bluuebunny/crossref_metadata_2025 huggingface dataset:
A Simple, Robust Interpretation Method for Matrix Acidizing Treatments – Part 1: Theoretical Basis and Field Example
Authors: Chan Keng S. , Flamant Nicolas C. , Helou Husam N.
Published: December 2007

During matrix acidizing operations performed on oil wells, post-treatment analysis is often overlooked due to the lack of a simple, robust tool. Current methods rely on the equations used for well tests analysis, either steady state or transient, and focus on the computation of the skin factor. However this computed skin most often proves unreliable due to the complexity of the treatments, which routinely involve multi-layered reservoirs and advanced diversion techniques, i.e. phenomena that are not handled by the well testing equations. As a result, operators content themselves with providing clients with a plot of pump rate and wellhead pressure versus cumulative volume - or time. Unfortunately, any interpretation performed with this type of plot has more in common with divination than science.

In this paper, a new way to plot matrix acidizing treatment data is presented. The inverse injectivity index and its integral are plotted versus time. This is similar to Hall plots, which are used to monitor water injection wells, but it corrects for the rate changes occurring during acidizing treatments. The concavity of the integral then visually indicates whether the treatment is achieving stimulation or diversion, and this behavior can easily be identified despite the noise usually associated with any field measurements. As a side benefit, this technique also provides means to verify the average reservoir pressure value.

The theoretical basis of this new plot is discussed and its usefulness assessed using data generated by Pericles, a matrix acidizing numerical simulator. A field case is then analyzed and is used as an opportunity to once again emphasize the need for reliable bottom hole pressure data, whether measured or calculated, without which any analysis is inherently flawed.
Patterns of Variation of Permeability Anisotropy in a Carbonate Reservoir
Authors: Sahin A. , Menouar H. , Ali A. Z. , Saner S.
Published: December 2007

Most classical reservoir engineering concepts are based on homogeneous reservoirs despite the fact that homogeneous reservoirs are the exception rather than the rule. This is especially true for the Middle-eastern carbonate reservoirs, which are known to be highly heterogeneous. The realistic characterization of such types of reservoirs is not an easy task and must include the study of directional variation of permeability. The latter is incorporated in engineering calculations as the square root of the ratio of horizontal to vertical permeability, a parameter known as the anisotropy ratio. Approximate estimates of this ratio are often used in practice despite the fact that it is a crucial parameter in hydrocarbon production.

This paper addresses the pattern of variation of the anisotropy ratio in an Upper Jurassic carbonate reservoir located in Eastern Province of Saudi Arabia. Based on whole core data from several vertical wells, statistical distributions of horizontal and vertical permeability measurements as well as the anisotropy ratios were determined. These distributions for each well are characterized by a considerable amount of positive skewness. A comparison of the values of anisotropy ratios along the wells indicates that, except for a few extreme values, a certain amount of uniformity exists among the ratios for each well. However, lateral variations in the anisotropy ratios are frequently observed. This study also indicates that higher fluctuations in the anisotropy ratios can be correlated with low porosity intervals existing within the reservoir. Three statistical techniques, based on the arithmetic, geometric and harmonic means, and commonly used for combining permeability measurements, have been proposed for combining the anisotropy ratios.
Permeability and RRT Estimation from Conventional Logs in a Middle East Carbonate Reservoir Using Neural Network Approach
Authors: Karri Suryanarayana , Al Amari Khalid , Vissapragada Badarinadh
Published: December 2007

A complex carbonate reservoir of a giant on-shore structure in central Abu Dhabi consists of a porous limestone section divided by several stylolitic dense layers. The reservoir is heterogeneous. High and low permeability rock types co-exist without much variation in log characteristics. Permeability changes several orders of magnitude with no appreciable change in porosity. Reservoir rock types and permeability measurements are available from 30 cored wells. The reservoir is penetrated by more than 100 vertical wells where open hole logs of various vintages are available. Estimation of permeability using conventional logs in the intervals and wells where core data is not available has been a challenge for many years. In this study, relationships were built between core data and the open hole log suite using the Artificial Neural Network approach and were used in estimating reservoir rock types and permeability.

A typical character of the reservoir is that the log signatures are similar for several dissimilar rock types. Due to limitation on the vertical resolution conventional open hole logs alone could not differentiate these rock types. A synthetic variable cementation exponent ‘m' curve, gradient curves from Gamma Ray, Bulk Density and Neutron porosity logs supplemented the open hole logs in the neural network analysis in resolving the permeability variations. Nine neural networks were constructed to handle the complexity. Robust reservoir rock type and permeability estimation was achieved in two iterations. In the initial pass, fractional facies curves were estimated using a supervised classification network. These fractional facies curves were then used in initial permeability estimation with the help of a supervised estimation network. In the next pass, this permeability curve was used to estimate the reservoir rock types. In the final step, to fine-tune the magnitude of the permeability, the log derived reservoir rock types were included in network analysis and a final permeability curve with improved magnitude was generated. The initial permeability estimation was made separately on two vertical sub divisions of the reservoir interval and the reservoir rock type estimation was made on four separate vertical sub divisions based on the occurrence of various RRTs.

