Drilling in Gulf of Suez is difficult due to wellbore instability, lost circulation and time dependent shale stabilization related problems. Drilling troubles in this region generally originates from high earth stresses and abundant natural fractures and faults associated with this tectonically active region. Presence of salts and evaporates with depleting reservoir pressure of a mature field aggravate the problem. Operators in this region always experienced drilling difficulties which sometime leads to well abandonment and costs over-runs in millions of dollars. Planning for the present well was the first attempt to drill a horizontal section in the porous and oil bearing Asl sand. Therefore, the main objective of the current study was to assess the risk of wellbore instability which may occur while drilling and recommend the remedial action plan for any risk encountered.Wellbore stability analysis has a major impact on the well design and planning the orientation of trajectory for safe and stable well and successful drilling operation. Traditional drilling practices based on pore-pressure of the reservoir and fracture gradient does not necessarily proved successful especially drilling horizontal wells. The failure criterion works in a completely different way for a horizontal well compared to a vertical well in the vicinity. Therefore, a safe vertical offset well inevitably never assures that the identical drilling design and practice will safely drill a horizontal well. The stress distribution and direction works differently for a vertical and a deviated or horizontal well. A rule of thumb is that the drilling gets more and more difficult with decreasing width of safe and stable mud window as the well becomes deviated and the situation worsen as the well turns horizontal. Adding to this complexity the direction of the trajectory with respect to in situ stress distribution and variation poses a major role in drilling a safe horizontal well.The case history presents a geomechanical risk analysis for a planned horizontal dual lateral well. The study is based on stress regime and well failure with the significance of choosing proper mud weight and drilling parameters using a proper mechanical earth model from a nearby offset well. It includes an assessment of the major risks expected during drilling the horizontal section and also indicates magnitude of failure that can happen while drilling, based on the trajectory sensitivity analysis. The planned well was drilled without any wellbore stability related problem. The present study suggests the importance and benefits of a proper well stability study while drilling a horizontal well in a tectonically disturbed area.
Borehole instability, in most of the cases, is a direct reflection of earth's in situ stress state. It is well known that the stress distribution around the wellbore induces deformation depending on many factors ranging from wellbore pressure history and rock strength to the trajectory orientation. A stress direction map is generated for the GoS from observations of borehole breakout detected in multi-arm-caliper logs and other log data base, viz., electrical Images and sonic logs. In vertical wells, the maximum tangential stress around borehole can produce breakouts and their orientation indicates the direction of minimum in situ horizontal stress (Sh). In the case of deviated wells, a stress-tensor diagram defines Sh direction with reasonable accuracy, provided wells cover wide range of deviation angle and azimuth The current study indicates that Sh in GoS is aligned along two major trends. The main NNE - SSW trend, with average orientation of N10degE, exists in most of the region. The second trend is aligned NE - SW and observed locally at the central eastern and south-western part of GoS, with an average orientation of N50degE. Most studies of the structural and tectonic history of the GoS have identified two age significant orientations for this extensional rift. The early to middle Miocene rifting, responded to a Sh direction of N55–60degE (rift-climax). The younger stress fields of the Late Miocene and Pliocene times rotated progressively counterclockwise to a N15-25degE direction that persisted into early-late Pleistocene time. The dominant in situ stress orientation trend, identified in this study, therefore, is mainly controlled by this younger stress field of the GoS rifting. In situ stress directions have strong impact in drilling high angle wells in GoS. Proper placement of well trajectory with respect to in situ stress reduces instability in drilling. The paper exhibits example of directional sensitivity of well trajectory and successful drilling campaign based on the developed stress map. Introduction Since the global power scenario changes with increasing demand for oil, more and more complex trajectory wells, highly deviated and horizontal, are being drilled in the areas with minimum knowledge with scanty or barely minimum previously drilled data. Enhancing the production to its maximum level is the reason, though the drilling uncertainty is being pushed to a limit that causes unexpected drilling problems resulting in high NPT; the expenditure goes beyond expectation involving multiple side tracks or abandonment of the well in worst case. In all the brown fields those are now being developed; the original data quality is very poor or inadequate to extract meaningful results that could be used to formulate a drilling program, one of this being the stress orientation of the area. This is a significant and most important input to forecast the drillability of the well. And in most of the cases the wells are being drilled without the proper knowledge of stress pattern of the area. The demand of an in-situ stress map, therefore, is extremely important while drilling a deviated high angle well. The situation becomes exceedingly critical if the drilling is being carried out in a tectonically active region involving multiple faults and variable degree of displacement of the adjoining structures. While mentioning a stress map we are mainly concerned about the directionality of the minimum stress across the region.
