There are a lot of reports of formation induced damage of wells world-wide. Despite extensive literature on the subject, formation induced damage is not a standardized part of well design. One reason may be that the associated fundamental mechanism is not yet fully understood, which makes it difficult to implement in design rules. As a step towards practical design, this paper aims at improving the understanding of characteristic mechanisms of well formation interaction by analytical solutions to two simple cases. The first case considered is a vertical well in a compacting reservoir and is solved by elasticity theory. An elastic length parameter is derived, which is function of the axial stiffness of well and shear stiffness of formation. The well is then shown to follow the deformation of the compacting reservoir, with exception of a transient zone around the boundary to the overburden. The elastic length determines the size of this transient zone. Through the transient zone, the axial force reduces towards zero in the overburden. A learning is that in many cases it is sufficient to instrument the well casing or liner to measure reservoir compaction. The result also supports the finding that the high number of well damage in the deep overburden is due to another mechanism: shear deformation or slip of a weak plane crossing the well. This second case is also studied analytically yet based on plasticity theory. Input parameters to this model are shear and moment capacity of the well, shear strength of the formation and a load displacement characteristic of the formation. A general finding is that during such slip, the well is normally not able to resist, and it fails by exceeding the moment capacity at a distance from the shear plane.
The final and third case studied is ovalization of the cross section of a horizontal well due to pressure from the formation. This is a phenomenon occurring in salt and weak shale. It is a more complex interaction problem and a numerical simulation by finite element is used to solve it. A workflow is developed for an uncemented part of a horizontal well in a shale formation. Input parameters are in-situ stress, pore pressure and stiffness and strength of well and formation. Since the vertical stress is larger than the horizontal, the shear mobilization is largest to the side of the casing and shear failure starts there, initiating plastic deformation until contact and start of ovalization by reducing the lateral diameter of the well. By reduction of the mud pressure in the outer annulus, the contact area grows. Finally, the structural capacity of an ovalized casing with full formation contact is calculated. The formation is found to have some supporting effect and the resulting capacity is higher than the capacity of an ovalized casing without formation support.
The Bahrain Oil Field was the first oil discovery in the Gulf Region in 1932 and is now in a mature stage of development. Crestal gas injection in the oil bearing, under saturated, layered and heavily faulted carbonate Mauddud reservoir has continued to be the dominant drive mechanism since 1938. Thirty eight 40 acre 5-spot waterflood patterns were implemented from 2011 to 2012. These patterns were located in both the South East and North West part of the Mauddud reservoir with a maximum injection rate of 80,000 bbl/day. With less than 10% PV water injected as of December 2012, premature water breakthrough was observed in most of the producers. Rapid water breakthrough in Mauddud A (Ba) is attributed to presence of high permeability vugs and layers resulting in water cycling and poor sweep in the matrix leaving bypassed oil. Following recommendations from the 2013 partner Peer Assist, the South East and North West waterfloods have been converted from pattern to peripheral with downdip wells providing water injection. Peripheral re-alignment has arrested the production decline, reduced water cut and stabilized production.
Surveillance data such as bottomhole pressure data, production logs, reservoir saturation logs, temperature logs and tracer data form the basis of understanding waterflood performance. Additionally, an array of analytical tools were used for diagnosis and analysis. Amongst the diagnostic tools, the Y- function helped to understand water cycling and sweep; the modified-Hall plot assisted in understanding the high-permeability channel or lack thereof and the water-oil-ratio (WOR) provided the clue on fluid displacement. Additional plots such as the "X" plot, decline curve, Cobb plot, pore volume injected vs. recovery, Jordan plot, and Stagg's plot were generated to gain insight on the waterflood.
Based on the waterflood analysis, a field study was initiated in December 2016 by shutting more than 80% of water injection followed by complete shut-in in September 2017. The purpose was to reduce the water cut, improve production taking advantage of gravity drainage effect of gas injectors located up dip of waterflood areas. The implementation of water injection shut-in is still ongoing in the Bahrain Field and pressure/production performance is being closely monitored. Improved production performance is observed following water injection shut-in.
This study underscores the importance of modern analytical tools to diagnose and analyze waterflood performance. This understanding also paves the way for much improved learning to take appropriate actions and help devise long-term reservoir management strategy.
Recently, the miscible CO2-EOR tertiary process used in the main pay zone (MP) of suitable reservoirs has broadened to include exploitation of the underlying residual oil zone (ROZ) where a significant amount of oil may remain. The objective of this study is to identify the ROZ and to assess the remaining oil in a brownfield ROZ by using core data and conventional well logs with probabilistic and predictive methods.
