic reservoir management process selective bottom up approach in perforation with higher concentrations of HCl treatment and without diverter has been adopted in these reservoirs. To obtain a degree of diversion over the perforated interval, the acid was pumped at higher rate and with higher pressure. Adoption of this changed perforation and stimulation treatment has been proved to be the key enablers for improving zonal productivity.
Around 30 wells have been completed with this changed perforation strategy and treated with this new recipe and technique. Post stimulation test results are comparable to those wells treated with regular matrix stimulation. The PLT survey post acid wash treatment by this technique showed that zonal contribution has improved. This process in addition to being simpler is faster and cost effective. This paper presents the comparison between the two types of perforation and stimulation strategies vis-à-vis test results and also the QA/QC followed prior to pumping the acid.
Acharya, Mihira N. (Kuwait Oil) | Chakravorty, Sandeep (Schlumberger) | Rao, Dhiresh Govind (Schlumberger) | Joshi, Girija Kumar (Kuwait Oil) | Pradhan, San Prasad (Kuwait Oil) | Rao, Narhari Srinivasa (Kuwait Oil) | Singh, J.R. (Kuwait Oil) | Dashti, Qasem M. (Kuwait Oil)
The Deep carbonate reservoirs of North Kuwait are broadly divided into deeper assemblage consisting of diagenitically modified dolomitic layer and shallower fractured-laminated tight limestone and Kerogen units. It is a challenge to establish and quantify the known phenomenon of dynamic changes in the flow path characteristics and properties of the reservoir rocks, as the natural stability condition are altered by production of reservoir fluid. The parameters of the flow path characterization become more uncertain in case of deep HP/HT digenetically altered reservoirs and fractured-tight limestone with laminated kerogen, then similar to the North Kuwait deep reservoirs.
In this study an integration of static data such as, borehole image, core and petrophysical evaluation with time lapse dynamic reservoir parameters like production, pressure data from buildup and pressure transient analysis was carried out to understand the flow path characteristic changes. A deterministic approach has been used to characterize the reservoir flow system and to estimate the fracture aperture for each time step. Thus the time dependent alternations in the flow path properties such as reduced fracture aperture and linked causative phenomena have been studied with multiple scenarios.
A detailed inventory and analysis of various well intervention operations between the time lapse measurements was carried out to distinguish the natural vs. work over induced causatives of flow path changes. This has assisted proper calibration of fracture properties for the static conditions, dynamic simulation and history matching. This workflow has also optimized the application of appropriate reservoir health checkups and remedial interventions. Cases of two representative wells completed in each of the deep reservoir assemblages are presented as examples to demonstrate the study.
Rao, Narhari Srinivasa (Kuwait Oil Company) | Al-Ashwak, Samar (Kuwait Oil Company) | Kidambi, Vijaya Kumar (Kuwait Oil Company) | Laiq, Kamran (Schlumberger Kuwait) | Kurniawan, Hari (Schlumberger Kuwait) | Ali, Shujaat (WesternGeco) | Campbell, Al (WesternGeco) | Menkiti, Henry (WesternGeco) | Nutt, Les (WesternGeco)
Al-anzi, Ealian H.D. (Kuwait Oil Company) | Rao, Narhari Srinivasa (Kuwait Oil Company) | Al-Ashwak, Samar (Kuwait Oil Company) | Kidambi, Vijay Kumar (Kuwait Oil Company) | Al-ajmi, Neema Hussain (Kuwait Oil Company) | Rao, Jonna Dayakar (Kuwait Oil Company) | Al-ateeqi, Khalid Abdullatif (Kuwait Oil Company) | Al-Mayyas, Rawan (KOC) | Olderman, Allan Stefanic (Kuwait Oil Company) | Acharya, Mihira Narayan (Kuwait Oil Company) | Chakravorty, Sandeep (Schlumberger) | De Keyser, Thomas Lee
Deep, tight carbonate reservoirs of Pliensbachian, Sinemurian, and Hettangian Stages of the mid-Mesozoic Era are becoming very important in the continued pursuit of hydrocarbon prospects in North Kuwait. At present, a total of 21 wells have penetrated the targeted reservoir zones. Of these, 12 have been tested for hydrocarbon production covering a large area of about 1700 sq km. Further, six wells have produced oil and gas, with two deemed commercially successful.