Examples are provided showing a satisfactory prediction of both RRTs and permeability in cored and non-cored wells. The estimated permeability and RRTs were incorporated in the 3D geological model supplementing core data.
ESP Failures / Analysis / Solutions in Divided Zone - Case Study
Authors: Mubarak Hisham A. , Khan Farooq A. , Oskay Mehmet M.
Published: December 2007

This paper presents the performance of Electrical Submersible Pumps (ESPs) in the Wafra field within the Divided Zone Kuwait-Saudi Arabia and covers the period of 4 years started in 1998 - December 2001. Included are the historical failures of ESPs, analysis of failures and some solutions that have helped to reduce the number of failures and that in-turn increased ESP run lives/reduced deferred oil production/saved rig cost/time.

Main reservoirs in the DZ area producing with ESPs include Wara/Burgan sands and Ratawi carbonates. Some ESPs are also operating in heavy oil carbonate Eocene wells, Ratawi limestone in south Fuwaris and Marrat carbonate in Humma field. As these reservoirs are of different lithologies, depths and flow rates vary from 1,000 to 9,000 bfpd, thus all series/types of ESPs have been run.

Tables and plots demonstrate/analyze total number of ESP failures by each reservoir / equipment / components types, mean time to failure (Average Run Life), ESP listings. In summary, it is shown that ‘Mean time to failures’ (Average Run Life) significantly increased from 157 to 370 days within this 4 year period.

ESP completions in Wara sands presented the biggest challenge to reduce ESP failures due to high sand volume erosion. New material design for ESP defined as ARM-COMP "Abrasion Resistance Material- Compression" was utilized successfully. In other reservoirs, new designs of ‘Slim Hole’ ESPs, ‘Shrouded Motors’, ‘Pump Optimizations’, ‘Stimulation jobs to increase intake pressure’ have been used to overcome frequent burnt motor, pump plugging etc., problems. Also, ESP XGC Gas Handler for high GOR wells was successful.

Introduction

Divided Zone Group (DZG) pursue exploration and optimum development of hydrocarbon resources of the Divided Zone (On-Shore) through Joint Operations (JO). The Divided Zone field is producing using artificial methods (Sucker rod pump & Electrical submersible pump). The focus of this paper is on the performances of electrical submersible pumps in period of 4 years started in 1998 until Dec.2001.

Main producing reservoirs in the DZ area include Wara/Burgan sands and Ratawi carbonates. Some ESPs are also operating in heavy oil carbonate Eocene wells, Ratawi limestone in south Fuwaris and Marrat carbonate in Humma field. As these reservoirs are of different lithologies, depths and flow rates vary from 1,000 to 9,000 bfpd, thus all series/types of ESPs have been run.

The total number of ESPs installed at the end of year 2001 in DZ was 251 and the common reasons of ESP failures and approximate percentages are:Motor Failures (40%)Pump Failures (22%)Cable Failure (26%)Others (12%)

The following case studies for wells in Ratawi and Wara Reservoirs will be presented. These wells had ESP failures either due to high GOR or excessive sand production.Slim hole pump - Ratawi Reservoir - Case1.Shroud Motor & XGC gas handler- Ratawi Reservoir - Cases 2 & 3.Abrassion Resistance Material (Floater&Compression)-Wara Sand Reservoir Case 4 & 5.
A Full Field Simulation Model of a Giant Cabonate Reservoir in Kuwait
Authors: Matar Saad , Al-Medhadi Fahad , Meadows David , Al-Ajmi Hamad , Oosthuizen Ulrich R.
Published: December 2007

The Minagish Oolite is a thick undersaturated carbonate oil reservoir in the Minagish field in West Kuwait (Fig. 1) containing several billion STB. It is a mature but relatively undeveloped reservoir. Since discovery in 1959, it has produced 10% of its OOIP under a combination of natural depletion, gas re-injection and aquifer drive. Initial reservoir pressure had declined by about 450 psi prior to the Gulf war in 1990. The well blowouts following the war caused a significant pressure drop of another 700 psi. Following the blowout, plans were made to redevelop the West Kuwait fields and increase the production rate starting in 2001 and to sustain the plateau for at least 5 years. This strategy called for threefold increase in the production rate of Minagish Oolite reservoir. Since the existing well inventory and the loss of the gas re-injection facility could not sustain the desired plateau rate, additional field development was required.

