Yan, Linhui (Petrochina Research Institute of Petroleum Exploration and Development) | Chang, Yuwen (Petrochina Research Institute of Petroleum Exploration and Development) | Tian, Zhongyuan (Petrochina Research Institute of Petroleum Exploration and Development) | Li, Xianbing (Petrochina Research Institute of Petroleum Exploration and Development) | Wang, Li (Petrochina Research Institute of Petroleum Exploration and Development) | Zheng, Xuerui (Petrochina Research Institute of Petroleum Exploration and Development)
Lithology identification in basement rock reservoir is one of the most important basic work for reservoir evaluation. The identification results can directly influence the evaluation of reservoir physical property, oil-bearing property and the identification of effective reservoir.
The paper takes buried hills in Bongor basin as an example to analyze the characteristics of lithologic chemical components and mineral compositions by using core observation, slice identification, geochemical analysis. Based on lithology characteristics, the classification criterion of basement rock was determined. In addition, the log response characteristics were summarized and the corresponding logging identification method was established. The results show that the basement rocks of the research area composed of metamorphic rocks and magmatic rocks. With the decrease of the content of light minerals, the contents of Si and K were decreasing and the contents of Fe, Al, Ti, Mg and Ca were increasing. According to the mineral compositions and contents, the lithologies were divided into 2 categories and 9 subclasses. There were brecciated structure, clasfic structure, gneissic structure, banded structure, net veined structure, augen structure, massive structure and intrusive structure in electrical imaging logging. Meanwhile, logging response characteristics of basement rocks were divided into 6 types combined with the difference of density curve and compensated neutron curve. The established cross plots and summarized characteristics of logging curves and response values could provide a basis for lithology identification. The machine learning approach (SVM) can improve the accuracy of lithological identification.
Jin, Fu (CNPC Research Institute of Petroleum Exploration and Development & CNPC Drilling Research Institute) | Xi, Wang (CNPC Research Institute of Petroleum Exploration and Development & CNPC Drilling Research Institute) | Shunyuan, Zhang (CNPC Drilling Research Institute)
Located in south of Eastern Venezuela Basin, Orinoco Oilfield is the unique huge ultra-tight oilfield that has not been developed by scale in the world. The high-density tight oil is known for its high content of acids, heavy metals and asphaltenes with a viscosity of 1000-10000mPa·s. ML Block whose OOIP is 178*108bbl is situated in east of the oilfield, while cluster horizontal well drilling and cold production technologies are still under research there.
Based on precise geological researches numerical simulation was carried out to optimize cold production of ultra-tight oil with foamy oil flow patterns in horizontal wells, including optimization of well placement, well spacing and horizontal section length. The near-bit geo-steering drilling technology was applied on adjacent wells to test its performance, while an experiment was conducted with PVT apparatuses to examine the effect of pressure decline rates on foamy oil flow. A long core pressure depletion test was accomplished to reveal the effect of foamy oil flow on recovery factors.
Three-dimension cluster horizontal well drilling and completion technologies shall be applied to develop ultra-tight oil reservoirs in huge loose sandstones, with the near-bit geo-steering drilling technology that controls landing points and horizontal sections in real time, keeping the bit move ahead along the lower boundary of the reservoir. Therefore, recovery rates may be dramatically improved due to the gravity drainage of ultra-tight oil. The most appropriate spacing of horizontal wells (500-600m) and horizontal section length (800-1200m) were determined to achieve the maximum recovery rate. The experiment proves that the recovery rate improves as the formation permeability increases, which means the "worm hole" contributes to heavy oil extraction. Boreholes with relatively large diameters, extensive perforated holes and slotted liners may be used to complete wells. In order to take the most advantages of the foamy oil flow mechanism high displacement ESPs shall be used with the selected thinner squeezed at the bottom, otherwise PC pumps with the thinner added at the wellhead are recommended.
Cold production technologies applied in ML Block save the overall production cost by 15.2%, improving the ultimate recovery rate by 8.6%. The foamy oil flow theory is improved, while it is the first time to integrate foamy oil flow production technologies with cluster horizontal well drilling technologies and near-bit geo-steering drilling technologies. As a result, the overall production rate of tight oil was greatly improved and the average production life of wells was extended.
