Mendoza, Alberto X. (ExxonMobil Neftegas) | Gaillot, Philippe (ExxonMobil Exploration Company) | Yin, Hezhu (ExxonMobil Abu Dhabi Offshore Petroleum Company) | Nicosia, Wayne (ExxonMobil Upstream Research Company) | Guo, Pingjun (Exxon Mobil Corporation) | Mardon, Duncan (ExxonMobil Upstream Research Company) | Passey, Quinn R. (ExxonMobil Upstream Research Co.) | Wertanen, Scott R. (ExxonMobil Exploration & Production Surumana) | Zhou, JinJuan (ExxonMobil Upstream Research Company) | Fitz, Dale Edward (ExxonMobil Upstream Research Co.)
Over last several years, the ability to perform accurate, quantitative formation evaluation in high-angle and horizontal (HA/HZ) wells has been increasingly recognized as a high priority, unsatisfied need within the formation evaluation (FE) community. The industry has realized that the ability to drill extended reach wells has surpassed the ability to evaluate them. Well logs are often underutilized for geologic modeling and assessment applications due to lack of confidence in petrophysical analysis results.
In this paper, we introduce a state-of-art formation evaluation toolkit specifically developed for quantitative interpretation of high angle and horizontal well logs. Starting with wellbore images and standard triple-combo field logs, the workflow consists of: 1) three-dimensional (3D) and two-dimensional (2D) display modules for well path, wellbore images logs, scalar logs and dips to quality control (QC) the data; 2) a comprehensive image analysis module combined with log analysis to build a 3D geometrical earth model; 3) a depth coherence processing (DCP) module to effectively correct recorded borehole images of different logging tool sensors with different depths of investigation (DOI) back to borehole size (BS); 4) a 3D joint inversion module to accurately model and interpret gamma ray (GR), neutron, density, and resistivity logs, to build a common petrophysical earth model; and 5) an output module in which the common earth model is populated with bedding geometries and petrophysical property distributions.
The advanced formation evaluation toolkit described in this paper enables geoscientists to realize much more value than ever before from high-angle and horizontal well data, especially in thinly bedded reservoirs. The detailed description of the internal architecture and lateral petrophysical characterization of the reservoirs are essential for understanding stratigraphy and conditioning geological models. The improved estimations of the petrophysical properties yield more accurate estimates of reserves in place.
The field X is a brown heavy oil field producing under strong bottom water drive since the mid-1980. Production is from a combination of Amin aeolian and Al Khlata glacial reservoir sediments. At present, the development is focused on drilling horizontal infill wells. One of the biggest challenges is the unfavorable mobility contrast between the heavy oil and water causing early water breakthrough.
The Amin Formation, the primary reservoir, is characterized by a high net to gross ratio and an average porosity of 30 %. However the initial hydrocarbon saturation at the same porosity often varies by 20 % in different parts of the field. Furthermore, core measurements show an order of magnitude scatter in permeability at the same porosity, indicating the presence of different facies. In early studies these variations were attributed mainly to the grain size variations. A later petrographical study found that the abundance of clays and feldspars could also severely reduce permeability, but may retain high porosity.
In the current Study it was found that the rocks have variable radioactivity due to the presence of radioactive Potassium isotope associated with feldspars. A fare correlation was observed between the grain size and the content of feldspars from core. A novel approach to reservoir characterization integrating core and logs was developed leading to a major breakthrough in the reservoir characterization including:
• Enhanced permeability prediction using normalized Gamma Ray (GR) log as 3rd parameter;
• Facies identification using normalized Gamma Ray cut-off;
• Facies based Saturation-Height models.
This work is a good example of advances in reservoir characterization achieved by integrating core and log data. It results in better understanding of reservoir properties distribution, optimization of completions of new wells and improvement of further development scenarios. In particular, abnormally high gross production and high water cut in the north of the field is currently in line with new facies scheme.
In recent times the topic of well barrier integrity has become increasingly salient. Within the well completion arena, there have traditionally been two main alternatives for barrier plugs used for packer setting or temporary well abandonment; these are the metallic flapper or ball type isolation plugs. This paper describes the evolution of an innovative glass type barrier plug from its first appearance in the oilfield in 2004, to the deployment of third generation prototype systems into wells in the North Sea today.
