In order to get a full petrophysical evaluation from log-based traditional techniques in every location, the formation density is needed in wire-line log measurements; otherwise, with a limited amount of information in terms of porosity values, the reservoir characterization has more uncertainty. That is, the case study of the giant Bachaquero-02 reservoir, there is a lack of Rhob data in the spatial data sets that prevent a good assessment of the storage capacity in the petrophysical model and thus wrong original oil in place estimation. This paper, therefore, presents a solution to this problem; this work develops a methodology for predicting formation density values which establish a link between probabilistic interpretations from multi-mineral solution and deterministic predictions from multiple linear regression with the main objective of seeking a mathematical expression which describes the best fit for the Bachaquero Member and Laguna Member in each location. The manner of estimating formation density can vary according to the available data in well logs, as a first step, this technique uses classic lithology indicators from well logging such as gamma ray, spontaneous potential and resistivity index to calculate the most probable minerals in the rock with the purpose of assessing a probabilistic approach, the second stage is to create a prediction model with surrounding wells, the input data, which is the probabilistic outcome and measured logs, it is trained using a'least squares' regression routine that will find the best fit in the data for bulk density reckoning. A reliable formation density profile according to the lithology of the reservoir was obtained for each well. The model shows more than 0.9 of correlation coefficients between the density measured by wire-line services and the new bulk density reproduced in this method. Particularly, the Bachaquero-02 reservoir has a notorious heterogeneity along the stratigraphic column; the Bachaquero Member has different depositional environment and rock properties in comparison with Laguna Member which has poor quality reservoir rock. This workflow has the ability to incorporate reservoir heterogeneities in the probabilistic module without a problem. 2 SPE-191163-MS
Oil exploitation in the Bachaquero field in east Maracaibo Lake has been occurring for more than 50 years. Sandstone is the primary formation type, and nonconsolidated and poorly consolidated sands are common in this field. Complex mineralogy and fines migration have become root causes of production decline and formation damage. This paper describes a comprehensive approach to reservoir characterization that has contributed to the successful stimulation of the sandstone formations in the field.
Chemical stimulation, specifically matrix acidizing with hydrofluoric (HF) acid systems that are customized and tailored to reservoir characteristics, has proven to be effective at enhancing production in this field. The types of clays that are present include kaolinite, illite, smectite, chlorite, and mixed-layer clays; feldspars are also present. An adequate analysis of each well helps to ensure that HF acid dissolves the clays to restore permeability without promoting nonsoluble fluorosilicates precipitation through reactions with aluminosilicates. Variations in mineralogy determine fluid performance and make customized fluid selection necessary. The high presence of feldspars requires more conservative treatments to avoid undesirable precipitations.
Reservoir characterization and fluid tailoring has helped ensure treatment success, but other good practices also have been applied to help achieve production goals. The stimulation treatment design includes pumping formation-conditioning fluids before and after the main acid; using different types of organic solvents to dissolve asphaltene deposits in the well; performing near-wellbore (NWB), hydrochloric (HCl) acid, and HCl/organic acid blend preflushes and post-flushes to treat calcium carbonate and control the pH and iron precipitation in the reservoir; achieving short-term clay inhibition and long-term clay stabilization; and using other fluids, such as relative permeability modifiers (RPMs) for water-control applications and diversion of treatment in laminar reservoirs with petrophysical heterogeneities. All of these combined practices have resulted in successful stimulation of the field.
This paper discusses in detail this comprehensive approach to reservoir characterization applied successfully in wells in the Bachaquero field. The workflow includes candidate analysis, from reservoir description and mineralogy and formation damage mechanism identification to stimulation treatment design, laboratory fluid systems tailoring, placement and diversion techniques, pretreatment operational task fulfillment, field execution, quality control, and post-job evaluation through analysis of records and statistics.
