bluuebunny/crossref_metadata_2025_split huggingface dataset:
Critical Evaluation of the Ensemble Kalman Filter for Reservoir Parameter Estimation under Incorrect and Uncertain Prior Models
Authors: Jafarpour B. , Tarrahi M. 
Published: September 2011

Abstract
The Ensemble Kalman filter (EnKF) has recently been proposed for history matching reservoir models. The performance of EnKF depends on the representativeness of the initial ensemble of reservoir properties. The initial ensemble is commonly constructed by assuming a known geological continuity model such as a variogram. However, since geologic continuity models are derived from incomplete data and imperfect modeling assumptions they are subject to uncertainty. Neglecting this important source of uncertainty can lead to systematic errors and questionable estimation results. In this paper, we evaluate the performance of the EnKF under varying levels of uncertainty in the variogram model parameters. We first attempt to directly estimate variogram parameters from flow data and show that the complex and non-unique relation they have with the flow data provides little sensitivity for an effective inversion with the EnKF. We then assess the performance of the EnKF for estimation of gridblock permeability values under uncertain and incorrect initial variogram parameters and show that any bias in specifying variogram parameters tends to persist throughout the EnKF analysis although locally reasonable permeability updates may be obtained near observation points. We consider the uncertainty in the variogram model parameters and account for the full range of structural variability in the initial permeability ensemble. We show that under this assumption the EnKF update results are quite promising. We conclude that, in applying EnKF to realistic problems where the level of prior uncertainty may not be known, it is preferable to overestimate the uncertainty in geologic continuity and initialize the EnKF with a wide range of variability in property description than to overlook the variogram uncertainty at the risk of introducing systematic bias that cannot be corrected by the EnKF updates.
Optimization of Completions in Unconventional Reservoirs for Ultimate Recovery – Case Studies
Authors: Snyder Daniel J. , Seale Rocky 
Published: September 2011

Abstract
Over the last decade, an industry wide shift to unconventional plays has occurred due to advances in technology allowing for the recovery of previously uneconomic reserves. The primary objective of completions in these unconventional reservoirs is to increase the effective surface area of the well to maximize reservoir contact. Horizontal drilling and multi-stage fracturing are two technologies which have accomplished this. The two main methods of horizontal, multi-stage completions currently used in unconventional reservoirs are: cemented liner "plug and perf" and open hole, multi-stage fracturing systems.
This paper provides an introduction to unconventional reservoirs, describes the main methods of horizontal, multistage completions, and discusses how the choice of method can affect good fracturing practices as well as long-term production. Case study examples are presented from a variety of unconventional reservoirs included including shale, tight sandstone and carbonate formations.
Operators working in a number of unconventional reservoirs, such as shales and other tight rock formations are experiencing faster than expected production decline rates, resulting in reduced long-term, ultimate recovery. This may be in part due to the abandonment of good fracturing practices, developed over the past 50 years, with the advent of horizontal, multi-stage fracturing. Issues such as near wellbore conductivity, flowback, and fracture tortuosity that can have a significant effect on the long-term production of wells need to be considered when choosing a completion method, particularly for unconventional reservoirs.
Unconventional plays are becoming a significant part of the oil industry today and will become a bigger part in the future. It is important that the reservoirs are completed optimally to make them as productive as they can be. The information provided in this paper is applicable to unconventional resource plays worldwide.
Use of High-Strength Coiled Tubing in High-Pressure/High-Temperature Perforating Operations
Authors: Makris K. , Barclay D. A. , VanArnam W. D. 
Published: September 2011

Abstract
A major operating company (operator) demonstrated interest in the development of high-strength (&gt;125 ksi yield) coiled tubing (HST-CT) for use in high-pressure/high-temperature (HP/HT) fields in the central North Sea. The operator's objective was to perforate underbalanced (in base oil) with coiled tubing (CT) using up to 1,700 ft of guns in one run. The combination of the low hydrostatic pressure of the perforation fluid and high bottomhole-pressure results in high wellhead pressures. Therefore, the CT had to be strong enough to avoid collapsing while pulling the weight of the bottomhole assembly (BHA) and the string inhole from the working depth of a live well. Because of this, the CT needed to be carefully designed to minimize its weight without compromising its strength. Computer simulations were run for string designs made from the currently available materials but none were found suitable for the operator's objective, so the operator pursued the development of HST-CT.
A major CT manufacturer working in conjunction with a material supplier developed the product and, allied with a service company, proved the product suited the needs defined by the operator.
It was decided to run the HST-CT in a newly drilled well in the same area but in a shallower reservoir with sub HP/HT conditions as a test bed to determine any potential issues with the use of HST-CT in reverse-deployment perforating operations. This job was performed with a shorter BHA and a lower wellhead pressure than true HP/HT conditions. Some of the operational testing normally conducted in Houston, Texas was performed in Aberdeen, Scotland because of job-time restrictions.
The test results, operational details of the CT job, and observations and conclusions derived from the use of the HST-CT developed are discussed in this paper.
This project involved the first offshore application and the first commercial application of HST-CT worldwide. The importance of this project is highlighted by the increased number of unconventional reservoirs being perforated, thus increasing the need and demand for stronger CT.
Two-Stage Cementation of A Multilateral Junction, After Floating In The Casing
Authors: Brooks Robert 
Published: September 2011

