Today's wireline technologies have surpassed previous boundaries to help operators focus and refine their production optimization efforts. Improved measurements and reduced costs are just part of the story.

Asset optimization is the leading business driver in today's oilfield. And for those focused on the reservoir, production optimization is the goal. With about 70% of the world's fields being more than 30 years old and the demand for energy increasing over current levels - 16 billion bbl more needed annually by 2020 - the drive to maximize recovery from known productive formations, as well as identify and complete bypassed pay surrounding existing wells, is fundamental.

Anticipating oilfield aging, significant research and development has been and remains focused on adapting and creating new technologies and services to help meet the full range of production optimization needs. In just the wireline arena alone, today's breakthrough technologies provide: reservoir analysis behind casing both near the wellbore and deep into the reservoir; deliver clean, reliable perforations; and analyze multiphase flow in any well scenario.

Analysis behind casing

What could only be dreamed of a generation ago is a reality today. Resistivity, bulk density and formation pressure are now being measured behind casing. The latest formation evaluation technologies offer cost-effective, valuable data:

• When looking for bypassed or additional pay in older wells;
• When monitoring saturation, depletion and reservoir pressure;
• As an alternative to open hole logging of development wells;
• When managing wells with high drilling risk by logging after setting casing; and
• As a complement to both open hole and while-drilling logs.

These data acquisition capabilities are applicable in any age well, and, importantly, are far less costly than drilling a new well.

Among the advanced technologies available to analyze formations behind steel casing is a formation resistivity measurement that is used to calculate saturation out to more than 10 ft (3 m) from the wellbore - a significant improvement over the 9-in. range typical with the standard pulsed neutron approach. The new technology works by introducing electrical current to the casing and measuring the voltage drop, which is proportional to formation resistivity, that occurs as a small amount of current escapes into the formation. Cased hole resistivity tools operate at speeds up to 240 ft/hr (61 m/hr) and provide resistivity measurements deeper into the formation than their openhole equivalents. A Wyoming operator is enjoying increased productivity by pinpointing unswept zones using this cased hole resistivity tool for water flood surveillance.

The openhole, integrated wireline density measurement has been characterized to provide the same density measurement behind casing. An extensive data set was collected in numerous formation blocks and casings with cement sheaths to engineer a density algorithm exclusively for cased hole logging of bulk density. At the same time, a cased hole formation porosity tool is available that measures both formation porosity and sigma. Use of an electronic neutron generator and borehole shielding in this tool enable higher-resolution measurements with fewer environmental effects than the standard chemical source tools, especially in shaly formations.

Acquiring resistivity, density and neutron measurements behind casing now enables the use of traditional openhole interpretation methods, such as Archie saturation, for cased hole evaluations.

Meanwhile, accurate measurements of formation compressional and shear slowness, which are key for evaluating the mechanical properties needed for oriented perforating and fracture design, are provided by the dipole shear sonic imaging tool. The resulting data also aid gas, fracture and anisotropy detection, assist with permeability evaluation, and contribute to geophysical interpretations.

A unique tool, the cased hole dynamics tester collects fluid samples behind casing and measures multiple formation pressures. The tool drills a small hole through casing and cement and into the formation. After testing and sampling, it plugs the hole, ensuring casing integrity. This approach provides a cost-effective method to optimize recompletion plans, enhance old or incomplete log data, assess pay zones and evaluate wells for their economic potential.

While each of the techniques mentioned can be used alone, they also are combinable, supported by customized software that prepares composite interpretations of the measurements.

Imaging deep into the reservoir

The most exciting wireline evaluation technique available today, in either cased or open hole, is actually an extension of the induction logging process. Deep electromagnetic imaging allows wireline evaluations to be made at the reservoir level, versus only a few feet from the borehole, which is useful for locating bypassed pay, imaging water and steam floods in time-lapse fashion, establishing well correlations, and mapping interwell structure. The measurements can be taken between two steel-cased wellbores separated by up to 1,000 ft (305 m) or between two openhole wells that are up to several thousand feet apart. Cross-hole data are interpreted by numerical inversions to give a map of interwell electrical resistivity. In the near future, deep electromagnetic measurements and well log data will be combined to create 3-D images of saturation across the reservoir.