The entire workflow to characterize these reservoirs is focused on delineating faults and associated fractures in individual wells. Detailed seismic study and volume curvature maps, revealed the existing fault and fracture corridors. Sub-seismic faults and subtle reverse faults with fractures were detected by log correlations and borehole image. Due to paucity of cores in these zones, descriptions of cuttings samples were used to identify faults and fracture zones, based on the presence of large euhedral crystals in the midst of cryptocrystalline dolomite, suggesting the percolation of hydrothermal fluids through fractures.
Many of the wells were drilled with an overbalanced mud system, leading to near-borehole porosity and permeability damage to the rock matrix and to the fracture system. Damage to natural fractures intersecting the well can prevent their detection, leading to missed potentially productive intervals. Mobility of hydrocarbons in these tight, fractured carbonate reservoirs depends upon (i) wells intersecting a natural fracture system that is sufficiently permeable and connected to a large volume of reservoir rock and (ii) the near-borehole area not having suffered irreversible damage due to overbalanced drilling. In summary, the proposed reservoirs are very tight carbonates (average 3 pu porosities) and a fracture play is considered to be the key factor in production. Acid stimulation produced multifold increases in productivity. Most of the wells were drilled overbalanced, which has negative impact on the producibility due to formation damage.
Chakraborty, Subrata Kumar (Schlumberger Oilfield Eastern Limited) | Rao, Narhari Srinivasa (Kuwait Oil Company) | Al-ajmi, Neema Hussain (Kuwait Oil Company) | Al-Ashwak, Samar (Kuwit Oil Company) | Kidambi, Vijay Kumar (Kuwait Oil Company) | Pattnaik, Chinmaya (Kuwait Oil Company) | Stelzer, Hermann (Schlumberger Oilfield Services)
Al-Eidan, Ahmad Jaber (Kuwait Oil Company) | Rao, Narhari Srinivasa (Kuwait Oil Company) | Al-Awadi, Mishari Ameen (Kuwait Oil Company) | Al-Ajmi, Neema Hussain (Kuwait Oil Company) | Pattnaik, Chinmaya (Kuwait Oil Company) | Al-Ateeqi, Khalid Abdullatif (Kuwait Oil Company) | Al-Houli, Maryam Mohammed (Kuwait Oil Company) | Kidambi, Vijay Kumar (Technically Write Consultants LL) | Dekeyser, Thomas
Exploration activity during the last eight years, targeting Jurassic carbonate reservoirs in North Kuwait, has culminated in the discovery of six major tight gas fields, encompassing an area of about 1,800 sq km with a reservoir gross thickness of about 2,200 ft. These fields are the first free-gas fields in Kuwait, which were put on early production during 2008. The reservoirs are characterized by dual porosity, dominated by low porosity and permeability, in deep HP/HT conditions, with wide variety of hydrocarbon fluids ranging from black oil to gas condensate with sour gas. Typical per well production rates are up to 5,000 BOPD/BCPD and 10 MMSCFPD, making them an excellent commercial success.
Despite the limited number of 38 well penetrations to date in this large Jurassic complex, understanding of the depositional model has improved over time through careful integration and detailed interpretation of log, core, and seismic data. Based on these studies, a depositional model incorporating sabkha, tidal flats, lagoon, backshoal, shoal, shoreface inner shelf, outer shelf, and slope/basin depositional environments has been built. Hypersaline brines were generated in the lagoon and seeped downward, selectively dolomitizing the underlying strata, creating secondary porosity and permeability, and significantly improving the reservoir characteristics in some of these fields. From the early phases of exploration, the role of natural fractures in enhancing the production from these reservoirs was recognized. Accordingly, well data acquisition is designed to maximize reservoir understanding. Delineation and development well placement are optimized to penetrate the most heavily fractured corridors, through a combination of seismic attributes and Discrete Fracture Network modeling, constrained by available well data.