Core and log data from three wells located in the East Seminole Field in Gaines County, Texas, were used to identify the MP and ROZ in the San Andres Limestone, and to predict oil saturations. The core measurements were used to calculate probabilistic in-situ oil saturations within the MP and the ROZ as a function of depth. Well logs, in combination with core data and calculated saturations, on the other hand, were used to develop two expert systems using artificial neural networks (ANN); one to identify the ROZ and MP, and the other to predict oil saturation. These systems were also supported by a classification and regression tree (CART) analysis to delineate the rules that lead to classifications of zones.
Results showed that expert systems developed and calibrated by combining core and well log data can identify MP and ROZ with a success score of more than 90%. Saturations within these zones can be predicted with a correlation coefficient of around 0.6 for testing and 0.8 for training data. The analyses showed that neutron porosity and density well log readings are the most influential ones to identify zones in this field and to predict oil saturations in the MP and ROZ. To explain the relationships of input data with the results, a rule-based system was also applied, which revealed the underlying petrophysical differences between MP and ROZ.
This new predictive approach using machine learning techniques, could potentially address the challenges that previous studies have come up against in defining the ROZ within the formation and quantifying remaining oil saturations. The method can potentially be applied to additional fields and help reliably identify the ROZ and estimate saturations for future resource evaluations.
In this work we present the evaluation of field scale commercial miscible EOR for a giant offshore carbonate reservoir. The current assessment leverages the prior multi-year evaluation assessing the attractiveness (‘do-ability’) and prize for miscible EOR within each geologic area and reservoir within the field.
Two geologic areas, the "heterogeneous" and "homogeneous" geologic regions of the Crestal area in the largest reservoir were identified as the primary targets for field scale miscible EOR based on the prior multi-year assessment and are the focus of the current assessment. Commercial field-scale miscible development is predicated on a 1-to-1 line drive waterflood with 1km spaced extended reach horizontals with 10,000 ft completions and 500m spaced infills in the more heterogeneous area of the field.
Given the offshore environment, base development utilizes four artificial islands which support facilities for field development and serve as drilling centers. Commercial development with miscible EOR will be implemented in a ‘phased’ (staged) approach where the initial developments will be from the Central Island in the late 2030s followed in succession by the South Island in the early 2040s and North Island in the 2050s.
This assessment uses a novel phased approach where a ‘Tapered WAG’ scheme is used. By utilizing a phased (‘staged’) approach, multiple development concepts can be evaluated expediently. In this work we (i) show an efficient field assessment process for miscible EOR, (ii) illustrate how subsequent patterns can be brought on-line over time as part of a Tapered WAG process and (iii) demonstrate the importance of commercial assessment as part of field development planning and pilot assessment.
The full field assessments included here allows multiple development strategies to be assessed expediently and facilitates the selection of an injection solvent (hydrocarbon gas or CO2). Full field economics have been completed and hydrocarbon gas has been identified as the target solvent. Hydrocarbon gas has the benefit of being commercially attractive and is compatible with existing facilities. Currently two pilots are in preparation to assess the incremental recovery beyond waterflood and assess the commerciality of miscible EOR in a giant offshore carbonate reservoir. Key learnings in this paper include (i) an efficient methodology to assess phased field development for miscible EOR, (ii) incremental recovery of miscible EOR beyond waterflood, (iii) metallurgy and facility requirements for the two solvent schemes and (iv) a preliminary assessment of field scale commerciality including economics for the two solvents, hydrocarbon gas and CO2.
Steele, Edward (Met Office, FitzRoy Road, Exeter, Devon, EX1 3PB, United Kingdom) | Neal, Robert (Met Office, FitzRoy Road, Exeter, Devon, EX1 3PB, United Kingdom) | Bunney, Christopher (Met Office, FitzRoy Road, Exeter, Devon, EX1 3PB, United Kingdom) | Evans, Benjamin (Met Office, FitzRoy Road, Exeter, Devon, EX1 3PB, United Kingdom) | Fournier, Nicolas (Met Office, FitzRoy Road, Exeter, Devon, EX1 3PB, United Kingdom) | Gill, Philip (Met Office, FitzRoy Road, Exeter, Devon, EX1 3PB, United Kingdom) | Mylne, Kenneth (Met Office, FitzRoy Road, Exeter, Devon, EX1 3PB, United Kingdom) | Saulter, Andrew (Met Office, FitzRoy Road, Exeter, Devon, EX1 3PB, United Kingdom)
AbstractMarine forecasts are essential to operational planning, with decisions able to be guided by a host of different weather products spanning a period of days, weeks and even months ahead. The correct selection and subsequent application of these different types of weather products has the potential to save many thousands of dollars per day in operational downtime, however this is only possible when the science underpinning these marine forecasts is properly understood by the user. In the current economic context, this is especially relevant to the offshore industry – whose use of forecasting technology is traditionally very conservative, and therefore whose planning is often more reactive – allowing large savings (e.g. mobilization / demobilisation costs) if robust decisions are made as early as possible. Two established methods for the interpretation of probabilistic data based on cost-loss and weather regime analysis are described and applied to ocean wave forecasting. It is suggested the selection of methods will be dependant on the timescales of interest, with the cost-loss analysis optimised for supporting decisions at timescales on days to weeks ahead and the weather regime analysis optimized for supporting decisions at timescales of weeks to months ahead. The application of these methods are illustrated from the point of view of a North Sea asset manager planning the mobilization of equipment / personnel under conditions of calm weather, and the protection of equipment / personnel under conditions of severe weather. For such a user, efficient operational planning will be best supported by the use of marine forecasts across all such timescales, from days to months ahead. It is intended that this will enable more informed decision-making, and help reduce operational costs, by promoting increased confidence in longer-range forecasts than are typically used by the offshore oil & gas and marine renewable energy sector.
