Defining the reservoir by its porosity, permeability, fluid content, water saturation, and behavior is one of the predrilling goals of exploration and production teams, with the aim of optimizing reservoir production over the life of the field.
Advances in Reservoir Characterization
Very few 2D measurements of any kind are taken from inside a wellbore, except for temperature and reservoir microseismic activity. Distributed temperature sensing (DTS), which uses fiber-optic cables, can provide a full temperature profile. However, to obtain pressure, flow, or fluid information, results are extrapolated and mathematically calculated with an inferred answer. Thus, it does not measure the reservoir directly, continuously, and in real time.
Some oil companies are moving toward time-lapse, 4D, seismic techniques that can identify infill drilling opportunities, predict flood fronts, and help avoid early water breakthroughs, despite the cost and complexity of these techniques. Seismic surveys are mostly for the identification of rock formations and subsurface faults, and much of the fluid analysis is inferred from these rather than directly measured.
Knowledge of the position and thickness of the oil rim in particular is critical for effective reservoir observation. To achieve maximum production, it is important not only to be able to measure the effects of injection operations on a reservoir, but also to monitor long-term reservoir movement and behavior. The only alternative way to keep track of the position of an oil rim is with periodic logging surveys, which can be both inaccurate and expensive.
Advances in seismic survey techniques have enabled much greater analysis of fluid chamber and reservoir movement. The gradual development and deployment of this type of technology has taken more than 50 years from single-point seismic source methods using an electronic transponder to 4D methods, which can map the movements, fluids, and sensitivities in the reservoir.
A Clearer Vision
With dwindling hydrocarbon reserves and increasing oil and gas demand, innovation in characterizing reservoirs is crucial to sustaining supply. Reservoir geophysics incorporating the latest software and reservoir characterization techniques is bringing significant improvements to production in existing fields.
Production geoscientists are heavily involved in characterizing reservoirs, particularly describing how reservoirs have been affected by ongoing production processes. The combined use of time-lapse and multicomponent data, such as seismic, logging analysis, and DTS, has had a dramatic impact on reservoir understanding that has led to improved production over the life of the field.
Knowledge and understanding of how production processes can be modified and enhanced in accordance with ongoing reservoir activity has become close to a reality. Continuous, real-time fluid modeling with high resolution is notoriously difficult because of the range of porosities and geometries. Nonetheless, advances have improved the ability of geoscientists to determine and examine a range of reservoir characteristics, including:
- Contents, fluid movements, and behavior
- Static pressure
- Flow and porosity
- Changes such as water saturation, reservoir depletion, temperature, and pressure that occur as the reservoir is produced
- Subseismic resolution faults and other production barriers
Most production logging and pressure measurement may be straightforward in naturally producing wells, but with approximately 95% of the world’s oil wells unable to produce naturally, the technology and tools required become more complex.
The use of a pump can create problems for instrument systems. The producing zone is usually below the pump, an area in which few analysis tools can be used while the pump is running. The size of the pump may also mean that few production logging tools can be run into the wellbore. Thus, analysis of activity below the pump is virtually a blind exercise. The development of cable-based sensors in conjunction with new ground fault immune technology for monitoring electric submersible pumps (ESPs) allows complicated data to be transmitted from below an ESP.
The demand for a device that can work below the pump and provide a real-time measurement of fluids has long been a holy grail for the reservoir engineer. A technology that will measure the position of the top and bottom of the oil rim in observation wells and potentially allow fluid measurement below pumps is under development.
A View From the Wire
Reservoir characterization models are used to simulate the behavior of the fluids in the reservoir under different conditions to determine the optimal production techniques to maximize production. The Zenith True Oil Rim Measurement (ZTORM) system provides reservoir engineers with dynamic brine/oil and gas/oil measurements for the first time, enabling accurate observation of reservoir levels, depletion, and fluid behavior, and steam flood applications for monitoring purposes. The information helps engineers protect the reservoir against water coning.