Improvement of oil recovery and reduction in water-cut in a matured field requires precise time lapse saturation monitoring. Behind casing resistivity, an important member of the comprehensive analysis behind casing services suite, provides the required answer by acquiring deep resistivity information through casing for subsequent formation evaluation. A time lapse saturation figure could be generated immediately after the acquisition which is extremely instrumental to take an immediate decision. The technology is well known in the industry and already proven beneficial in many occasions. In favorable conditions, this is the most effective methods available to date for saturation monitoring as being the deepest through casing measurement in terms of radial investigation. The case study presented in the paper describes a successful water shutoff operation and improved oil recovery from the Bahariya formation in the western desert, Egypt. The well was drilled in early 2006 followed by logging and testing and was put on production immediately after completion. During the production time of little more than one year, the oil rate decreased by 70% with 84% water-cut. This significant fall in oil output with sharp early water break through, a common phenomenon in this mature field required a proper shutoff operation. The present example discusses in detail the reason in choosing resistivity behind casing acquisition and time lapse formation evaluation to monitor present saturation profile. Comparison between original and present water saturation level immediately detects the depleted zones and the degree of depletion across perforated intervals indicates the interval contributes most of the produced water. In the present example the cased-hole resistivity acquisition shows departure (lowering) of resistivity values against some perforated intervals. The zones were isolated after identification of the intervals that showed considerably lower resistivity values. This is considered as an indication of rise in oil-water-contact or lengthening of transition zone. In the present case, the remedial measures for water shut-off operation based on time lapse evaluation have enhanced oil recovery from 540 Barrel to 2250 Barrel oil per day with significant reduction in water cut from 84% to 0.5%. The results were encouraging enough for the operator to reinvestigate the production performance and identify wells where this technique could be meaningfully utilized. The current analysis and workflow offers a fairly good understanding about the reservoir's production scenario, monitoring and future actions for enhanced recovery. Introduction The different techniques of reservoir monitoring and maintenance, enhanced/improved oil recovery, evaluation of bypassed oil, identification of fluid movement etc. depends primarily on the saturation estimation at different stages of the life of a reservoir. Time lapse monitoring of a reservoir is estimated at different wells in the field using incessantly developing and evolving through casing measurement techniques. The ability to detect hydrocarbon behind casing is therefore vital. Two parallel and complementary methodologies have been evolved over the years and numerous patents and articles have been published. Nuclear emission based techniques were the forerunners in this category. Thermal Decay time measurements (TDT) and Reservoir saturation estimation using Carbon-Oxygen ration have been used regularly since its inception. More recently the resistivity measurement through casing opened new vistas in estimating saturations under suitable conditions efficiently and accurately (Béguin et al., 2000, Bellman et al., 2003). The technology proves to be exceedingly effective and beneficial in taking quick decision to perform real-time operations, for example, successful water shutoff operation for the current scenario. Analysis behind casing (ABC) is the presently active methodology that encompasses various techniques used for comprehensive analysis behind casing for complete and elaborate formation evaluation. Cased hole formation resistivity (CHFR) is one of the major components of this category of logs that has been used successfully on many occasions wherever the technology is introduced in the world.