The addition of discontinuity data into geomechanical numerical models can allow for a better understanding of the behavior of the overall rock mass. For this study, LIDAR data has been collected at the Kartchner Caverns site in Cochise County, Arizona. The discontinuity data collected at this site can be divided into two types. The first type is the overall trends of small to medium-sized discontinuities such as short joints and small fracture faces that appear consistently in the LIDAR point clouds. When added to the numerical model, these fractures can be applied throughout the entire model as joint sets. This is done by inputting statistical information representative of all of the discontinuities in the set such as the mean discontinuity plane orientation, a measure of orientation scatter such as the Fisher constant, and statistical information about joint spacing and joint roughness. The second type of discontinuity information is data collected on a single specific discontinuity such as a major fault or a persistent bedding plane. Because the interaction of these large scale features with one another and with the overall geometry of the cave openings may be relevant to the overall stability of the system, it may be advantageous to model these discontinuities individually, and input them into the numerical model in their actual location.
Discontinuities commonly occur in rock masses. In order to ensure proper rock mass classification, safety, and long-term stability in many modern geomechanics projects, the acquisition of geotechnical information about the discontinuities is required. Geometric information about rock mass discontinuities such as dip, dip direction, location, and information related to the persistence or extent size of the discontinuities are important indices to evaluate. Finding a comprehensive, rapid and accurate method for accessing the geometrical and location information is a foundation of rock mass stability assessment. This information can be and has been traditionally measured using a manual compass and a tape measure. However, the rock masses of interest are often located in difficult to access places, and due to poor geological conditions, surveying and mapping may be difficult and dangerous. The traditional methods for engineering geological surveying and mapping work are a time-consuming, labor-intensive, costly, and highly difficult problem. To solve these problems, engineers both in the US and abroad have utilized new technologies, such as 2D digital photo interpretation, 3D digital photogrammetry, and terrestrial laser scanning (TLS). The use of 3D laser scanning technologies provides an effective, practical and advanced technical means, to solve the problems mentioned above.
Vogel, M. (Shell Global Solutions US Inc) | Hanson, J. (WaveForce Technologies LLC) | Fan, S. (Shell Global Solutions US Inc) | Forristall, G.Z. (Forristall Ocean Engineering Inc) | Li, Y. (Shell International Exploration and Production Inc) | Fratantonio, R. (RPS ASA) | Jonathan, P. (Shell Projects & Technology)
Estimation of environmental and complex structural responses, such as fatigue for risers on deepwater floating production systems, is a critical and generally computationally intensive process. Long term damage estimates require the determination of host vessel motions used for riser stress calculations. In principle, riser stress could be calculated for each of a large number of directional sea states, a considerable computational burden. However, it might be possible to identify a representative subset of directional sea states for vessel motion and subsequent riser stress analysis, such that estimated fatigue characteristics (from the full set of sea states and the subset thereof) were equivalent. This would be advantageous as it would require considerably less computational effort.
In this work we use non hierarchical K-MEANS cluster analysis to partition a large set of directional wave spectra for contiguous sea states at a location offshore Brazil, corresponding to a period of approximately 2 years into a number of clusters. We adopt the set comprised of cluster centroids only as representative sea states for efficient characterization of the environment and structural response.
We demonstrate that the representative sea states provide an efficient basis for estimation of overall sea state bulk, wind sea and swell characteristics. We evaluate the effect of cluster size on the performance of the representative sea states using custom built visualization tools utilizing the Kolmogorov-Smirnov test statistics. The representative sea states are further used as input for a VLCC-class FPSO vessel motion analysis. For heave at the turret, roll motions, and relative vessel heading, distributions of vessel motions from analysis of representative sea states are in excellent agreement with those from analysis of all sea states. Guidelines for the application of the methodology are provided.
The relentless exploitation of hydrocarbon resources to meet ever increasing demand has compelled upstream companies to focus on deep and ultra-deepwater. Flow assurance is one of the most critical aspects in field development. The hydrocarbon product needs to be transported from a remote well to the process facility, without experiencing significant energy losses to the environment. Some of the oil discoveries in Indian deepwater are having high wax content; high Wax Appearance Temperature (WAT) and pour point flow assurance is a major challenge in low ambient temperatures.
