5.4. Production Logging Tools
Extraction of raw materials such as gas, oil or water from the ground requires a systematic process that minimizes the costs and maximizes on the profit to be accrued from the extraction process. For the processes to achieve their intended goals, drillers must conduct routine tests on the reservoirs to check for any dynamics and deviations within the well that might affect the production and performance of the system. These routine tests are collectively known as reservoir monitoring processes. And, they work by acquiring data through various acquisition systems known as performance monitoring logs. These logs quantify on a real-time basis the production process and pressure distribution across the system (Peng and Zhang, 2007). In addition, analysis of fluid dynamics analysis with an aim of correcting any deviations are carried out. Some of the parameters analyzed include temperature, pressure, and velocity of the fluid among others. As part of dealing with the dynamism of the well/reservoir parameters, reservoir monitoring involves the use of data logs and acquisition systems. The paper here discusses the use and significance of data logs such as temperature log, pressure-density logs, spinner logs and multi-array logs in reservoir monitoring. It will also describe how they are used in the real-time monitoring of the reservoir as part of performance measurement and optimization.
5.4.1 Temperature Log
One of the most important parameters in reservoir monitoring is the temperature gradient. It is a parameter primarily governed by the proximity of the reservoir to the mantle of the earth and the relative heat/thermal exchange capacity and conductivities of the structures forming the lithosphere, which the reservoir is part of. These temperature gradients vary from one basin to another and thus it is prudent to use loggers that monitor any anomalies or variations detected in the temperature (Liu et al., 2017). Temperature logs are used in the identification of these variations and record data either through analog or digital means. The temperature log tool comprises of a cage, that has an opening connected to the reservoir fluid, located at the bottom end. The cage houses a thermistor, which acts as a sensor to the surrounding temperature of the fluid in the reservoir. Platinum rods are preferred as the sensors in the temperature logs because of their ability to offer electrical resistance which varies proportionately and linearly with the temperature over a given range (Liu et al., 2017). Each time a variation is noted, a log is entered and recorded as a pulse. Therefore, the equipment works based on the principle that voltage across the platinum sensor of the temperature log varies as the electrical resistance of the sensor.
It is important to use temperature logs in reservoir monitoring as temperature gradient provides crucial information about the structure of the reservoir and the geologic structure holding it. It is also applied in finding zones that produce or take up fluid. This is important in evaluating the hydraulic fracture treatment to be conducted as well as the location of the casing leaks and circulation zones. Owing to the fact that temperature takes a considerable amount of time before dissipation, temperature logs thus reflect the behavior of reservoirs over a given period of time compared to other parameters.
5.4.2. Pressure-Density Log
Although different techniques are explained in theory, in practice, the pressure-density logs work by predicting the pressure gradient that exists within the vertical pipes in which fluids such as oil, gas and water flow. Fluid density obtained via scattering methods in which gamma rays from a Caesium-137 are utilized help in the estimation of density in those vertical pipes (Hyne, 2014). These densities are then mathematically calculated using algorithms in the logs to come up with pressure logs on a real-time basis when two or more phases of fluids are encountered.
Based on the fact that pressure-density gradient relates to the depth and rock structure of the reservoir, the information obtained by pressure-density logs is crucial in the identification of cracks/porosity of the rocks in the reservoir and the water-filled gaps within the rocks. And, this is important in knowing the nature of drilling to be conducted optimally and with minimal risks associated with the drilling of wells/reservoirs. The determination of pressure-density gradient in the production and well-injections in reservoir monitoring is crucial in helping drillers gather information and data on the effects of pipe-sizes, flow-rates and the pressures they exert on each other and the fluid (Liu et al., 2017). This information alongside density is also used in the estimation of the gas injection rates and the power required in lifting oils or fluids in the case of operations such as gas-lifts.
5.4.3 Spinner Logs
While pressure and density remain the key component or parameters in well-injection, the fluid velocity plays a critical role in helping drillers and geologist in knowing the nature of drilling to conduct as well as the type and size of pipes in bringing the fluid onto the surface. This information is required on a real-time basis to help them make an accurate and prudent decision on the same. Information on the fluid velocity is acquired through spinner logs (Darling, 2005). The spinner logs have fan-blade like objects that rotate at given velocity. These spinners come into contact with the fluid within a well and the extent of change in velocity of these spinners as a result of contact is then related mathematically to the effective fluid velocity on a real-time basis (Bakulin et al., 2015). The spinner logs are used to help drillers in the decision making during injection and production in wells/reservoirs.
5.4.4 Multi-Array Logs
In conventional drills, use of single production logs may not be adequate, and this may jeopardize the performance and production process. Therefore, as part of dealing with the inadequacies of using a single log tool especially in horizontal and deviated wells is by having multiple sensors known as multi-array logs (Bakulin et al., 2015). These logs provide a comprehensive analysis of the characteristic of downhole flow systems and their conditions (Liu et al., 2017). Each sensor serves a particular function such as identification of velocity, temperature, density, water capacity and even angle of well inclination. Most multiple array logs combine logs such as resistance array tools, spinner arrays, and gas hold up tools to optimize the well performance. They are used in the determination of flow behavior/regimes at different stages and points within the cross-section of the well. In addition, water entry point and velocities can be identified on a real-time basis using multi-array logs.
Bakulin, A., Jervis, M., Colombo, D., Tsingas, C., & Luo, Y. (2015, December). Bring geophysics closer to the reservoir-A new paradigm in reservoir characterization and monitoring. In 2015 SEG Annual Meeting. Society of Exploration Geophysicists.
Darling, T. (2005). Well logging and formation evaluation. Elsevier.
Hyne, N. (2014). Dictionary of petroleum exploration, drilling & production. PennWell Corporation.
Liu, Y., Li, J., & Wang, Y. (2016). Monitoring Technology for Wellbore of Injection-Production Wells in Gas Storages.
Peng, S., & Zhang, J. (2007). Engineering geology for underground rocks. Springer Science & Business Media.
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