Traditional ball or flapper type plug systems need to operate in two states: open and closed. This functionality typically necessitates the use of dynamic seals, which also have to compensate for the pressure differential applied across the plug. Plugs built in this manner can be prone to malfunctions in the dynamic seals and have limitations as to the pressure differentials that can be applied to them when opening. Additionally as the balls or flappers themselves are traditionally manufactured using metallic alloys, in the event that a plug fails to open the only alternative is milling, which if successful, will still leave a restriction in the well limiting options for future well interventions.
Glass barrier plugs have to operate in two slightly different states, solid or shattered. When the plug is run in hole the glass is in a solid state with pressure integrity maintained using static elastomeric seals. Once well operations have progressed to the stage when the plug needs to be opened, a preinstalled trip saver can be activated which would shatter the glass and open well communication. Operating in this manner avoids the use of dynamic seals thereby increasing plug reliability. Other major advantages are that the differential pressure applied across the plug when opening has no effect on the plugs functionality and since the plug is made out of glass, in the event of a trip saver malfunction the plug can be opened using a shoot down tool, a spear, or milled within approximately 10 minutes using a wireline tractor (Welltec, 2011) leaving a full bore ID for future well interventions.
This paper describes how BP Norway and TCO used the lessons learned from two generations of Glass Barrier Plugs (GBPs) to develop a system with increased debris tolerance, improved redundancy and a larger inner diameter.
Cinar, Yildiray (The University of New South Wales) | Arns, Christoph (The University of New South Wales) | Dehghan Khalili, Ahmad (The University of New South Wales) | Yanici, Sefer (The University of New South Wales)
Resistivity measurements play a key role in hydrocarbon in place calculations for oil and gas reservoirs. They are a direct indi-cator of fluid saturation and connected pore space available in the formation. Carbonate rocks, which host around half of the world's hydrocarbons, exhibit a wide range of porosities with scales spanning from nanometres to centimetres. The often sig-nificant amount of microporosity displayed by Carbonate rocks emphasizes the necessity of an adequate characterization of their micro-features and their contribution to hydrocarbon in place. In this paper we examine upscaling methods to probe for-mation factor of a fully saturated carbonate sample using an X-ray CT based numerical approach and compare to experimental measurements.
Three-dimensional high-resolution X-ray CT enables the numerical calculation of petrophysical properties of interest at the pore scale with resolutions down to a few microns per voxel. For more complex and heterogeneous samples however, a direct calculation of petrophysical properties is not feasible, since the required resolution and a sufficient field of view cannot be obtained simultaneously. Thus an integration of measurements at different scale is required. In this study a carbonate sample of 38mm in diameter is first scanned using the X-ray CT method with a resolution of 26µm. After accompanying experimental measurements on the full plug, four 5mm plugs were drilled vertically from this sample and X-ray CT images of these plugs acquired at resolutions down to 2.74 µm. We calculate the porosity of the sample (macro- and micro-porosities) using the phase separation methods and then predict the formation factor of the sample at several scales using a Laplace solver. The formation factor is calculated by using a general value of m=2 as cementation factor for intermediate porosity voxels. We compare to experimental measurements of formation factor and porosity both at the small plug and full plug scale and find good agreement.
To assess the degree of uncertainty of the numerical estimate, we probe the extent of heterogeneity by investigating the size of a representative elementary volume (REV) for formation factor. We find that for the considered heterogeneous carbonate sam-ple, formation factor varies considerably over intervals less than a centimetre. Our results show that this variation could be explained by different cementation exponents applied at the micro-voxel scale, with the exemption of one plug, for which the cementation exponent would have to be unreasonably low. These cementation factors are derived by direct comparison be-tween numerical simulation and experiment. We conclude that for one plug an error in experimental measurement might have occurred. The numerical approach presented here therefore aids in quality control. Excluding this plug in the upscaling proce-dure improves the agreement with the experimental result for the whole core while still underestimating formation factor. Al-lowing for a constant m=2 in the simulation at the small scale and using directly the resulting relationship between porosity and formation factor in the upscaling process leads to an overestimation of formation factor.