Abstract
This paper presents a practical solution for successfully floating in the casing and performing second stage cementing of a multilateral junction. The drilling and completion objectives required a successful cement job on the 9–5/8 in (244 mm) × 10–3/4 in (273 mm) casing that would: Isolate the reservoir at the 9–5/8 in (244 mm) shoe.Isolate the multilateral junction and prevent hydrocarbons from entering the upper annulus during production.Provide support for the 9–5/8 in (244 mm) casing during exit window milling operations.
Due to the distance between the 9–5/8 in (244 mm) shoe and the multilateral junction point it was identified early in the planning stages that the chances of success for a single stage cement job completing these objectives was minimal.
With this in mind it was decided to perform the 9–5/8 in (244 mm) cementing in 2 stages. A multilateral well with a TAML Level 4 junction requires a cement sheath that provides 360 degrees of annular isolation. Full cement coverage also contributes to constructing a high-strength TAML Level 4 junction. Most junctions are placed at a depth which is considerably shallower than the motherbore casing shoe depth. Achieving full cement coverage at the junction depth may not be possible over long intervals. This scenario may require a secondary cement program. This is to ensure competent cement around the multilateral junction for support during the milling phase and isolation from the annulus in the event that a 360° phased perforation had to be performed on the whipstock.
Due to the trajectory of the wellbore it was necessary to float the casing in-hole using a "casing flotation technique." Air was trapped inside the casing and the first stage cement job was performed as normal using a 2-plug operation. The second stage cement job was subsequently carried out through a locking Port Collar with Metal-to-Metal (MTM) seals.
This completion method was utilized in an Upper Captain Sand dual-lateral infill well in the North Sea. After the casing had been successfully run to depth and the first stage cement job completed, a Port Collar operating tool was deployed on drill pipe and the cementing Port Collar was opened allowing the second stage cement job to be carried out. After displacement of the cement the operating tool was picked up to close and lock the Port Collar. A successful pressure test across the Port Collar confirmed this. All objectives were met; a world first was achieved for this combined operation. Since this first well was completed several other wells have similarly been completed. As a further development to this, a number of operators have expressed the need to have the MTM cementing port collar qualified to meet the requirements of ISO/DIS13679.2, which defines various ‘connection application levels’ (CAL), with CAL III now being the most severe.
Mitigate Drilling Risks in Highly Depleted Reservoir Fields by Intelligently Planning Well Trajectory for a CO2 Sequestration Project
Authors: Fang Zhi 
Published: September 2011

Abstract
A highly depleted gas field in the southern North Sea has been selected as a potential CO2 storage site. Data from drilling of production wells shows that wells experienced substantial tight spots, overpulls, cavings, mud losses, even pack offs, and considerable time was spent in reaming tight holes and circulating excessive cuttings. To mitigate the drilling risks for new CO2 injection wellbores, a geomechanical study was undertaken to understand the drilling issues and to quantify a practical mud-weight window.
The study used offset well drilling and wireline log data to derive field stresses, formation pressure, rock strength and elastic properties. A practical workflow was developed to characterise the interaction between pressure depletion and fracture gradient changes. In this particular case the results showed that the fracture gradient was as low as 9.3 ppg, and the wellbore collapse pressure in the overburden shale was highly dependent on the well trajectory. A vertical well could be drilled safely with a hydrostatic mud weight of ~8.6 ppg. A wellbore deviating at more than 65º would require mud weight approaching 9.3 ppg, the fracture gradient, to prevent wellbore failure. This leads to a tight mud-weight window for drilling management. If an operation mud-weight window of 0.5 ppg is required, the well inclination should be below 65º if it is planned towards the minimum horizontal stress Shmin orientation, or less than 45º if towards the maximum horizontal stress Shmax azimuth to mitigate drilling risks.
This paper describes a geomechananical workflow that assists in understanding the interaction between pressure depletion and fracture gradient changes from which it is possible to quantify a mud-weight window for safe drilling. The workflow description shows how the approach can help mitigate drilling risks and minimise drilling costs by optimising well trajectories and therefore play a significant role in the well planning and drilling management of depleted reservoirs.
Continuous Energy Efficiency and Green House Gas Emission Surveillance and Control
Authors: Cramer Ron , Goh KC , Iyer Mahesh , Wali Nnamdi , Spence Bill , Kessler Guus , Kanten Roy 
Published: September 2011

