Defining Technology for Exploration, Drilling and Production October 2007

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RESERVOIR CHARACTERIZATION

WIRELESS SEISMIC ACQUISITION

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RESERVOIRCHARACTERIZATION

While overall increased drilling activi-ties and challenging drilling applications are capturing the headlines, there is a quieter, but potentially more significant movement within the oil and gas indus-try. This new direction emphasizes using information to manage the reservoir for optimal reserves recovery over the full life cycle.

Today, despite the many advances in reservoir characterization technologies and techniques, the recovery factor for oil reservoirs still averages less than 40%. It is estimated that in the US there are in excess of 374 billion bbl of stranded oil with almost 100 billion bbl recoverable. Extrapolated on a global basis, bypassed oil con-stitutes perhaps the final fron-tier for the industry, with well over 1 trillion bbl in play. With crude prices high, and new ex-ploration involving significant expense and risk, there is a great deal more emphasis on opti-mizing the recovery rate from existing and new reservoirs.

VERTICAL INFORMATION GAP

The inadequacy in reser-voir information is revealed by reviewing the resolution and coverage of presently available reservoir life-cycle informa-tion sources. Fig. 1 provides an overview of the reservoir information ranges covered by the various available tech-nologies. A quick review shows that most of the information is available at a scale suitable for reservoir identification and its bulk characteristics.

While conventional 3D processes generate a workable aerial view of the reservoir, the technology provides little

information about a reservoirs vertical heterogeneity. In addition, while such conventional processes as well logging, core sampling, and production data analysis do generate information about vertical heterogeneities, they are limited to within a few inches of the well. New, deep investigative resistivity and sonic log tools add only a few feet more to the area measured around a well.

What is missing is real information concerning the vertical distribution of heterogeneities between wells. Statistical methods do not provide the hard data

necessary to power reservoir understand-ing and effective management strategies.

CROSSWELL IMAGING Crosswell imaging provides signifi-

cant data to help fill the vertical infor-mation gap between wells, Fig. 1 green zone. Offering a resolution many times greater than any surface method, this technique provides a detailed look at ver-tical heterogeneities between wells.

The typical surface seismic view, Fig. 2, provides limited information about the reservoirs presence, perhaps its thickness

and a few bulk properties. The detailed insidethe-reservoir view indicates a multi-layer, faulted reservoir. The resolu-tion difference in the two seis-mic methods is shown by the comparative views.

Crosswell imaging employs a seismic source and receivers, both placed in wells at the res-ervoir level. To image reservoir intervals as thin as 0.5 m to 1 m, the system passes special-ly-engineered sound waves through the area of interest, from source to receiver, Fig. 3. The change in the sound waves, as they pass through the interwell section, provides both horizontal and vertical information.

This technique is designed to provide a detailed reservoir view for up to a kilometer between wells. In addition, multiple surveys can provide reservoir continuity and con-nectivity data across a field. This detailed data is readily integrated into an existing geo-logic and reservoir model, re-sulting in a highly effective and comprehensive tool for long-term reservoir management.

Information-driven life-cycle reservoir managementBetween-well data from crosswell imaging is one key to optimizing recovery.

Raul Ortiz, Raul Ortiz and Associates

Fig. 1. Available technologies provide a range of reservoir information covered at a scale suitable for the reservoir and bulk characteristics identification.

Fig. 2. The typical 3D surface seismic view (left) provides limited reservoir information, while crosswell imaging (right) reveals a faulted, multi-layered reservoir.

OCTOBER 2007 World Oil

Reprinted from: October 2007 issue, pgs 99-102. Used with permission.

Understanding the between-well data is critical to locate left-behind oil. This inter-well information also identifies the production mechanisms within the reservoir and can reveal op-portunities for optimizing pro-duction.

INTEGRATING TECHNOLOGY

No single technology is a panacea for optimum reservoir management. Rather, global integration of all information sources will be the key to a fu-ture revolution in reservoir life-cycle management. Effective reservoir management will be driven by more and better data throughout the reservoirs life. In fact, operating companies have aligned themselves into cross-functional teams to better manage diverse information.

In this new paradigm, reser-voir monitoring is also provid-ing a more continuous flow of information about the reser-voirs condition, becoming the driver for proactive remedial reservoir action. Armed with current data and an in-depth understanding, an asset team is now able to anticipate a reser-voir problem before it reaches the well. Many common prob-lems, such as water encroach-ment and gas leakage, can be proactively addressed before they become critical.

RESERVOIR LIFE CYCLEThe key to improved reser-

voir management and a higher recovery rate is more and bet-ter data about the reservoir, its structure and properties. To effectively acquire quality data, priority and budget must be given to information-generating activities at all stages of a reser-voirs life cycle: exploration, discovery, de-lineation, production, secondary recovery, tertiary recovery and abandonment.