Accurate time-lapse saturation information is the key to making the right decisions on completion strategy, maximizing oil recovery and reducing water cut. This paper presents a case study from the Bahariya Formation, a heterogeneous fluvio-marine channel deposit in the Western Desert, Egypt. All the wells considered in this paper showed significant water production. To identify the main water-producing zones and the bypassed oil, all the wells were logged using a through-casing formation resistivity tool. One well was also surveyed with pulsed neutron capture logs. Based on the log results, depleted zones were identified, and the intervals contributing most to the water production were isolated. Water cut was significantly reduced. In some wells, the saturation analysis revealed that the stacked reservoir zones had variable levels of depletion and that the depletion was not necessarily related to the distance to the original oil-water contact. In these wells, the water shutoff leaves oil behind, and a different completion strategy was recommended. The results from the resistivity and nuclear measurements are discussed in detail with respect to environmental effects. This case study demonstrates that through-casing formation resistivity measurements provide more robust answers compared to neutron measurements in the studied environment. The deeper depth of investigation is extremely valuable, as the wells cannot be logged under dynamic conditions, and fluid reinvasion is always present. Moreover, in view of increasingly high rig rates and limited rig availability, the simple nature of the processing and interpretation of the through-casing formation resistivity log enables fast decisions. These examples from the Western Desert illustrate how analysis behind casing provides critical information to maximize oil production and facilitate water shutoff decisions. Introduction The field studied in this paper is located in the Western Desert of Egypt. It has been producing oil since 1992 from the Bahariya Formation, a heterogeneous fluvio-marine channel deposit. In 2006, the oil production started to decline with a sharp increase in water cut. Four wells were selected by the operator for water shutoff operations. In each of these, the water cut was more than 70%. In each well, through-casing resistivity was acquired. A pulsed neutron capture (PNC) tool was run in one of the wells. On the basis of this data, immediately after the resistivity run, a decision was made about which zones had the highest water saturation and needed to be isolated. This was done by setting a bridge plug. The same rig was used for the logging and the setting of the bridge plug. The water shutoff operations successfully increased the oil production. The purpose of this paper is to demonstrate the potential of the through-casing resistivity measurement compared to its nuclear counterpart, since its deep depth of investigation gives more immunity to reinvasion. The interpretation is fast and hence allows making an almost real-time decision on water shutoff operation so that it can take place immediately after logging. Saturation Measurements Behind Casing Three main types of data are used in saturation monitoring: through-casing resistivity, pulsed neutron capture, and neutron inelastic capture measurements. Aulia et al. (2001) provide a comparison of the applicability of each measurement in different environmental conditions.
As the global power scenario changes with increased demand for oil and gas, remote and challenging (deepwater offshore, high pressure-high temperature, high-angle wells) locations are drilled in an ever-demanding exploration effort with minimum or no experience in the area. To meet the demand and associated financial implications, operators are drilling high-angle wells that require special care from well stability, completion, and formation evaluation, to name a few. Complex reservoirs require complex trajectory and therefore proper well placement. To place a well accurately, the evaluation of acquired data requires real-time interpretation. In Egypt, Petrobel is following the trend and has been drilling deviated wells to enhance production in the Mediterranean by using a new logging while drilling (LWD) platform for formation evaluation. The data delivered by this service includes not only traditional measurements such as resistivity, density, neutron porosity, gamma ray, and caliper, but also induced spectroscopy and sigma. The sigma measurement is used for saturation evaluation independent of resistivity. The availability of two independent saturation estimations provides additional confidence. Spectroscopy provides formation mineralogy and accurate clay fraction, which enhances shaly sand interpretation. Formation evaluation in real time provides accurate estimates of hydrocarbon in place and allows for comprehensive decision making. The field is approximately 70 km from the coast of Alexandria, Egypt. The main reservoir is predominantly sandstone with gas and condensate. Highlighted in this study:Real time interpretation based on capture spectroscopy and sigma combined with the traditional measurements.Lithologic and mineralogic evaluation using capture gamma-ray spectroscopy measurements on LWD in real time.Independent gas saturation estimates from both resistivity and sigma measurements on LWD in real time.Real time formation evaluation requires minimum inputs from user, making it a powerful tool for operators in the decision making process. Introduction The new LWD platform is a substantial improvement over the previous versions of LWD tools that were much longer than the current version where one single collar 26 ft long tool combines both drilling and formation evaluation measurements (Figure 1). Measurements include resistivity, density, porosity, gamma ray, dual caliper and two new measurements to the LWD world which are elemental capture spectroscopy measurements that provides detailed lithology information in real time and formation capture cross section (sigma)1 providing an alternative to resistivity for fluid saturation calculation in saline formations in real time. The tool also can be run without chemical sources where the density measurement is derived from the interaction from neutrons with the formation which are generated from the pulsed neutron generator (PNG, also known as minitron) which is an electronic source of generating neutrons. The tool, therefore, could be run with no chemical sources or with only the cesium source. The AmBe chemical source for neutrons has been replaced by the electronic source PNG. The dual ultrasonic caliper 180 degrees apart permits borehole caliper measurement even when sliding and provides high resolution caliper measurement at 16 different azimuths while rotating.
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