Beyond offshore West Africa where modern densely-sampled data from ships and satellites have played a key role in current understanding of passive margin evolution, Africa is in general rather unevenly known, especially in the subsurface in more remote areas. The GIS-based Exploration Fabric of Africa (EFA, the ‘Purdy project’) was designed to address that problem. It includes structural features such as faults and basin outlines but at a very high and often generalized level, divorced from their underlying genetic linkages. We have undertaken to compile a more detailed tectonic synthesis aimed to integrate understanding of the oceanic margins with the continental realm. This is an overlay to EFA with a variety of public domain, published, non-exclusive, and derivatives of proprietary work at a closer and more detailed level, importantly guided by known patterns of structural styles. Potential field (gravity and magnetic) data provide guidance in locating, extending, and connecting key mapped features; we then rely on the kinematic patterns to predict missing details in a testable interpretation. The result is a detailed structural features map that can function as a framework within which to target and prioritize both conventional and unconventional activity by operators and licensing/regulatory organizations. We illustrate the process in theory and in practice along the Central African Rift System (CARS), where data is sparse. This fault linkage systems approach has flagged underexplored areas where unmapped structure is likely that could, for example, be targeted with hi-resolution geophysical data. A similar system to CARS appears to cross southern Africa from Namibia to Tanzania - a “Southern Trans-African Rift system" or STARS. Exploration in the eastern Owambo Basin resulted in the mapping of a pull-apart basin from depth-to-basement inversion of high-resolution magnetic data and subsequently studied with structural modeling. Thinking in terms of such fault and structural systems, this ‘Kavango Basin’ can be related along strike to the Karoo Basins in Eastern Africa via features such as the Omaruru lineament, implying the possibility of a fairway of extensional basins and shears across the continent that are not obvious in existing low-resolution data. STARS represents a blue-sky frontier concept for both conventional and nonconventional exploration potentially offering new exploration leads, the ultimate objective of big picture work.
Kang, Lin (China National Offshore Oil Corporation (CNOOC) Ltd, Tian Jin Branch) | Wu, Kui (China National Offshore Oil Corporation (CNOOC) Ltd, Tian Jin Branch) | Zhang, Zhongqiao (China National Offshore Oil Corporation (CNOOC) Ltd, Tian Jin Branch) | Guo, Naichuan (China National Offshore Oil Corporation (CNOOC) Ltd, Tian Jin Branch) | Fan, Jianhua (China National Offshore Oil Corporation (CNOOC) Ltd, Tian Jin Branch)
S-shaped strike-slip faults are superposition products of strike-slip and extensional effects, and the deformation zones of the active wall developed along which have been verified as hydrocarbon enrichment areas in many cases of petroliferous basins. Therefore, it is essential to predict the distribution of deformation zones caused by S shaped strike-slip fault. However, there haven’t been methods that can accurately predict them. In this paper, an innovative modeling method is proposed. Firstly, the S-shaped strike-slip fault and strata on its both walls are modeled, strength of strike-slip and extension on which can be adjusted by varying oblique slip angles. Then the active wall of S-shaped strike-slip fault featured by strike slip-extensional, tensional-shear, and extensional-strike slip properties are conducted with forward modeling, based on results of which the distribution model of deformation zones of the active wall can be established. Meanwhile, strata deformation attributes are integrated, which demonstrates restraining and releasing zones as areas favorable for migration and accumulation of oil and gas.
In this study, Liaoxi NO.3 fault, an S-shaped strike-slip fault with extensional-strike slip properties, in the Liaodong Bay, Bohai Bay basin is selected for the case study subject to verify the simulation results. According to interpretation results, on the active wall, fault anticline traps formed in the restraining uplift zones are mainly distributed in the transition part of left to right bends along the fault, while fault block traps formed in the releasing subsidence zones are located in the transition part of right to left bends. Currently, drilling in the restraining uplift zones has received good business effects, which conform to the simulation results. Generally, the forward modeling technology can effectively predict the distribution of restraining and releasing zones, and it can further provide fundamental guidance to the exploration well site deployment.