Abstract
The purpose of this paper is to document Shell's experiences and learnings in the effort to better track and reduce Green House Gases (GHG) and improve Energy Efficiency in our downstream manufacturing and upstream production operations. The paper is based on Case Studies from various Operating Units in Shell upstream and downstream operations, as well as outlining further development plans.
In Shell operations we seek to minimize GHG emissions by continuously monitoring, displaying and reporting associated Key Performance Indicators (KPI's) and quickly alerting operators of changes to trigger remedial intervention. Reduction of GHG emission is also achieved by improving cross validated and mass balanced tracking of our process streams. This ensures that manufacturing and production processes are operated efficiently and transparently.
Continuous GHG monitoring also allows automatic compilation of emissions by source which can then be automatically reported as part of the normal daily reporting cycle. The resulting emissions figures and associated KPI's are then prominently displayed in the Daily Production Report. The daily emissions totals are also stored and trended to flag more subtle and/or gradual changes. In this way GHG emissions data and performance information are made available to operations staff and management to facilitate awareness and corrective actions when appropriate.
Sensitivity Analysis for Dual Porosity and Dual Permeability Systems to Small and Large Scale Heterogeneities in a Naturally Fractured Carbonate Reservoir
Authors: Correia Manuel G. , Schiozer Denis J. , Santos Marcos S. 
Published: September 2011

Abstract
The best way to analyze the sensitivity of dual porosity (DP) or dual permeability (DK) systems is his interaction with small and large scale petrophysical features using a fine grid, since in a coarser scale, small scale heterogeneities are always integrate in an upscaled background matrix. Two significant petrophysical features in carbonate reservoirs were applied over a background matrix, defined by, low permeability but with separate vugs (SV) randomly distributed with high porosity and permeability: (1) tectonic fractures (nearly vertical) and (2) touching vugs (nearly horizontal), were defined as large scale fractures, with high permeability and low porosity. These features are combined and tested in two cases. In the first case, the flow is controlled by touching vugs and tectonic fractures. In the second case, the flow is controlled only by touching vugs. The two cases were tested with and without SV in the background matrix. In both cases, oil recovery factor (ORF) is higher in presence of SV. ORF in DK system is also higher than in DP system. In Case 1A, the ORF for the DP system is nearly equal to the case 1B, showing that the flow rate between matrix blocks is highly important in the presence of SV, leading to the necessity of a DK system application. Without SV, ORF is nearly equal for DP and DK systems thus disregarding the necessity of a DK system, given the higher flow simulation time. The front of injected water in second case is nearly horizontal and homogeneous in fracture component resulting in a higher flow rate between SV to fracture network. The heterogeneous front of injected water in first case reduces the significance presence of SV. This work gives a valuable sensitivity of DP/DK systems in the study of small and large scale heterogeneities.
Upscaling Technique Applied to Naturally Fractured Carbonate Reservoirs
Authors: Correia Manuel G. , Maschio Celio , Schiozer Denis J. , dos Santos Marcos S. 
Published: September 2011

Abstract
The purpose of this study is to develop an upscaling technique, applied to naturally fractured carbonate reservoirs, adjusting fracture blocks components and well indices according to our small scale fracture behavior using mini-models. Two significant petrophysical features of carbonate reservoirs were applied over a background matrix, defined by low permeability but with separate vugs (SV) randomly distributed with high porosity and permeability: (1) tectonic fractures (nearly vertical) and (2) touching vugs (nearly horizontal), were defined as large scale fractures, with high permeability and low porosity. These features are combined and tested in three cases, with the same background matrix. In the first case, the flow is controlled by high permeability thin facies and tectonic fractures; in the second case, the flow is controlled by touching vugs and tectonic fractures and in the third case the flow is controlled only by touching vugs. A fine grid cell size of 0.8x0.8x0.4 meters is used as reference for the upscaling method. All fracture grid blocks in coarser cell size (3×3×2 meters) were adjusted in order to calibrate the fracture direction according to our fine grid fracture behavior, combined with a well index adjustment. In order to isolate the matrix heterogeneities effects, the same fracture calibration procedure is applied to a homogeneous matrix using the same fracture network in order to validate the fracture upscaling. It was possible to adjust all reservoir parameters (field average and well pressure, oil recovery factor, etc.) and reduce the flow simulation time from days to a few seconds. With this methodology we can transfer small scale heterogeneities from different sources to a coarser scale without losing information.
Application of the Ultrasonic Technique for Monitoring and Measuring the Phase Fractions of Liquid-Gas-Solid Mixtures
Authors: Goncalves J. L. , Paiva T. A. , Carvalho R. D. , Venturini O. J. 
Published: September 2011