The system is a cable-based sensor that measures the fluid contacts anywhere along its length. It can be lowered into live observation wells (Fig. 1) using a crane and wellhead equipment to allow rigless deployment.
The technology delivers high-resolution measurement for oil and water simultaneously in real time and provides a new measurement capability. At present, the system is designed for observation wells in which the fluids are settled, allowing it to measure the depth of the water and oil simultaneously. It can be used as a permanent or semipermanent installation, and the information that it provides includes
- Brine/oil interface level
- Oil/gas interface level
- Reservoir sump pressure
- Reservoir sump temperature
- Gas cap pressure
- Gas cap temperature
- Confidence level on measurement
- Status register
In general, the data measured by the system is relatively simple and is transmitted digitally from downhole to the surface. As always, the most complex information will involve the behavior of the well. In some cases, it may be beneficial to reproduce the signals that the system has read. For this, Zenith has a test rig in which downhole conditions are simulated so that the reproduced readings are validated by observation.
The system can be run in hole with a mobile crane on an existing observation well with perforations and can be installed on a live well. The installation can be done on a turnkey basis without the need for a workover rig. The device is designed for a long service life and can be employed over lengthy periods.
For observation wells with a simple cased hole, the system is supplied with a surface actuated anchor for easy deployment and retrieval.
The system’s cable is able to respond to the fluids that surround it. It does this over its whole length, so it must be fully immersed in the fluids being examined. The cable is made of advanced polymer, steel, and wire and has a tensile core and a hard outer shell for pressure sealing.
The electronics that transmit the signals from the system’s two pressure sensors are based on technology that has been used by the oil industry for more than 20 years and by Zenith for 9 years with very high success.
Viability and Reliability
Developed over the past 5 years, the system has been extensively proved in a test rig at the company’s headquarters in Inverurie, Scotland. A simulated oil rim was created thousands of feet beneath the surface. The position of the rim could be moved under computer control, thus allowing the sensor systems to be evaluated for operating range and accuracy resolution.
Results confirmed good long-term stability and accuracy of all fluid interface measurements, as well as very good resolution and sensitivity, measured to the scale of a few centimeters. Features have been added to improve the system’s gas/oil interface sensitivity and the mechanical design allows the option of rigless deployment. The materials used to build the system were developed to enable its use in harsh environments.
Providing permanent pressure gauges at the top and bottom of the sensor array gives the operator wellbore measurements, as well as level information, which can be used to cross check fluid level readings from the sensor cable. The level information can include a pressure gradient survey at installation, if needed.
The system is capable of measuring more than two fluid interfaces, and there is ongoing development to extend the system’s capabilities. A successful feasibility study has been carried out, and the system has measured a slug of oil floating upward in a pipe and settling on water at a 4,000 ft depth measured along a wire. Thus, the system is able to view and image moving fluids. Several near-term field trials are in discussion with operators.
Conclusion
Globally, the size of hydrocarbon discoveries is decreasing while demand for oil and gas continues to grow. In many countries, the rate of production exceeds the rate of reserves replacement. Yet sufficient reserves may be there, if improved technology can enable them to be located or extracted more efficiently.
Reservoir geophysics combined with the latest software and reservoir characterization techniques is bringing significant improvements to production in existing fields. The continuous improvement of production efficiency and extension of field life will likely require a more continuous process of reservoir monitoring and depletion analysis that incorporates much higher levels of detail than before.
The ZTORM system allows direct empirical measurement of reservoir fluids in real time, taking responses directly from the different characteristics of the oil, water, and gas. It is a continuous, real-time 2D tool that has the potential to run below the pump and is not as expensive or complex as 4D seismic. The fluid level system is unique so the measurement techniques will be new to the oil industry. While this poses a challenge, the new level of information obtained on reservoir characteristics and behavior has the potential to bridge the gap between reservoir analysts and production engineers by providing a detailed, real-time visual image of how fluids migrate and flow throughout the reservoir.