The Middle Minagish Oolite Formation is 450 to 550 feet thick interval of porous limestone reservoir, composed of peloidal/skeletal grainstones with lesser amount of packstone, oolitic grainstone, wackstone and mudstone in Umm Gudair field, West Kuwait. It is characterized by small scale reservoir heterogeneity, primarily related to the depositional as well as diagenetic features. Capturing reservoir properties in micro scale and its spatial variation needs special attention in this reservoir due to its inherent anisotropy. Reservoir properties will depend on the level that we are analyzing on reservoir (millimeter to meter scale). Here we used Electrical Borehole Image (EBI) and Nuclear Magnetic Resonance (NMR) to capture small scale feature of Umm Gudair carbonate reservoir and compared them with core data
In present work, reservoir properties (including texture, facies, porosity and permeability) interpreted by the EBI shows good match with NMR driven properties and core data. Textural changes in image logs also match well with pore size distribution from NMR logs. Further highly porous zones which are considered either due to primary porosity or vugs match with larger pores of NMR logs and these corroborates with also core derived porosity. A good match has been observed between EBI, NMR and cored derived porosity. Permeability calculations have also been made and compared with core data. A detail workflow has been developed here to interpret reservoir properties on un-cored wells, where only low vertical resolution data is available. This technique is quite useful to identify the characters and mode of origin highly porous zones in reservoir section which are generally not identifiable by low resolution standard logs. This workflow will allow us to interpret the heterogeneity at high resolution level in un-cored wells, as results are validated with integration of EBI, NMR and core data.
Oil and gas producers have shown renewed interest in developing reservoirslocated both onshore and offshore within the Arctic regions of Alaska, Canadaand Russia. In many cases, the hydrocarbon reservoirs are known to be overlainby a massive permafrost interval that extends over depths of up to 700 m belowthe surface active layer. These conditions create unique design and operationalchallenges for production and injection wells from the perspective of ensuringthat well integrity will not be compromised by the inevitable thaw subsidenceof the permafrost soil layers.
The permafrost soil layers surrounding arctic wells will thaw gradually withtime due to wellbore heat loss. As the thaw zone advances radially outward fromeach well, the ice-to-water phase change within the pore space of thefrozen/partially frozen sediments will lead to changes in the permafrost soilproperties and to the loading conditions within the thaw column region. Thesechanges will result in soil deformations (including both vertical settlements(subsidence) and horizontal displacements) which can, in turn, inducesignificant well casing strains that need to be considered in selecting thewell design and layout. The magnitude of the soil deformations that occurthroughout the permafrost interval are highly dependent on the depositionhistory, insitu temperature and the physical and mechanical properties of theindividual soil layers. Therefore, in order to accurately predict the soildeformations and resultant localized casing strain levels, it is essential toobtain reliable data to properly characterize the lithology (soil types) withinthe permafrost interval, as well as the frozen state and the relevantmechanical and thermal properties (both frozen and thawed) of individual soillayers. This paper describes the various information and geotechnical test datathat has been used to establish the thaw and deformation response of differentpermafrost soils at a number of arctic locations for the purpose of evaluatingthe effects of thaw subsidence loading on wells. Overall, the paper serves tohighlight the importance of collecting the appropriate geotechnical data toallow thaw subsidence-induced ground deformations and associated casing loadingconditions to be properly considered at the well/project design stage.
CO2/CH4 exchange in a sandstone-hosted methane hydratereservoir was executed in the field, following several years of laboratoryexperimentation. Reservoir simulation and laboratory data informedfieldtrial design, including use of a cell-to-cell model that included correctliquid/vapor/hydrate phase behavior of methane, carbon dioxide, nitrogen, andwater.