To achieve the production target, a multidisciplinary team was formed to evaluate options. The recommended plan required the drilling of additional producers and installing a field-wide peripheral waterflood. The reservoir, however, presented a number of significant challenges to waterflooding, such as the presence of a substantial and not well defined tarmat near the oil/water contact, and uncertainties of lateral and vertical heterogeneities. In 1997 a full-field simulation model was developed, but this model didn't capture the water movement properly because of insufficient reservoir data at that time. As new core was obtained, a refined reservoir description was developed. Building on lessons learned from the previous full-field model and sector models, a new full-field model was developed which significantly improved well-by-well history matches. Although containing twice as many grid cells, the new model ran up to four times faster than the previous model by making use of the Analytical Aquifer option within the model, improved relative permeability curves and other model refinements.

This paper traces the history of the field and the systematic evolution of the development plan. The reservoir simulation efforts including modeling strategy, history matching events, prediction runs, future direction and challenges are also addressed.
Numerical Simulation of Non-Darcy Flow Utilizing the New Forchheimer's Diffusivity Equation
Authors: Belhaj H. A. , Agha K. R. , Nouri A. M. , Butt S. D. , Vaziri H. F. , Islam M. R.
Published: December 2007

Recently, it became evident that non-Darcy flow occurs not only in gas reservoirs, fractured reservoirs and multi-permeability systems within oil reservoirs experienced non-linearity due to non-Darcy flow behavior. Most reservoir simulators currently used encountered false predictions due to their dependency on the traditionally used diffusivity equation.

This paper introduces alternative diffusivity equation to replace the one derived from Darcy's law. The new equation was derived from the commonly known as Forchheimer's equation which is basically Darcy's equation plus an inertia term to account for high velocity fluid flow in porous medium.

Mathematical derivation of the diffusivity equation based on Forchheimer equation has been presented in a previous paper by the authors.

The newly derived diffusivity equation has been numerically simulated. Correlations used to estimate the non-Darcy coefficient "β" have been comprehensively reviewed; nine correlations found suitable for use in this study for technical reasons. A new dimensionless number (Be) relating β, velocity, density and viscosity has been introduced to differentiate between Darcy and non-Darcy flow in porous medium for any rock type and any flowing fluid. Evidences show that this new dimensionless number cannot be considered a declaration of turbulence flow in porous medium rather the energy loss is contributed to the nature of both flowing fluid and the porous medium. The point of deviation from the Darcian behavior to the non-Darcian behavior has been found at Be = 0, for practical use it has been determined that Be = 0.0526 at 5% deviation from Darcy's linear trend.

A range of permeability from 1 md to 1000 md with porosity changing accordingly has been verified with the new model, velocity as low as 0.0001 cm/sec and as high as 700 cc/sec has been tested as well. Both Darcy and non-Darcy behaviors have been identified for the domain of testing, and the numerical model has proven of good agreement in all cases.
CFD Modelling of Drag Reduction Effects in Pipe Flows
Authors: Koskinen Jukka , Manninen Mikko , Pättikangas Timo , Alopaeus Ville , Keskinen Kari I. , Kolehmainen Eero
Published: December 2007

CFD modeling of drag reduction (DR) by polymer additives dissolving in hydrocarbon was carried out in pipe flows and in a rotating shear viscometer. Two-layer turbulence model describing the damping of turbulence by the DRA molecules in the near-wall regions was applied. The von Karman constant in the near-wall region was used as the model parameter. Extensive measurements of the DR effect for a rotating viscometer were performed at different Reynolds numbers, apparent molar masses and concentrations of DRA. The model with only one fit parameter was able to reproduce the experimental results both in pipe flows and in the rotating viscometer. Experimental results were utilized in relating the model parameter to the relevant physical properties.
Reducing Pressure Drop in Offshore Pipelines by Controlling the Viscosities of Pressurized Emulsions
Authors: Kokal Sunil , Alvarez Carlos
Published: December 2007

Viscosities of emulsions can be substantially higher than the viscosity of crude oil especially if the emulsion is tight. High pressure drops could develop in offshore pipeline networks due to this problem. It is shown that by adding a demulsifier offshore the viscosities of the emulsions can be reduced substantially. This paper describes the results of a study that was initiated to determine the viscosities of pressurized crude from an offshore field in Saudi Arabia. The viscosities of the emulsion were measured and determined to be very high. Several tests were conducted to determine the effect of different variables on pressurized emulsion viscosity including temperature, pressure, watercut, and demulsifier dosage. The paper presents some very interesting results for emulsions measured under pressurized conditions (for the first time). The viscosity of the emulsion can be reduced substantially by adding a demulsifier. The demulsifier breaks the emulsions into oil and water thus reducing the mixture viscosity. The results can significantly impact the transportation of crude oil from this offshore field and the required surface/subsurface facilities to produce it.
A Simple Approach to Numerical Analysis of Complex Well Tests
Authors: Al-Thawad Faisal M. , Issaka M. B. , Agyapong Dominic , Banerjee Raj
Published: December 2007

The inability of analytical models to handle the increasingly complex well and reservoir structures encountered in well tests has necessitated the move to numerical well testing tools. Complex well geometries, such as slanted and multilateral wells, completed across multilayered reservoirs with faults and fractures, present situations analytical models are incapable of handling.