Abstract
In many industrial applications, especially in the oil industry, the requirements of multiphase flow measurement pose numerous technological challenges as it oftentimes involve harsh media, strict safety regulations, access difficulties, long distances, and aggressive surroundings. The fluids produced from oil wells most often emerge as a multiphase mixture of oil, natural gas, water, and a variety of solids (sand, hydrates, and asphaltenes). It is generally recognized that multiphase flow metering (MFM), i.e. measuring the flow rates of the individual phases in a multiphase flow, could have many advantages in terms of costs, layout of production facilities, well testing, reservoir management, subsea and downhole metering. In this regard, the ultrasonic technique has been receiving increasing attention in the past years because it is noninvasive, fast responding, and suitable for operation in harsh environments. In the present paper, initially a review is made of selected commercially available separation-type and in-line MFM systems; the multiphase flow parameters measured in each system and the techniques used as well as general comments regarding the system performance are provided. Next, ultrasonic attenuation and transit time data are presented for oil-continuous oil-air-sand and oil-air-water mixtures in steel pipes. A discussion is also presented on attempts to sort out the air and sand concentrations in water-air-sand mixtures from ultrasonic signals. It is hoped that the data presented will help establish the potential of the ultrasonic technique for use in MFM systems in combination with or replacing specific instruments already in use.
Unique Axial Oscillation Tool Enhances Performance of Directional Tools in Extended Reach Applications
Authors: Al ali A.. , Barton S.. , Mohanna A.. 
Published: September 2011

AbstractThe effect of having an axial oscillation generator tool (AGT) in the drill string has been demonstrated to be the difference between the premature termination of drilling a section and successful completion of the section to planned TD. The AGT tool has traditionally been used in commercial applications where sliding with steerable motors has been problematic.Axial oscillation in the drill string is generated through a series of pressure pulses coming from one part of the AGT. The pressure pulses act on the pump open area of the shock tool, generating the axial motion required to reduce friction, improve the weight transfer to the bit, reduce reactive torque (and thus overall) downhole torque. The hydraulic vibration also enhances the performance through reducing friction. Combined together, the hydraulic vibration and the axial vibration have proven extremely effective in reducing friction and improving weight transfer with steerable motor assemblies. This had led to increase step-out capabilities, improved tool face control, reduced vibration, and higher penetration rate.This paper provides an overview of the tool technology and application with motors. However, it also focuses on expansion of the tool application with both conventional rotary and also rotary steerable BHA’s. Test results discussed include; A study on rotary drilling with AGT to evaluate and quantify improvement in weight transfer to the bit, revealing significant improvement in DWOB and reduced torque-at-bit.Overview of the compatibility of the AGT with rotary steerable system (RSS) and MWD tools, in particular the dynamic of the AGT which influences the weight transfer. Significant improvements in drilling performance are revealed when combining AGT and RSS.Case studies are included, displaying the breadth of benefits with the AGT with all forms of drive systems (rotary, motor, and RSS).
Heavy Duty Wireline Fishing Operations in a HP/HT Sour Gas Well: A Case Study
Authors: Purwagautama Gustioro , Afandi Idi Yusup , Rachman Syahriza Ghany , Vincent John M. 
Published: September 2011

Abstract
HP/HT gas wells, especially those in South Sumatra, Indonesia, present many challenges to exploitation operations due to a 408°F bottomhole temperature, a 10,500psi bottom hole pressure at some 11,000ft, and sour gas components of 34% CO2 and 100 – 200ppm H2S.
During the final stages of the completion of one such well, after setting the production packer a wireline run was made with .108″ slick line to recover the plug body from the seating nipple at 11.406ft. While pulling out and with the plug at 6000ft. the well became live without warning, blowing the tool string up the hole and, to contain the well, the drilling BOPs were closed shearing the landing joint which in turn closed the sub surface safety valve. Subsequently the well was successfully completed and prepared for wireline fishing operations.
The wireline fishing operation went on for several days using .108″ wire, during which 45 runs were made with the recovery of only approximately 550ft of wire and so it was decided to call in heavy duty wireline fishing (HDWF) expertise to review the situation and make recommendations as to how to proceed. This resulted in a commitment being made to mobilize personnel and equipment from Aberdeen and the Middle East to carry out the required operations. In the meantime a total of 11 further runs were made in the well using coiled tubing which resulted in recovery of the upper part of the tool string and some additional wire. Fishing operations were recommenced using 7/32″ Supa 75 braided cable with two two/man crews providing 24 hour operation and a project leader located at the clients office to provide for direct liaison. After a total of 17 runs over 6 days of continuous operation, the fish was completely recovered and the well made ready for production.
In this paper the authors will review the situation that was faced, the remedial work done to complete the well, the early fishing operations and the subsequent input of HDWF expertise. They will conclude with a detailed account of the successful HDWF operation – an excellent example of how careful planning, experienced personnel, dedicated project management and a collaborative relationship between the client and service company can lead to a successful outcome.