Simple & Accurate Device for Water-Cut in Oil Measurement

Transcription

1 Simple & Accurate Device for Water-Cut in Oil Measurement M. Habli M. Meribout A. Al-Naamany A. Maashari Electrical & Computer Engineering Department Sultan Qaboos University P.O. Box 33 Muscat 123 Oman K. Al Busaidi A. Al Ismaily Petroleum Development of Oman P.O. Box 81 Muscat 113 Oman Abstract This paper presents a simple, accurate and inexpensive measuring device for water-cut in oil. The new device is based on the relationship between the water-cut in oil & water mixture and the pressure of a sample from the mixture. Experimental results show that the device can attain very high accuracy that can reach up +/- 0.4% and it can be used to measure a full range of water cut levels (0-100%). Keywords: Water-cut, Oil, Pressure, Simple, Accurate, Inexpensive 1. Introduction Oil producing institutions in many parts of the world are still facing issues with the produced oil, which consists of high percentage of water and gas. Knowing the percentage of each phase is essential for oil production. Pipelines from different nearby wells carry the oil to a central pipeline that feeds into main reservoir. Produced oil from each well is usually monitored using BS&W instruments in order to control the quality of the oil that is going to the main reservoir. BS&W instruments determine the water cut or the percentage of the water in the produced oil. Conventional BS&W instruments are based on measuring the capacitance or impedance of the oil [1,2]. However, such measurements are not accurate enough and can t be used when water cut exceed certain limits. Other devices such as radioactive and microwave [3,4,5] are harmful and they require a special operation procedure. Fiber optics sensors [6,7,8] are also available and can provide up to +/- 1% accuracy. However, these devices are expensive. This paper presents an inexpensive, simple and accurate device which is based on the pressure of a sample from the oil and water mixture. Section 2 includes the proposed device concept. Mathematical formulations are in section 3. Experimental setup is presented in section 4. Section 5 has experimental results analysis. Results are in

2 section 4. Finally the conclusions are in section The Proposed Water-Cut Measurement Method The proposed measurement method of the water-cut in a mixture of oil and water is based on the pressure of a sample from the mixture. Consider the system in Fig. 1.The system consists of an apparatus where the pressure of a liquid s sample can be measured. The needed sample from a liquid is of a fix quantity or height H s. In principle, for water and oil mixture, the mixture s sample will consists of H o and H w, where: H o + H w =H s (1) The pressures of the oil and the water are given by Eqs. 2 and 3 respectively: P o =ρ o *H o *g (2) P w =ρ w *H w *g (3) Where P = pressure in kilo Pascal; g = gravity in kg.m/s 2 ; H = height/level in meter (Pressure Head); and ρ = density in kilogram per cubic meter. The subscripts (o, w and s) stand for oil, water and sample respectively. The total pressure P t at the bottom of the container is equal to: P t =P o +P w +P a (4) where P a is the atmospheric pressure. Subtracting P a from Equation 4, yields: P s = P o +P w (5) P s is the pressure of the mixture s sample which reflects the heights or the fractions of both oil and water in the sample. This system can be calibrated to indicate the fractions of water and oil for different mixtures. 3. Mathematical Formulation Substituting Eqs. 2 & 3 in Eq. 5 yields: P s =ρ o *H o *g+ρ w *H w *g (6) Eqs. 1 and 6 are two independent equations in two unknowns, H o and H w. The sample quantity H s in Eq. 1 is known and fixed. On the other hand, the sample s pressure P s in Eq. 6 will be measured pressure. For known ρ o and ρ w we can solve for H o and H w as: H o = [(P s /g)-ρ w *H w ]/( ρ o -ρ w ) (7) And H w =H s - H o (8) This device can also be used to find ρ o and ρ w from the pressures of two samples of known heights. For example, Eq. 6, for the first sample can be written as: P s1 =ρ o *H o1 *g+ρ w *H w1 *g (9) Similarly, for the second sample, Eq. 6 is written as: P s2 =ρ o *H o2 *g+ρ w *H w2 *g (10) Eqs. 9 & 10 can be solved for: ρ w =[( P s2 /g)-ρ o *H o2 ]/H w2 (11) and 2