Using a numerical well test package, we demonstrate the power of numerical solutions to overcome some of these challenges, by matching pressure transient data from complex wells completed in heterogeneous reservoirs. Three field cases are presented in this paper; a vertical well intersecting a fracture/fault network, a 45 degree slanted well intersecting a finite conductivity fracture and a dual-lateral well in a tight reservoir. An integrated approach was used to match the pressure responses in all cases.

Our experience shows that finding the optimum grid settings necessary to model the flow regimes created by these complex features, is by far the most critical factor in arriving at a satisfactory match of the well test data. Without an optimum grid, strange pressure responses can be generated, creating an artifact on both the simulated pressure and derivative plots.
An Integrated Approach to Interval Pressure Transient Test Analysis Using Analytical and Numerical Methods
Authors: Jackson R. R. , Banerjee R. , Thambynayagam R. K. M.
Published: December 2007

In this paper we demonstrate that reservoir parameters such as horizontal and vertical permeability's may be determined in-situ from the analysis of interval pressure transient tests, and the results verified via accurate analytical and numerical modeling and simulation coupled with gradient based history matching techniques. From evaluation of simulated and field cases, and results of numerical experiments, we propose an integrated workflow for analysing wireline formation tester interval pressure transient tests using analytical and numerical methods. This approach includes simulation model generation, reservoir simulation, interactive sensitivity analysis and an automated history matching procedure for optimisation. An objective function with appropriate weightings for different data items is defined interactively. Weighting may include prior knowledge or statistically determined error variance in input data, sensor resolution, data uncertainty and regression schemes for data sets with responses of disparate magnitude.

The MDT (Modular Dynamics Formation Tester)* tool provides the capability to conduct fluid sampling, controlled local production, standard transient and vertical interference tests. Such interval tests usually comprise of a drawdown and buildup–typically the tool is stationed in the borehole and two packers are set to hydraulically isolate a section of the formation. Observation probes or multiple probes mounted above the packers provide the capability of vertical interference testing within the near-wellbore formation.

The methodology of analysing MDT transient data is similar to conventional pressure transient tests. However, specialised mathematical models are used for matching pressure and flow rate measurements. Conventionally, analytical models have been used in analyzing these tests. While computation of analytical models are fast, they may be too simplistic or inadequate for many test configurations. On the other hand, numerical models are more realistic and flexible. However, their use is more complex. Therefore, in our interpretation methodology we profess an integrated approach.

Although gradient based techniques have been used in reservoir simulation for history matching of pressure and production performance of reservoirs for some time, our work focuses on their application to MDT interval pressure transient tests in combination with numerical well testing methods.
The Workflow for 4D Seismic
Authors: Kawar Ra'ed , Hatchell Paul , Calvert Rodney , Khan Mashuir
Published: December 2007

In this paper we will present the Shell workflow for 4D seismic. This workflow contains several stages: feasibility study, acquisition design, data processing, data interpretation and final feedback loop of the results to the geological and dynamic reservoir models; which in turn are used for field development planning and well proposals. Furthermore, we will also show some case studies from a variety of fields ranging from Carbonates to Sandstone and Turbidities.

The first stage of this workflow is the feasibility study that is based on measurements of rock properties and the currently used reservoir simulation model. In the case the results of the feasibility study prove to be positive we proceed to the next step. The second step involves the survey design, acquisition and in-house processing. Parallel to this process, the reservoir simulation scenarios and history matching are carried out. With these stages finished seismic data interpretation takes place. The results of the seismic data interpretation reveal the most appropriate reservoir simulation scenario. Further fine-tuning for the selected reservoir simulation and geological scenarios is then carried out to match the 4D seismic. Our experience with the fields we have processed so far has proved to find some by-passed pockets of hydrocarbons. In this paper we would therefore like to share our experience in these fields.
A Case Study on Redevelopment of a Giant Multilayered Carbonate Reservoir Based on Modification of Ongoing Water Flood Program through Integrated Reservoir Modelling Studies
Authors: Yatindra Bhushan , Amitabh Pandey , Srivastava Sanjiv K. , Lohar B. L.
Published: December 2007

Mumbai High North L-III reservoir is a highly heterogeneous multi-layered carbonate reservoir located on the continental shelf of Western India in Arabian Sea in about 250 ft of water depth. It is a saturated oil reservoir having about 4 billion barrels of initial oil in place and 1.7 trillion scf of initial gas cap gas in place. The field was put on production in 1976. Peripheral water injection was started in 1984. Couple of years later, up-dip water injection was also initiated in addition to peripheral water injection. The oil production rate from the reservoir peaked at about 120,000 bopd in 1986. The oil production rate in 1999 end was about 60,000 barrels of oil per day at about 60% water cut. This paper discusses various redevelopment options for the Mumbai High North L-III reservoir in detail, based on a detailed reservoir characterisation and reservoir simulation study for the field.