Introduction and Historical background
Majority of wells in Kuwait are targeted for shallow Cretaceous reservoirs for conventional oil through fields discovered during the pre-70s (Fig.1). The exploration for deep Jurassic play was impacted due to lack of conventional reservoirs; particularly in the North Kuwait areas and very few deep wells were drilled targeting Jurassic reservoirs (Fig. 2). During the 90s' country wide coverage of 2D and 3D seismic data acquisition was carried out (Fig. 3). Detailed mapping and geo-scientific studies were undertaken utilizing these data sets (Fig 4). These studies resulted in a Paradigm shift in Play concept from conventional reservoirs to search for fracture plays during the late 90s' and early 2000. The focused exploration for fractured tight carbonate Jurassic reservoirs in the ensuing years resulted in the discovery of commercial quantities of oil and gas in the six fields Sabriyah, Raudhatain, Bahra, Northwest Raudhatain, Umm Niqqa and Dhabi in the North Kuwait area encompassing approximately 1800 sq.km. (Fig. 5)
Rao, Narhari Srinivasa (Kuwait Oil Company) | Al-Kandari, Abdullatif Y.A.M. (Kuwait Oil Company) | Kidambi, Vijay Kumar (Kuwait Oil Company) | Al-Ashwak, Samar (Kuwait Oil Company) | Al-Qadeeri, Bashar (Kuwait Oil Company) | Pattnaik, Chinmaya (Kuwait Oil Company)
Understanding fracture corridors is the primary driver for successful development of fractured carbonate reservoirs. This assumes further significance if the carbonate reservoir is characterized by very low porosity and permeability; producibility of the reservoir is purely dependent on the presence of natural fractures. Distribution and type of natural fractures is a function of palaeo & present day stress, structural elements, regional tectonics and diagenetic history. Direct detection of fractures is below the resolution of conventional seismic data. However, through a combination of seismic derived attributes integrated with well data, it is possible to better understand the distribution of fracture swarms.
Kuwait Oil Company (KOC) is currently engaged in an early phase of development of a tight fractured carbonate North Kuwait Jurassic gas play. Considering the limited well control, field development is heavily reliant on seismic data for fracture characterization. This paper presents our current understanding of the relationship between fractures observed in the well data and structures, faults and lineaments interpreted on seismic data. In addition to conventional seismic analysis a suite of seismic attributes including Dip, Coherence, Edge and 3D Volume curvature were used for mapping structures, faults and minor lineaments. Well-wise and field-wise analysis of relationships between seismic derived attribute-pattern and fracture orientation was established. The understanding between these two different sets of data has helped in locating potential zones of sweet spots for placing successful delineation and development wells. These seismic attribute volumes were also used as soft constraint for building the Discrete Fracture Network (DFN) model for populating the fracture network in the reservoir model. The data presented in this paper are from the Raudhatain, Sabriyah and North West Raudhatain (NWRA) fields for the Najmah-Sarjelu part of the Jurassic section.
Chakraborty, Subrata Kumar (Schlumberger Oilfield Eastern Limited) | Rao, Narhari Srinivasa (Kuwait Oil Company) | Al-Awadi, Mishari Ameen (Kuwait Oil Company) | Kidambi, Vijay Kumar (Kuwait Oil Company) | AL-Houli, Maryam Mohammed (Kuwait Oil Company) | Al-ateeqi, Khalid Abdullatif (Kuwait Oil Company) | Perrin, Christian (Schlumberger) | Ghorayeb, Kassem (Schlumberger)
Dedicated exploration efforts targeted on the Jurassic reservoirs in North Kuwait culminated in the discovery of six separate fields, encompassing an area of approximately 1,772 km2. These reservoirs are known to hold commercial accumulation of gas, gas condensate and volatile oil and are currently in an early phase of development. The producing Jurassic reservoirs belong to the Marrat, Sargelu and Najmah Formations of Toarcian to Tithonian ages. These reservoirs consist of tight carbonates with several other complex lithologies and are naturally fractured with fractures acting as a driver to production in the Najmah and Sargelu formations and fractures acting as production assist in the Marrat Formation.