Most concepts for producing methane from hydrate deposits rely ondepressurization, heating, or chemical melting. These techniques resultin dissociation of hydrate into its water and gas constituents. Effectiveexchange of CO2 for CH4 in the crystalline hydratelattice, without dissociation, was long deemed an improbable recovery strategybecause experimental results on bulk hydrate samples indicated very slowreaction kinetics. Recent laboratory tests documented enhanced exchangekinetics and efficiency, attributed to the increased surface area present inporous media. A series of laboratory tests ranging from simple gas-richsystems to more complex gas-deficient / water-rich systems guided the design ofa field test program. Ignik Sikumi #1 was drilled in 2011 on the AlaskaNorth Slope, designed specifically for testing CO2/CH4exchange in hydrate-bearing sandstones.
Ignik Sikumi #1 was drilled vertically with chilled oil-based mud to a depthof 2600ft. Four hydrate-bearing sandstones were encountered, andpetrophysical evaluation indicated the Sagavanirktok "C Sand" hosted thehighest hydrate saturations. These sandstones occur in the subsurface atreservoir conditions similar to temperatures and pressure conditions of labtests. Reservoir modeling with conventional simulators and in-housecell-to-cell models guided both equipment design and test parameters. Anticipated low injection rates and cryogenic injectant required the design ofspecialized pumping equipment. Operations at Ignik Sikumi #1 re-commencedin December 2011. Following perforation, over 200,000 scf of mixedCO2/N2 gas was injected. A short unassisted flowperiod was followed by extended production testing via jet pumping. Results from the production test will be shown.
CO2/CH4 exchange is a novel approach to recovermethane from sandstone-hosted hydrates. Field trial has validatedlaboratory results and reservoir simulations, and has proven thatCO2 can be injected into naturally occurring sandstone-hostedhydrates. Subsequent flowback/drawdown testing produced injectants(nitrogen, carbon dioxide, and tracer gases) methane, water, and very finesand.
This paper presents a method for determining the Archie saturation exponent, n, from a single, nonequilibrium centrifuge step. The input measurements include detailed 3D saturation distributions from magnetic resonance imaging and the DC conductivity of the sample under examination. The latter is obtained by making use of a patented 4-contact cell. The sample is modeled as a 3D conductivity network and a specially developed algorithm based on random walk (RW) is used to compute its overall conductivity in a very short time. The value of the n exponent is determined by matching the measured conductivity to the calculated one. The entire analysis takes one day. Examples demonstrate the method and details of the impedance cell and the RW algorithm are provided.
This article, written by Senior Technology Editor Dennis Denney, contains highlights of paper SPE 146732, "Geosteering With Sonic in Conventional and Unconventional Reservoirs," by Jason Pitcher, SPE, Jennifer Market, and David Hinz, Halliburton, prepared for the 2011 SPE Annual Technical Conference and Exhibition, Denver, 30 October-2 November. The paper has not been peer reviewed.
Permeability provides a measure of the ability of a porous medium to transmit fluid and is significant in evaluating reservoir productivity. A case study that compares different methods of permeability prediction in a complex carbonate reservoir is presented in this paper. Presence of siliciclastic fines and diagenetic minerals (e.g., dolomite) within carbonate breccias has resulted in a tight and heterogeneous carbonate reservoir in this case. Permeability estimations from different methods are discussed and compared. In the first part of the paper, permeability measurements from conventional core analysis (CCAL), mercury-injection capillary pressure (MICP) tests, modular formation dynamic tests (MDTs), and nuclear-magnetic-resonance (NMR) logs are discussed. Different combinations of methods can be helpful in permeability calculation, but depending on the nature and scale of each method, permeability assessment in heterogeneous reservoirs is a considerable challenge. Among these methods, the NMR log provides the most continuous permeability prediction. In the second part of the paper, the measured individual permeabilities are combined and calibrated with the NMR-derived permeability. The conventional NMR-based free-fluid (Timur-Coates) model is used to compute the permeability. The NMRestimated permeability is influenced by wettability effects, presence of isolated pores, and residual oil in the invaded zone. new modified Timur-Coates model is established on the basis of fluid saturations and isolated pore volumes (PV) of the rock. This model yields a reasonable correlation with the scaled core-derived permeabilities. However, because of the reservoir heterogeneity, particularly in the brecciated intervals, discrepancies between the core data and the modified permeability model are expected.