A 780 layer fine grid geo-cellular model was prepared taking input from core data, log data and 3D seismic. This fine layer model was up-scaled to a sixteen layer model. A full field reservoir simulation study was carried out using the sixteen layer coarse up-scaled geological model. Saturation height functions were used to distribute initial saturations in the coarse model. Layer wise pseudo relative permeability curves were generated for use in reservoir simulation.

Various redevelopemnt options studied indicate that there is a scope to improve the overall oil recovery from the multi-layered MHN L-III reservoir through infill drilling, enhancement and redistribution of water injection and proper completion strategy. A shift from line drive to pattern water injection is found to be highly beneficial.
Skills Shortage ‘The Way Forward’
Authors: Rengers Frederik , Scholten Jan
Published: December 2007

Concerns are raised of a personnel and skills shortage in the Oil & Gas industry.

Two solutions can mitigate the problem: 1. Make more efficient use of the present pool of people 2. Promote the Oil & Gas industry at the universities and to young professionals in other industries.

This paper will describe why there is a reason for concern and how new technology will be part of the solution to overcome a personnel shortage.
Planning, Executing and Analyzing the Productive Life of the First Six Branches Multilateral Well Drilled Underbalanced in Brazil
Authors: Lage Antonio C. V. M. , Sotomayor Gabriel P. , Vargas Angela C. , Rodrigano Roberto , da Silva Paulo R. C. , Lira Humberto L. , Filho Paulo Silva
Published: December 2007

The present work describes not only the main aspects related to planning and executing six multilateral (ML) horizontal branches drilled underbalanced (UB) in Carmópolis field, but also presents an analysis of the initial productive life of this well. All the main engineering information used for making decisions while planning and at well site, drilling performance and production evaluation are included.

As the targeted formations are consolidated, UBD did not bring any special concern in terms of wellbore collapse. In order to achieve the required bottomhole pressure, nitrogen was mixed with a synthetic base mud (SBM) after flowing down through the annular space between the 9 5/8″ and 7″ casings and passing through a 1″ aperture in the 7″ casing. After that, the mixture went up through the annulus between the 7″ casing and the drill string. This concentric casing approach for injecting gas made possible the use of conventional MWD, but it brought some concerns about the maintenance of the UB condition while operating.

The first ML well UB drilled in Brazil successfully combined the following different technologies: (1) ML level 2; (2) UB condition, and (3) medium radius build-up drilling system. It also involved four different parties where the operating company worked together with three service companies to build a team and achieve operational goals.

As the reservoir pressure was not sufficient to sustain the flow of oil to the surface, this paper also addresses the design of the pilot well, which was specifically prepared for operating with a sucker rod pump (SRP). Finally, from the perspective of the reservoir engineers in charge of developing this field, the oil production is presented and evaluated.
Reservoir Criteria for Selecting Underbalanced Drilling Candidates
Authors: Xiong Hongjie , Shan Dan
Published: December 2007

Candidate selection is the key to the success of underbalanced drilling (UBD) projects. It is a question yet to be answered why sometimes UBD significantly improves productivity compared with conventional overbalanced drilling (OBD) practices but sometimes doesn't. To answer this question, we have performed an extensive reservoir engineering study to understand the mechanisms of productivity improvement from UBD. A single-well numerical model has been built that simulates the UBD process and the OBD process (while taking into account drilling filtration invasion, mud cake, and permanent formation damage) and their impact on production under a variety of reservoir conditions. Production rates are predicted for both UBD and OBD cases. Then, an incremental net present value (NPV) model has been used to analyze the economic benefits of UBD operations (if any) for all study cases. This paper describes our study methodology, models, simulation results, and analysis results and summarizes the reservoir criteria for successful UBD operations.

Reservoir properties play the most important role in the success of UBD operations. The study clearly demonstrates that UBD works for some reservoirs but not for all reservoirs. The study results provide a set of reservoir criteria that can be used as guidelines to choose potential UBD candidates.
Drilling Near Balance and Completing Open Hole to Minimize Formation Damage in a Sour Gas Reservoir
Authors: Harting Thomas , Gent Jonny , Anderson Tim
Published: December 2007

Peak production of 15 MMCFD was restored in a sour well (17% H2S) drilled and completed near-

balance to prevent reservoir damage. The ACG #6B well is in the Whitney Canyon Gas Field located in southwest Wyoming. The Mission Canyon formation is the primary reservoir. The current bottom hole pressure is less than 1100 psi at 12,457 feet TVD. Severe damage occurred to the original ACG #6A wellbore and the reservoir during an attempted work-over to replace production tubing.