A 3D geological model was developed for these reservoirs. The static geological model consists of a matrix model capturing the facies, and other matrix reservoir properties like porosity, permeability and water saturation; and the fracture model captures the fracture properties like fracture porosity, permeability and matrix to fracture coupling parameters. The model framework was guided by seismic horizon interpretation from 3D seismic data while the model vertical resolution was optimized to adequately capture the log defined reservoir properties. Electro facies were interpreted at wells with extensive core calibration and were used to develop a 3D facies model capturing the worked out depositional environment. This facies model was used to develop the porosity, permeability and water saturation models using the processed wireline log data, core plug based measurements. The 3D fracture model was developed capturing the areal distribution of fractures using the calibrated 3D seismic volume curvature attribute as soft data and constrained vertically by hard data from image log and cores from wells, thus bringing robustness and reliability for planning the development wells. This 3D geological model was optimally upscaled depending on the aerial well density for use of flow simulation studies.
The North Kuwait Jurassic Complex (NKJC) consists of six fields, namely, Raudhatain, Northwest Raudhatain, Sabriyah, Umm Niqqa, Dhabi and Bahrah, extending over an area of approximately 1,772 km2 in the northern part of the State of Kuwait (Fig. 1). The major reservoir stratigraphic units are the Najmah/Sargelu and Marrat (Upper, Middle and Lower) formations (Fig. 2) separated by major flooding surfaces (Dey A. K., et.al. 2008). These reservoir stacks are composed of very tight fractured limestone and some porous dolomite streaks occurring at subsea depths about 13,000 to 16,000 ft. These reservoirs are envisaged to share a single aquifer in the upper part of the Lower Marrat, leaving most of the Lower Marrat in the water leg and they contain multiple fluid types at near critical conditions (gas-condensate and volatile-oil) with average temperatures of 275 oF, initial pressures averaging 11,000 psia, and saturation pressures ranging from 4,300 to 6,200 psi. The reservoirs' flow regimes vary between purely fracture-driven in the Najmah/Sargelu Formation to a combination of matrix-fracture driven in the Marrat Formation. Thirty two deep, high-temperature and high pressure wells have been drilled in these six fields and typical production per well is approximately 5,500 bopd oil and 15 MMscfd gas.
Deviation from exploration for conventional, Albian age, clastic plays towards carbonate plays of a stratigraphic nature has prompted the use of plan view - horizontal seismic displays in the interpretation process as an additional means of data visualization enhancement.
Significant improvement in accuracy and speed have been attained with the use of horizontal displays in the mapping of minor lineaments and faults with no visible, or minor throw on vertical seismic sections. Mapping practices include work on horizontal slices and slabs, which can be either horizon or time consistent. This approach to seismic interpretation has allowed the inclusion of seismic Attributes, Spectral Decomposition, Edge detection, Coherence, Dip and Azimuth analysis, and other display enhancements into the interpretation process.
Carbonate plays of the Lower Cretaceous are of emerging interest as exploration targets. This type of play relies on fracture porosity, which is developed in close association with faults. Generally poor seismic data quality prohibits accurate fault identification on vertical sections. Here it has often been necessary to employ circumspect methods including the use of horizon volumes. These may even be based on vertically offset, but well defined marker horizons that approximate the curvature of the studied event. Methods of this nature have been employed in the identification of stratigraphic features such as the edge of a Lower Cretaceous oolite play proximal to the Minagish field in southern Kuwait.
Horizontal displays of seismic attributes were also used in the mapping of the extent of a flat spot in the Upper Cretaceous, and of a very shallow, Lower Miocene channel system in northern Kuwait. In the two latter cases it was extremely difficult to maintain lateral pick consistency among consecutive vertical sections.
We conclude that for these types of play, the level of accuracy attained in the interpretation of faults, stratigraphic features, i. e. onlap edges, channels and direct hydrocarbon indicators on horizontal data volumes, widely surpasses what can be reasonably achieved on vertical data displays. We anticipate that interpretation and study of horizontal seismic attribute and other signal enhancement displays will play an increasingly important role in the future subsurface mapping in Kuwait.