Waterflooding is an improved oil recovery (IOR) method commonly used worldwide. Some of the world's largest waterfloods are found on Lake Maracaibo, in western Venezuela. One such project is the Bachaquero-02 (Bach-02) which was initiated in 1967 as a peripheral water injection in an unconsolidated sandstone reservoir with average permeability of 350 mD and average oil gravity of 15 API. This waterflood project has been the subject of various studies that include conventional assessments, integrated full field and simulation studies. This paper presents a diagnosis of the impact that waterflooding has on Bach-02 heavy oil recovery, in view of a new approach that reveals mechanisms of heavy oil displacement by water. The findings of this field study are in accordance with empirical, experimental and simulation investigations recently reported in technical literature. Voidage Replacement Ratio (VRR) and Water Oil Ratio (WOR) analysis in this reservoir show particular behaviors associated with fluid flow characteristics in the reservoir; namely, periods of VRR less than 1.0 which activate mechanisms that stabilize WOR at values close to 1.0 and result in increased oil recovery. These mechanisms are explained by recent investigations as the flow of water-in-oil emulsions, and in situ formation of foams that benefit from chemical changes brought about by periods of underinjection.
Major heavy oil accumulations are found in the tertiary sandstones of the Lagunillas Formation of the Costanero Bolivar Field, located in Lake Maracaibo, Venezuela. The hydrocarbons are found in poorly consolidated shaly-deltaic sands, at depths of around 1200 ft to 2350 ft. The oil ranges from 10 to 18 degrees API, with viscosities ranging from 400 cP to 10000 cP. The formation water salinity is below 5000 ppm and variable within the reservoir, after years of injection of fresh water and steam to increase recovery. Consequently there is today no correlation between water cut and resistivity and the differenciation between oil and water with conventional petrophysical techniques is inaccurate. Conventional log analysis has limited potential since the resistivity shows identical values in both oil and water bearing levels.
Deciding on a completion strategy from an inaccurate saturation computation is a major challenge. Additionally, the free water presence reduces the net pay and rapidly increases the probability of water production in this high oil viscosity environment. Therefore an accurate assessment of free water and oil viscosity is a critical factor in the economics of the field. The present work incorporates dielectric and molecular diffusion measurements, showing significant progress in detecting free water from oil and defining the most prospective intervals.
Movable oil and fresh water are clearly identified using dielectric polarization at multiple frequencies. The dielectric measurement provides the water-filled porosity, while the magnetic resonance identifies the irreducible versus free water within that volume. This allows predicting the likelihood of producing hydrocarbon or water in areas with high oil saturation. In conclusion, the integration of dielectric polarization and diffusion information at multiple depths into the reservoir enable to distinguish oil from free and bound water and to estimate the oil viscosity, a result impossible to obtain with conventional logs in these environments. This integrated methodology allows accurate reservoir characterization and definition of the production potential of these heavy oil sands, leading to improved completion decisions.
The development campaign in Lagunillas sands now has a new workable technique to reduce uncertainties and to optimize heavy oil production.
The first recorded deliberate attempt to stimulate recovery from an oil reservoir by hydrocarbon gas injection was in the Macksburg field, Washington County, OH, ' long before water injection was used for secondary recovery purposes. For almost 60 years, most secondary recovery projects included some form of immiscible gas injection, and its use continued even after the advent of new methods and materials. In spite of this, it was the late 1940's before serious attempts were made to develop quantitative techniques for describing reservoir performance under gas-injection operations, especially with regard to depleted oil reservoirs. Before then, such efforts were directed primarily toward describing the water displacement process. As a result, techniques used to describe the performance characteristics of immiscible gas injection consist of modifications to methods originally developed for describing performance of water-injection operations, even though there is a fundamental difference in the basic displacement mechanisms of the two fluids.
Once an oil exporter, California now depends on imports for more than 60% of its oil supply. This paper examines the oil production outlook for each of California's major oil sources, including California itself. Oil production trends, published geological and engineering reports, and proposed developments in California's supply area are reviewed to define supply trends, especially for the medium-to-heavy, sour crudes that are processed in California's refineries. Refinery upgrading capacity is already highly developed in California, thus it is assumed that a competitive advantage in heavy, sour crudes will continue, although refining heavy oil releases more carbon dioxide.
About a quarter of California's imports are from Alaska, the rest from foreign sources including Saudi Arabia, Ecuador and Iraq. Before foreign sources became so important, California's refining industry processed California's own crudes and Alaska's North Slope crude. Like those crudes, oil from northern Saudi Arabia, southeast Iraq, and Ecuador is also sour and medium to heavy, ranging from 16 to 35° API and from 2 to more than 3% sulfur by weight. By far the most important sour crude development in California's supply area is Saudi Arabia's 900,000 BOPD Manifa project, originally slated for completion in 2011 but now facing delays. Manifa contains oil that ranges from 26 to 31° API and from 2.8 to 3.7% sulfur. Over the longer term, Alaska will continue to play an important supply role if the Chuckchi and Beaufort Seas live up to expectations.