Recently drilled wells suffered massive lost circulation. Continued pressure depletion raised concerns that the ACG #6B sidetrack was undrillable using conventional overbalance drilling techniques. lf drilling was successful, setting and cementing production casing through the reservoir carried high risk of getting stuck. Failure to get casing to bottom would result in a poor performing well as the highly damaged rock left open hole would be unproductive.

Formation damage from the expected loss of 50,000 barrels of drilling fluid would require a fracture stimulation to regain productivity. To prevent these massive fluid losses while drilling and completing the well, the asset team considered near balance drilling and open hole completion the best option. This technique, if successful, would maximize the well productivity and eliminate casing across the reservoir, perforating, and massive acid stimulation. Drilling near balance would also significantly improve the drilling ROP and reduce nonproductive time caused by massive losses

Utilizing near-balance drilling and an open hole completion, allowed a simplified acid wash stimulation vs. a typical acid frac on offset wells. Near-balance drilling was preferred to underbalance drilling to manage the risk of produced sour gas at surface.

Production results from the well exceeded the prediction of 10 MMCFD. Current well production is in excess of 11 MMCFD and peak production rates were nearly 15 MMCFD. Minimizing formation damage provided the productivity and resulted in decreased completion costs.
Does Underbalanced Drilling Really Add Reserves?
Authors: Cade Randall , Kirvelis Ringys , Nafta Minijos , Jennings Jeffrey
Published: December 2007

Underbalanced drilling in its various forms has been used in the oil and gas industry for more than 20 years. A growing body of technical publications documents the mechanics and advantages of underbalanced drilling (UBD) technology, including details related to casing sizes, circulation rates, depths, rate of penetration, etc. There are also many mentions in the literature of resultant well productivity improvements. In contrast, there is little written in the literature about what contribution UBD makes to reserves and ultimate recovery.

This paper provides a basis for considering the reserve contributions of UBD technology. The six reservoir case histories presented here document tangible reserves attributable to the three key reservoir-related benefits of UBD: improved reservoir access, reduced skin damage, and better ability to evaluate the reservoir while drilling.
Underbalanced Data Acquisition: A Real-Time Paradigm
Authors: Saeed Saad
Published: December 2007

Formation damage and reservoir attributes can only be effectively evaluated with high quality, pertinent data. Underbalanced operations require a complex array of techniques that enable data to be effectively collated and managed. With the further advent of networking technologies the real-time transmission of this data is also now a viable option, leading to a myriad of benefits from accurate diagnosis and evaluation to real-time planning. This paper aims to describe data acquisition in underbalanced operations, commencing with a conventional acquisition setup, moving to a real-time operational case study.

Underbalanced data acquisition has its roots in the traditional well testing setup, with the basic surface separator being standard. But due to the complexity of the operation many specializations and innovative modifications have been implemented, making it an entity in itself. The paper describes a typical underbalanced data acquisition setup. It then traverses to the latest high technology realm of real-time acquisition and transmission, which enables data to be acquired in the field and transmitted directly to the client's office. System setup, architecture (client – server) and limitations will be discussed and evaluated. The advantages of real-time data acquisition will be shown as well as current caveats and possible conceptual technological innovation to overcome these limitations.

The future of underbalanced data acquisition is also taking a fundamental shift. Its amalgamation with the data analysis fields (i.e. non-simple processing) is leading to a fundamental shift in acquisition setups. These trends will be examined along with challenges and current industry direction.

In the underbalanced sphere, the marriage between data collection and transmission within a real-time framework plays a vital role in optimizing results. In the past, data acquisition design and implementation has been relegated to the last phase of many operations. This paper aims to bring data acquisition, its current direction and setup to the forefront. Formation damage and ultimately recovery are important entities, but it is only with good quality data, available in a timely manner, that these can be representatively evaluated and the well understood.
Underbalanced Horizontal Drilling Yields Significant Productivity Gains in the Hugoton Field
Authors: Devaul Tijuana , Coy Alejandro
Published: December 2007

The Hugoton Field located in the Texas Panhandle is one of the largest gas producing fields in the lower 48 states. Up until now, these shallow reserves have been exploited by drilling either vertical wells with air or long reach horizontal wells with conventional overbalanced techniques. Due to the low formation pressures, in the range of 600 psi, depletion is a major issue and wells drilled in conventional overbalance have not exhibited a consistent productivity trend. These low pressures have also resulted in severe lost circulation problems during drilling with resultant formation damage and productivity deterioration. An additional challenge has been the close proximity of water to the productive formation, which effectively eliminates the use of fracturing as a stimulation option.