3 ρ o = [(P s1 /g)-(h w1/ H w2 )*( P s2 /g)]/[h o1 -(H w1 * H o2 )/H w2 ] (12) 4. Experimental Setup The experimental setup consists of a piezo resistive pressure sensor. The sensor is attached at the bottom of an apparatus to measure the pressure of a sample. The output of the sensor is connected to an amplification circuit as shown in Fig. 2. The H s for all the experiments is set to 25cm. The pressure sensor that is used in the experiments is a differential type, i.e. the output of the sensor indicates P s as in equation 5. A number of experiments were conducted to test the accuracy and the capability of the device. The water used in the experiments is tap water. The oil used is motor oil. 4.1 Experiments In the first experiment, a full range is conducted starting from 25cm water and zero oil and ending with zero water and 25cm oil with a 1cm steps. Fig. 3, shows the results. From the results it is clear that since oil has less density, the pressure dropped as the oil level increased. The results show almost a linear relation between the oil level and the voltage. The second experiment, is designed to test the accuracy of the device and if it can detect small changes in the mixture. In this experiment, we started with 25cm water and zero oil. We increased the oil in the mixture from zero to 0.8cm in 0.1cm or 1mm steps. Fig. 4, shows the results. From the results it is clear that the device can detect the 1mm change in the mixture of 25cm. This change is equivalent to +/- 0.4% accuracy. A third experiment is conducted and was also to test the accuracy of the device at another range. The range taken is from 20cm water and 5cm oil to 19cm water and 6cm oil in 1mm steps too. The results are shown in Fig. 5. In the fourth experiment, another mixture range is taken. We started with 5cm water and 20cm oil to 3cm water and 22cm oil in 1mm steps. Results are shown in Fig Analysis of the Experimental Results From the experiments that were conducted, we can conclude that the device can be used and trained to detect water-cut in an oil and water mixture. The results of the first experiment show a clear change in the output voltage as the percentage of the oil increase in the mixture. The other three experiments that were conducted show that the device can detect small changes at different oil level ranges. In all the ranges that were considered the device was able to clearing detect 1mm change of oil in the mixture. The 1mm change in a 25cm level is equivalent to +/- 0.4% accuracy. One other very important conclusion is that the device can operate over full range of water-cut from 0-100%. 6. Conclusions An inexpensive, simple and accurate measuring device for water-cut in oil is presented. The device is based on the relationship between the water-cut in oil & water mixture and the pressure of a sample from the mixture. Experimental results show that the device can attain very high accuracy that can reach up +/- 0.4% and it can be used to measure a full range of water cut levels (0-100%). 3

4 7. Acknowledgment This work is sponsored by Petroleum Development of Oman, PDO. Conference Proceedings. 10th Anniversary. Advanced Technologies in I & M., 1994 IEEE, page(s): vol References 1. Smit, Q. Mortimer, B.J.P. Tapson, J., General purpose self-tuning capacitance sensor [for oil recycling and soil moisture measurement application], Instrumentation and Measurement Technology Conference, IMTC/98. Conference Proceedings. IEEE, page(s): vol.2 2. Lucas, G.P. Flow rate measurement in vertical oil-water flows using conductivity sensors and a void fraction wave model, Advances in Sensors for Fluid Flow Measurement, page(s): 13/1-13/3. 3. Wu Qun Deng Shaofan, A high-accuracy microwave sensor and calibration for measuring three-phase saturations in cores, Instrumentation and Measurement Technology Conference, IMTC/ Wu Qun Deng Shaofan A microwave technique for measuring three-phase saturations in cylindrical cores, Precision Electromagnetic Measurements, Digest., 1994, page(s): Beckwith, T. G. et al. Mechanical Measurements, Addison Wesley Publishing co. pp , G. Betta, LM. D Apuzzoand al, An Intrinsic optical fiber sensor, Imeko TC4, pp , T. G. Giallorenzi et al, Optical fiber sensors challenge the competition, IEEE Spectrum, K. T. V. Grattan, Recent advances in fiber optic sensors, Measurement, Vol. 5, pp ,

5 P s = P o +P w H o ; P o =ρ o *H o *g H s = H o +H w H w ; P w =ρ w *H w *g Sensor Sensor Fig. 1a, A pressure measuring apparatus Fig. 1b, A sample will consist of H o oil and H w water Fig. 2, The piezo resistive pressure sensor and its amplification circuit. 5

6 Oil Versus Voltage (Full Range) Fig. 3, Oil versus voltage Oil Level Versus Voltage Fig. 4, Oil versus voltage 6

7 Oil Level Versus Voltage Fig. 5, Oil versus voltage Oil Level Versus Voltage Fig. 6, Oil versus voltage 7