Middle East production is not increasing, yet oil cargoes from the Middle East have to pass growing Asian markets to reach California. Alaska and Mexico also supply oil to the Pacific Basin, but are facing production declines. The effect of rising Asian demand on Pacific Basin oil markets is already visible, with significant amounts of oil coming to California from Atlantic Basin sources such as Angola, Brazil, and Argentina.
The US West Coast pipeline system is separate from the integrated East Coast, Gulf Coast and Midwest system, so energy security issues for the West Coast may differ from those of the country as a whole. There are policy options that could affect California's oil supply security, including increased oil development in Alaska or offshore California, development of additional oil pipeline outlets on Canada's Pacific Coast or substituting natural gas for oil if possible. All of these policy options are currently the subject of political debate.
Historical Oil Production Trends in California's Supply Area
Historical oil production trends are of interest because, unlike reserve estimates, they are readily verifiable factual information. Another issue with published reserve data is the quality of the supporting information; Alberta produces a detailed annual reserves report, while Saudi Arabia and Iraq publish only national aggregate figures. All of the oil production volumes reported in this section are from the annual production survey of the Oil and Gas Journal or the annual report of the Alaska Division of Oil and Gas and do not include natural gas plant liquids.
Iraq's oil production peaked in 1979 at 3.43 million BOPD. In 2007 it was 2.09 million BOPD, but production levels had been affected by internal instability and were higher in 2008.
Interactions of solid mechanics and fluid flow have been studied by numerous researchers for the past several years. Different methods of coupling such as full and iterative coupling have been used. Nevertheless, the accuracy and the large run time of the coupled solid-mechanics fluid-flow model are outstanding issues that prevent the application of the coupled model in full-field studies. In this work, a novel relationship of porosity as a function of pressure, temperature, and mean total stress is developed for iterative coupling of stress and flow. The new formula not only improves the accuracy of the coupling but also reduces substantially the number of coupling iterations. The latter feature decreases significantly the CPU time. The new approach was implemented in a modular, iteratively coupled system. The rapid convergence provides the equivalent of a fully coupled method that is necessary to investigate complex coupled problems. The main advantage of this type of coupling is that a geomechanics module can be easily coupled with different reservoir simulators. The paper gives some comparisons of results obtained by the new porosity formula with another formulation.
After 70 years of continuous exploitation in the Bolivar Coast region of Lake Maracaibo, Venezuela (Fig. 1), advanced reservoir depletion is causing drilling problems such as severe loss of circulation, hole instability, and extensive formation damage. In an attempt to optimize drilling performance, several operation practices were implemented through 1997, such as air-, mud-, foam-, and oil-base mud systems, etc., without achieving the expected success.
Because of the challenges of working in these mature fields, the operator and the oilfield services company created a services alliance contract called PRISA (a Spanish acronym for Integrated Drilling and Workover Alliance Services). This paper describes the drilling technologies and best practices used to improve drilling performance in these mature reservoirs, the learning curve evolution, and process improvement. The results included a cost reduction of 12% (in investment cost/barrel), a production increase of 4%, a 58.7% reduction in drilling time, and a drilling cost reduction of 3.6% (Figs. 2 through 3B). In addition, nonproductive time (NPT) has been reduced from 24.1% in 1999 to 3.8% in 2002 (Fig. 4).
In 1999 the operator entered into a 10-yr contract with an oilfield services company to drill and work over wells in mature areas of Maracaibo Lake. In this alliance the operator will share risks and involve the services company in the operations performance through an incentive-penalty scheme with the objective of reducing operation times and costs, protecting the environment, maximizing reserves recovery and introducing technologies to achieve the desired goals.
The engineers pooled their knowledge and expertise to build multipurpose green rigs designed for well construction and work over on Lake Maracaibo environment. Likewise, each rig has a sophisticated treatment system to recycle wastewater into water suitable for industrial use. The document presents how this system can avoid pollution and the wastes are sent onshore to be taken to a zone designated for their final disposal or recycling and the sophisticated system that these rigs have to operate with zero discharge of any pollutants in Lake Maracaibo.