Previously, wells were drilled vertically using underbalanced techniques (UB) but the limited kh that resulted had little positive effect on productivity. Nevertheless, the operator decided to drill three horizontal wells using UB, as a case study to make a definitive evaluation of its effectiveness in the Hugoton Field. All three wells were successfully completed and achieved a significant increase in production when compared to offset wells drilled conventionally. The results achieved were of added significance considering the low cost environment in which the wells were drilled, where use of conventional techniques typically resulted in marginal economics.

In this paper the authors will detail the well design and the UB model considerations, which took into account the geological, reservoir depletion and water proximity challenges. They will present a detailed evaluation of the incremental production rates of the order 300 – 400% that were achieved when compared to offset wells drilled and completed conventionally. They will conclude that the use of Horizontal-UB techniques in the Hugoton Field is indeed a highly viable means of achieving significant production improvement and may well provide a powerful tool for the continued exploitation and prolonged economic life of the field.
Multilayer Reservoir Model Enables More Complete Reservoir Characterization During Underbalanced Drilling
Authors: Hunt James L. , Rester Stephen
Published: December 2007

In order to effectively analyze reservoir inflow data from a well drilled underbalanced, a true multilayer reservoir model is required. In a previous paper,1 a single layer analytical model was developed to handle reservoir inflow resulting from the changing wellbore geometry during the underbalanced drilling process. A method to analyze data acquired during underbalanced drilling for reservoir permeability was developed and presented. The resulting reservoir model was shown to be unique in the industry. However, it was recognized that a multilayer model was needed to fully analyze the data and generate a more complete reservoir evaluation during underbalanced drilling.

This paper presents a true multilayer model that describes inflow from multiple reservoir layers encountered during underbalanced drilling. The model is fully transient and is based on the single layer model presented in previous work. The model accounts for the lengthening of the wellbore through any number of reservoir layers having similar or different properties and the resulting flow into the wellbore. Results from validation of the model are presented. The model enables formation evaluation during the underbalanced drilling process.
A New Downhole Tool for ECD Reduction
Authors: Bern P. A. , Hosie Dave , Bansal R. K. , Stewart Donald , Lee Bradley
Published: December 2007

Wellbore pressure management is a critical part of normal drilling practices, where static and dynamic fluid pressures are used to contain formation pressures and to assure wellbore stability. Excessive fluid pressure while circulating can create problems including reduced operating margins between fracture and pore pressures and, in the extreme, lost circulation. To address these problems, an Equivalent Circulation Density (ECD) Reduction Tool (RT) has been developed.

The ECD RT is designed to counter the frictional pressure effects that exist while circulating. The tool is expected to have a broad range of drilling applications including the narrow pore/fracture pressure margin deepwater environment, wellbores prone to instability, pressure depleted reservoirs and extended reach wells.

The tool has the potential to:

Improve wellbore stability. Extend hole intervals and reduce casing requirements. Improve rate of penetration (ROP). Reduce lost circulation. Reduce differential sticking. Improve hole cleaning in extended-reach drilling (ERD) wells through the use of higher flow rates.

This paper describes a new downhole tool for ECD reduction, which is run as an integral part of the drill string. A prototype tool has been built, to operate inside 10-3/4″ to 13-3/8″ casing strings, which has undergone testing in a flow loop and in two experimental wells. The design features of this prototype and the test results obtained so far are discussed in this paper.
The joining and splicing of internal cable CT strings to enable deeper Coiled Tubing Deployed ESP completion
Authors: Ivey Martin S , Macleod Alistair I
Published: December 2007

Coiled Tubing conveyed Electrical Submersible Pump completions (CT-ESP) have now been established as a cost-effective means of improving production in pressure depleted reservoirs, with low installation and work over costs. However, due to max lift limitations, and crane down grading practices associated with offshore platforms, the use of CT-ESP’s in many cases cannot be considered, because it is simply impossible to lift a CT-ESP reel of the required length on board the platform. This paper describes a worked solution (1) for a conventional CT reel that exceeded the platform crane lift capacity. A worked solution (2) for a CT-ESP re-completion, followed by a proposal (3) for a complete CT-ESP solution, with a focus on overcoming the max crane lift issue, thereby allowing deeper CT deployed ESP completions in the future.
Field Validation of a Coiled Tubing Fatigue Model
Authors: Behenna F. R. , Myrick D. D. , Stanley R. K. , Tipton S. M. , Hammond W. A.
Published: December 2007

Tracking the bending and internal pressure history of coiled tubing (CT) at the reel and guide arch, as it is run in and out of wells during field operations, is accepted throughout the industry as the preferred method for predicting the fatigue damage distribution along the string throughout its life. A mathematical model, relating bending strain and hoop stress (from internal pressure) to cumulative fatigue damage can be applied to this recorded field loading history. Such models determine when tubing should be retired or when tubing cuts should be made in order to maximize the utilization of the pipe while minimizing the chance of pipe failure. The problem of CT fatigue has been studied extensively in the lab, using constant pressure bend cycling and/or axial coupon testing. However, models resulting from experimental programs have seldom been correlated in the literature with CT behavior under field loading conditions.