A number of solutions like chemical injection (CI), mechanical insulation (Vacuum Insulated Tubing (VIT), Pipe-in-Pipe (PIP) etc.), Pigging, Piggy back line (PBL) and Electrical heating of flowline (EHFL) are the available means of mitigation to address these flow assurance challenges. In addition to high ‘steady state’ thermal insulation performance, the system has to provide good transient cool down behavior to prevent wax or hydrate formation during shut down and to minimize the time required for start-up. Through this paper an attempt has been made to generate a matrix for identifying the likely option for the means of mitigation individually or in combination, both for steady state and transient scenarios.
The various options have been studied for suitability for flowline under different flow scenarios like steady state, shutdown, restart, and turndown and for redundancy. Emerging options are further technically analyzed qualitatively, based on the extent of field use, robustness, tangible & intangible benefits etc. and evaluated to arrive at a suitable flow assurance strategy. For field X located in 600–700 meters of water depth, the combination of PIP and CI with EHFL as a fall back is emerging as the most suitable option. During initial start-up, the flowline can be preheated using electrical heating and the sample taken for identification of suitable inhibitors and for arriving at the required dosage. During shutdown and restart scenarios electrical heating of flowlines can be adopted to prevent the fluid from gelling.
The solution matrix presented in this paper will help in evaluating the available options and to arrive at the best suitable flow assurance strategy and means of mitigation, to any field during all operating scenarios.
Notwithstanding the fact that commercial oil discovery has been fifty-plus years in Nigeria; the Nigerian natural gas industry is known to be a developing industry. Thus, understanding the huge potential this industry presents to its economy, the Nigerian government has sought to implement policies that will bring about development in this industry. The paper seeks to examine the policies and the legal framework (if any), in order to ascertain to what extent the policies have been able to develop the industry and if not, why? In addition, the paper examines the adequacy or otherwise of the present legal framework. The paper finds that the successful and sustainable development of the natural gas industry hinges on the existence of adequate legal framework, as policies can only serve as aspirations and sought-after-goals without the backing of sufficient legal framework to support their implementation. The paper finds that in light of shale gas development in Nigeria's investors cum market geography, time is fast running for Nigeria as there is the urgent need to address the situation as Nigeria is gradually losing not just investors but her natural gas market as well.
In this paper we evaluate the use of an automatic moment tensor inversion algorithm on passive seismic data from the Eagle Ford for its usefulness in evaluating the microseismic source mechanisms as well as potential applications for real time processing by comparing it to a hand-picked source mechanism inversion method. Hand picking involves manually picking first arrival p-wave amplitudes for a subset of events and utilizing a grid search for a pure shear or DC (double couple) strike, dip, and rake source mechanism solution (Aki and Richards, 1980). A least squares inversion (Sipkin, 1982) is utilized for the full moment tensor solution. This provides a general fit solution that is representative of all events. The solution fit is verified by applying polarity reversals to correct for the radiation pattern at the surface such that the first p-wave arrival becomes consistent. This is repeated until enough general solutions exist to properly correct move-outs for all events, classifying them into discrete mechanism groups. Automatic picking utilizes a form of full waveform inversion to calculate moment tensors for every microseismic event individually.
The dataset analyzed in this paper exhibited two distinct source mechanism solutions from the hand picking process, one dip-slip style class of events and a strike-slip class extending away from the well as a long coherent trend. This trend was dominated by a strike slip shear mechanism solution causing it to stand out against the rest of the treatment and was interpreted as a sub-seismic fault. Clustering analysis can be used as an early alert system to show the emergence of such a fault or other geo-hazard during hydraulic fracturing operations.
Automatic calculation of moment tensors offers a distinct advantage over the hand picking method in that it enables fast and efficient evaluation of numerous microseismic events and their possible source mechanisms by computer with little need for analyst intervention. While hand picking source mechanism solutions results in a small number of discrete solutions fitted to the entire point set, automatic picking offers a full moment solution for all events. This facilitates a more detailed and accurate picture of stress and resultant fracture network both spatially and temporally. It also offers a much quicker response to large changes in source mechanism in real time allowing the processing to adapt to these changes as they happen. This eliminates the need for an analyst to pull data, pick a new source mechanism, apply it to the processing, and delaying results delivery by requiring reprocessing of data. Such detailed moment tensor analysis makes it possible to quickly build more accurate discrete fracture network models, meaning that engineers can begin to plug microseismic data into their modeling in near real time in a more deterministic way. The most direct application of this type of analysis is real time stress evolution analysis. Being able to determine temporal changes in the stress fields offers a look into how the treatment is changing the stress field as its happening.