Presentation Date: Tuesday, October 18, 2016
Start Time: 1:50:00 PM
Presentation Type: ORAL
Drilling through shale formation can be challenging and sometimes results in wellbore instability problems due to the reaction between hydrophilic shale and drilling fluids. The typical low permeability of shale, the presence of ions and charged surface of the constituent clay are factors which makes the problem of wellbore instability very complex despite efforts dedicated to the study by researchers. The study of wellbore stability in shale is quite important because 75% of all formation drilled worldwide are shale formations and 90% of all wellbore instability problems occur in shale formations costing the industry more than $1 billion USD/year (Chenevert, 2002; Zeynali, 2012); the lost time due to this challenge also account for over 40% of all drilling related non-productive time (Zhang et al, 2009) and these instabilities are responsible for 10-20% of the total drilling cost. A solution through this challenge is critical to the sustenance of the investment made by operating companies in the oil and gas industry. This will drastically reduce drilling cost, completion and workover cost as well as the accompanying downtime involved. It will also improve the net present value of operating company in the industry. Basically wellbore instability occurs when the mechanical stress induced by drilling into the formation exceeds the formation rock strength. Chemical interactions between the drilling mud and the in-situ shale affect the in-situ stress state of the formation hence the stability of the formation. Geo-mechanical models have been designed to tackle mechanical wellbore instability in the Niger Delta the challenge therefore is a chemical solution to the wellbore instability problems of the region. Oil-based muds have been known to overcome wellbore stability problems, but disposal challenges and environmental concerns have led to infrequent use. The challenge therefore is to formulate an environmentally friendly drilling mud having the inhibitive properties to tackle the wellbore instability challenges. This paper presents a review of studies carried out to characterize the mineralogy of shales and the salinity distribution of formation water in the Niger Delta depobelts with a view of designing a "balanced-activity" drilling fluid to help stabilize the formation during drilling. Results obtained from the reviewed researches showed that shale mineralogy characterization and formation water salinity distribution is critical in designing a balanced-activity drilling mud that can effectively tackle the problems of wellbore instability.
A stacked series of fault juxtaposed reservoirs across four fault blocks in an offshore field were being considered for further development, but communication across fault blocks remained an uncertainty even after 40 years of production from these reservoirs. Providing a reliable forecast associated with a development plan required understanding of the impact of these faults to the reservoirs' historical production and how it may impact future development opportunities. A full field simulation model containing all the reservoirs in the field, which can capture the impact of the communication across the fault blocks was built but due to the large number and coarse scale of grid blocks as well as mulyiple reservoirs, it was decided to use the results from the pressure-match work done on the full-field model and work with smaller, but finer scale, full-field simulation models with less number of reservoirs which were more likely to be communicating to produce a model which could answer these questions and provide a reliable forecast associated with the development opportunities. This paper describes a full field, three-dimensional, three-phase, black-oil simulation model of several reservoirs in Bayelsa field (not real name) which was used to investigate across fault block communication and identify further development opportunities in these reservoirs.
Lirong, Dou (CNPC International Limited) | Shrivastava, Chandramani (Schlumberger) | Chuanshu, Dai (CNPC International Limited) | Jingchun, Wang (CNPC International Limited) | Hammond, Nidra (Schlumberger) | Anoliefo, Chioma (Schlumberger) | Siddick, Mahamad (CNPC International Limited)
The complex interplay of different generations of fractures and the varying stress regime within the Bongor basin of Chad has posed serious problems in understanding the impact of these fractures on the producibility from the granitic reservoirs. These reservoirs correspond to Type I fractured reservoirs in which there is little or no primary porosity, and the fractures provide the essential secondary porosity and permeability.
The producibility of granitic reservoirs is strongly related to a few specific lithofacies where secondary porosity develops. The basement granites encountered in the wells have unique facies assemblages with differing characteristics controlling the reservoir properties. Three major facies; Unweathered Granite, Fractured (normal and intensely fractured sub-facies) and Leached Granites were delineated based on their geological and petrophysical characteristics, the Leached Granites being the best reservoir facies accompanied by fault breccia.
Knowledge of the orientation and variation in stresses within a basin is important for fractured reservoir characterization, stimulation and further exploitation. Two main generations (WNW-ESE and WSW-ENE) of fractures striking perpendicular to each other were identified. The WSW-ENE fracture set has the same orientation as the identified maximum horizontal stress which also corresponds to the strike of the regional Central Africa Shear Zone. This fracture set forms an integral part of the fracture network that give strong indications for future production from the basement. Slight rotations in in-situ stresses towards the south-eastern part of the basin may be indicative of possible reactivation of fault lines in response to migration of the Benue trough.
The placement of subsequent wells in the leached and fractured reservoir facies with preferred fracture orientation depends on several determinants, especially in light of the rotating stresses identified in the study; coming from an interplay of tectonic forces from the Central African Shear Zone arising from the West and Central African Rift system. This is particularly important for future production plans such as a stimulation treatment through hydraulic fracturing where specially engineered fluids are pumped at high pressure and rate into the reservoir interval to be treated, causing vertical fractures to open.
This study not only provides a methodology to characterize such complex reservoirs, but also suggests ways for optimal exploitation.
With a landmass of about 5.1 million square kilometers and a community of 15 member states, Western Africa is one of the worlds most endowed regions with energy sources. The region holds about 13% of the total proven gas reserves in the world, with Nigeria accounting for almost all that figure with 185tcf.