This paper presents such validation by correlating the experimentally determined cycles to failure of CT samples which have had extensive use in field operations, to predictions from an advanced predictive fatigue algorithm. The string, from which the samples were taken, had been removed from service based on criteria specified by the operating company for which the CT work was being performed. This stated that strings were to be retired after a given value of "running meters" had been reached, rather than on the actual bending and internal pressure history.

Excellent agreement is demonstrated between the actual remaining fatigue life of the tubing, determined experimentally, and the estimated cycles to failure predicted by the fatigue model. Therefore, it is shown that the string could have been safely utilized much longer than the current string retirement specification allows. Based on the results of this testing, the operating company has changed its criterion for CT string retirement to one that relies on tracking of bending and internal pressure history and subsequent predictive fatigue modeling based on this history.
Damage Removal in Screened Horizontal Wells
Authors: McCulloch D. J. , Mann J. , Macmillan P. , Ali S.
Published: December 2007

The requirement for screened and gravel packed completions in horizontal wells has led to the development of alternative stimulation fluids and placement techniques.

This paper describes a common damage mechanism in this type of completion and the remedial cleanup procedures developed.

While horizontal wells do not normally require stimulation for primary production reasons, it is often necessary to clean up the drill in fluid (DIF) filtercake.

A placement technique was developed that includes a coiled tubing, controlled injection technique (CIT) in combination with a true fluidic oscillator (TFO) to aid in placing a modified propriatory scale and DIF filtercake solvent (PSDS) fluid behind the screen into the gravel packed annulus.

The placement technique is supported with surface testing results of two standard jetting tools vs. a TFO for the placement of the stimulation fluid.

This PSDS fluid was specifically modified to improve the effectiveness of removing filtercakes layed down with a drill solids-laden DIF mud sample from the field. The presence of formation-drilled solids results in a much more tenuous filtercake than one created with a clean DIF.

The technique was developed for a long horizontal gravel-packed North Sea well, however the procedure has global applications.
Improved Nondestructive Inspection During Tubing Manufacture and Servicing
Authors: Stanley R. K. , Martin J. R. , McCoy T. , Rosine R. , Aulert C.
Published: December 2007

In SPE 74830, a paper presented at the 2002 SPE ICoTA Conference in Houston, an inspection program was presented that would take coiled tubing (CT) quality to the next level. The program was aimed at total prevention of strip and mill defects and to determine the capability of field inspection units for CT. From these tests and investigations, it was determined that a higher level of nondestructive examination (NDE) could be achieved. This paper covers the NDE work performed since SPE 74830 was published.

This work includes the design and operation of an electromagnetic tool for incoming strip inspection, and the results of strip inspection over a 4-month period. In addition, work included operational design of a phased-array shear wave ultrasound system capable of inspecting continuously tapered CT and the results over a 3-month period. The results of tests of butt-welds at an experimental stage using sophisticated ultrasonic technology are also presented.
Innovative Casing Exit System for CT/TT Applications: System Description, Validation Surface Test and Field Runs
Authors: Desai Praful , Dewey Charles , Cruickshank Brian , McGarian Bruce
Published: December 2007

Historically, in CT/TT application, sidetracking through the casing requires multiple trips to effectively produce a useable window for the next directional drilling assembly. The number of trips needed depends upon the performance of the window milling system and the comfort level of the system operator.

This paper presents an innovative one trip casing exit system especially tailored for the CT/TT operation. The system includes a hydraulically set anchor which can pass through a restriction and expands to a larger diameter, a whipstock, that incorporates a hydraulically activated hinge mechanism, which provides the optimum trajectory for the mill with multiple ramps for strategically guiding the milling tool, a lead mill with tightly spiraled blades dressed with cylindrical inserts for aggressive milling action and a retrieval tool in case the system is to be recovered.

The paper will also present a detail description of validation surface test equipment for a 4½" X 7" TT system, test procedure and test results. The test set up included a 90 ft. long ‘I’ beam test bed, triplex pumping unit, data acquisition system and video camera for recording the milled window profile and rat hole conditions.

The initial test objectives were (1) to functionally test the anchor and hinge mechanism; (2) test the TT Whipstock system inside a test fixture and successfully mill a window; (3) evaluate various mill configurations utilizing the TT Whipstock system.

The second test objectives were (1) to mill window in the 7"-26#, P-110 casing buried in a 3’ x 3’ x 40’ concrete block; (2) drift directional drilling assembly through the window consisting of 3⅛" O.D. mud motor with a 3.2 degree bent housing and 3¾" X 4⅛" Bi-Center bit below the motor; (3) Retrieve the system with a hook. All of the objectives were successfully achieved.

Additionally, the paper will detail field runs of above described system along with lesson learned, conclusions and way forward.