Many oil companies are faced with mature fields in need of revitalization. In the past, revitalization was strictly focused on well processes: workover, recomple-tion and other remedial action. Today, deep-investigative crosswell methods can drive reservoir revitalization.

Even though information is indispens-able in life-cycle reservoir management,

it is not without its cost. The extra time spent to test and analyze increases the price of drilling. In addition, there are risks to the well. These factors always lead to some educated compromise. However, a value proposition that can save a water-flood pilot, avoid a failed deep horizontal well or prevent premature water encroach-ment justifies the investment.

OPTIMIZED FIELD DEVELOPMENT

A May 2005 filing with the Wyoming Oil and Gas Commission by Ultra Petro-leum demonstrates the value of integrated

information for strategic devel-opment decisions about a tight gas reservoir. Ultra integrated information from crosswell imaging, 3D surface seismic, cores, logs, production data and microseismic operations to de-termine that it was feasible to use tighter infill spacing in its Jonah field in Utah in the west-ern US.

Crosswell imaging provided the unique high resolution information that allowed the companys asset team to direct-ly image thin sand channels in the reservoir, Fig. 4. From this, Ultra determined that the res-ervoirs average sand channel size would allow 10-acre well spacing. The imaging also con-firmed that hydraulic fracture dimensions, determined from microseismic monitoring, were conformable and constrained by the sand channel geology. Ultras information-based deci-sions eventually resulted in re-ported incremental recoverable reserves of 14.9 Tcf.

SECONDARY RECOVERY While secondary recovery

through waterflooding often yields half of the total recov-ery in a field, this process has been traditionally implement-ed without detailed reservoir knowledge. This is an impor-tant point, since the potential for reservoir damage can result from an inadequately planned and poorly executed water-flood program.

Despite this, waterflood pi-lots are often little more than injectivity tests. The success or failure of the pilot is judged

simply by what is recovered at the pro-ducer. Waterflooding is inherently a between-well operation, which can be thought of as crosswell injection.

An in-depth understanding of con-nectivity and continuity are crucial to understand the sweep in secondary re-covery. Inside-the-reservoir images are essential for better reservoir understand-ing, flood planning and monitoring.

A crosswell image can provide a clear geologic picture between wells and mon-itor both flood fronts and pressure be-tween wells over the projects life. And, while pressure monitoring and chemical

Fig. 4. Crosswell imaging provided high-resolution data to directly image thin sand channels in Ultra Petroleums reservoir.

Fig. 3. Crosswell imaging uses a seismic source and receivers placed in wells at the reservoir level.

OCTOBER 2007 World Oil

RESERVOIR CHARACTERIZATION

tracers are good indicators of what has reached a producer well, neither effec-tively describes the vertical characteristics between wells or determines swept and unswept intervals. By running a baseline

pattern, new information can assist in planning injector and producer well lo-cations and selecting depths for injection and production. Once water injection is initiated, repeat surveys can establish the front between wells, thief zones and ver-tical channels that may impact sweep.

A monitoring example from a hetero-geneous reservoir shows that a uniform sweep assumption is often not accurate, Fig. 5. In this case, the sweep was into a zone not completed in the production well. The lack of communication injec-tor-producer masked an opportunity to produce new oil and optimize waterflood pilot success.

Monitoring provides the information necessary for quick and effective remedi-al action within the secondary recovery process. Armed with in-depth interwell information, the operator can shut-off injection into thief zones, reorienting injection patterns to honor the observed sweep and recomplete wells to match the oil channels.

CONCLUSIONWith demand for oil and gas show-

ing no signs of slowing, and the cost of drilling escalating, it is imperative to

maximize economic recovery from res-ervoirs. Higher commodity prices, lack of investment opportunity in new explo-ration, constraints on margins in many international concessions, and declines in the major fields around the world are all driving the focus to existing reservoirs and optimized recovery through reser-voir life-cycle management.

New information technologies such as crosswell imaging are filling the criti-cal information gap within the reservoir between the wells and driving a new life-cycle approach to reservoir production and management. Operators are now equipped to address the new frontier in-side the reservoir and realize increasingly better recovery rates. WO

THE AuTHOR

Raul Ortiz earned a BS degree in petroleum engineering from the Polytechnic Institute of Ecuador and an MS degree in petroleum engi-neering from the Colorado School of Mines. Pri-or to starting his own company, he worked for Texaco in a range of executive positions in pe-troleum engineering, operations, new business development, and technology management. Ortiz is president of Raul Ortiz and Associates, an oil and gas reservoir consulting firm.

Fig. 5. This waterflood monitoring example shows that a uniform sweep assumption is often inaccurate; this sweep was into a zone not completed in the production well.

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