Connecting reservoir and wellbore

Clean, precise perforations enhance productivity and are the goal of any perforation job. However, until recently, perforation reliability was tough to ensure, as no uniform quality control standards were available to regulate shaped charge manufacturing. To rectify this problem, Schlumberger assisted the American Petroleum Institute in instigating a Perforator Design Registration Program and companion recommended practice, API RP19B. The program requires API observers to witness and verify shaped charge testing. As observations are completed, the resulting test reports and evaluation data sheets are posted on the API Web site.

Meanwhile, taking dead aim at the clean perforation goal, a new service that utilizes specialized perforation guns has been developed. Studies have shown that during underbalanced perforating, which is currently the technique of choice for maximizing productivity, the downhole pressure dynamics that occur within milliseconds of perforating with standard guns can vary widely and limit ultimate perforation performance. The new service provides independent control of the post-perforation surge and reduces dynamic wellbore pressure without requiring a high initial underbalance. This reduces pressure on the perforating string and can provide cost savings on nitrogen and completion fluids. Field use has demonstrated that this system delivers consistently cleaner perforations, eliminating the need for perforation clean-up jobs to stimulate production. An operator in Europe reports 100% production increases in gas reservoirs when applying this technique.

For perforation placement accuracy, of particular importance when hydraulic fracturing is used, a wireline oriented perforating tool is used to ensure the perforations are azimuthally oriented with the preferred fracture plane of the target zone(s). Precisely oriented perforations enable use of larger proppant sizes and/or higher proppant concentrations for aggressive stimulations, better proppant packs, improved fracture conductivity and reduced opportunity for proppant screenout. An operator in North America reports 120% improvement in its fracture stimulations and 100% placement of the desired proppant volume using this technique.

Understanding the flow

In producing wells, hydrocarbon production can often be increased by analyzing thoroughly downhole flow regimes and determining exactly where production is entering the wellbore. The traditional production logging tools that have been used to achieve this end recently have taken a performance leap thanks to innovative sensor design. Electrical and optical probes have been integrated into a compact logging platform that provides oil, gas and water flow-rate measurements for multiphase fluid analysis in vertical or deviated wells. These measurements can even be taken down to the furthest reaches of the borehole.

Additional sensors were added to this production logging platform to evaluate the exceedingly complex flow regimes present in high-angle and horizontal wells. This suite can differentiate water and hydrocarbon entries, detect the flow regime distribution, measure individual phase hold-ups and determine the flow rates of each phase in wells of any deviation. An integral part of this service includes real-time wellsite analyses for early results, followed by comprehensive post-acquisition evaluations. Together, these diagnostic services provide a complete understanding of well flow dynamics that boost production optimization design.

Moving forward together

Together, operating and service companies are meeting today's dual challenge of increased demand from maturing fields, a challenge that is requiring increasingly precise, sophisticated technological solutions. As needs are identified, solutions are found by consistently searching beyond the boundaries and collaborating in unprecedented ways. Where single-issue or single-discipline answers were once the norm, more integrated processes and solutions now are being applied to gain unparalleled efficiencies and improvements in productivity.

Without question, the exploration and production landscape will continue to change. Thus, to assume anything about the boundaries of a technology today is folly. Companies such as Schlumberger are persistently pushing the envelope and developing new solutions to today's challenges, such as the wireline advances reviewed herein.

In this environment, it is prudent to stay abreast of current capabilities and to routinely ask, "what is possible?" Just as we have now pushed past the boundaries of cased hole wireline technologies, what we perceive as the current technology boundaries will be overcome - and perhaps quicker than expected. Stay tuned.