While natural gas is fast becoming the worlds preferred choice of energy thus seeing an increase in its demand, the region is yet to fully harness the potential she possess in energy generation. West Africa still ranks amongst the regions with the lowest energy generation in the world. Major sources of energy are petroleum, natural gas and hydroelectricity and it is estimated that no more than 20 percent and in some countries as little as 5 percent of the populace have direct access to electricity. The figure falls to about 2 percent in the rural areas. This situation has become a major hindrance to the economic and technological growth of the region.
Thus, energy has become a major driver for regional integration in West Africa, which can assist many of her individual economies in overcoming the mismatch between energy demand and energy resource endowments. Furthermore, this can take advantage of economies of scale and help countries develop and gain access to low-cost energy to sustain future growth. Resource rich smaller economies would then export energy which can become engines of growth and development while larger economies energy imports may help postpone, reduce, or avoid capital investments in new production facilities and thereby overcome temporal cash flow problems.
Robust re-development and management of brown field depends on having an integrated reservoir model that can reliably replicate performance history, and predict future performance in form of acceptable and consistent production forecasts that can be validated with other methods. This often involves integration of all geosciences and petroleum engineering data in a relatively fine simulation model which, although captures the heterogeneities in the reservoir to some more detailed level, usually takes up so much CPU space and long model’s runtime. Typically, a shorter runtime can be achieved by building a coarser static model, and/or upscaling finer static model that will lead to the loss of some of the reservoir’s pecularities. Such coarser models are normally impacted by many shortcomings, most important of which is numerical dispersion that result in spurious gas cusping or water encroachment/coning issues. These problems are more critical in reservoirs with high dipping structure or in model with some random skewed or dipping cells. This paper demonstrates a robust methodology of managing these issues of long run time and not compromising on quality of result.
This case study was on a 67 ft oil-rim reservoir (20km X 8km) with huge overlying gas cap and long production history in the Niger Delta. The wells in this reservoir are characterized by some water coning and gas cusping problems.
In order to achieve shorter models runtimes while still preserving key reservoir details, a relatively coarse static model was constructed based on consistent geological concepts incorporating all relevant geosciences and Petroleum Engineering data. Preliminary attempts made towards using the coarse model to achieve acceptable history match failed, largely due to numerical dispersion issues arising from high dipping model cells around areas with slight increase in structural dip. The model was subsequently updated by “refinement” of the high dipping cells at the free water level interface only. The updated model, whilst still fast, achieved acceptable history match within the defined uncertainty envelop, which could only have been achieved by a very fine model that will take order of magnitude time to run.
Based on the model derived using this approach, more than 12 hours of model runtime was saved, the existing well behaviours were better replicated and few new drilling opportunities were identified. The use of this robust approach results in a better understanding of the subsurface and delivers new opportunities which will ultimately enable more optimal (faster decision making) management/development of the field.
Pan, Xiao-Hua (Research Inst. of Petroleum Exploration and Development, PetroChina) | Yuan, Sheng-Qiang (Research Inst. of Petroleum Exploration and Development, PetroChina) | Ji, Zhi-Feng (Research Inst. of Petroleum Exploration and Development, PetroChina) | Hu, Guang-Cheng (Research Inst. of Petroleum Exploration and Development, PetroChina) | Liu, Li (China Petroleum Engineering & Construction Corp, PetroChina)
With the exploration activities of China National Petroleum Corporation (CNPC) in Sudan, Chad and Niger, more and more 2D, 3D seismic data had been required, a series of exploration and appraisal wells had been drilled, and more of other geologic materials had been obtained, such as well logs, core, geothermal gradient data, geochemistry data etc. Based on these abundant data and integrated application of geophysics method, physical modeling, computer modeling and geological analysis etc., the authors carried out an in-depth study on the Western-Central African Rift Basins (WCARBs), coming up with an understanding that the Basins are of passive rifts different from the so-called active ones formed in result of mantle up-welling. The WCARBs present unique petroleum characteristics including low geothermal gradient, aggregate thin-bedded hydrocarbon source rocks in syn-rift sequence, small-scaled post-rift sequence and high-angle basin-controlling major faults etc., and as a result, exhibit petroleum systems with unique characteristics, such as late and long-lasting oil generating window, high oil-expulsion efficiency, and fault-block dominated traps with rare rollovers etc. The deep study and analysis of the basin structure, sedimentary features and petroleum system obviously enhance the exploration discovery of WCARBs, which is the key factor of the successful E&P activities of CNPC in WCARBs.