A joint field verification test showed that a digital level sensor was an acceptable alternative in oil custody transfer for tank truck sales. The sensor, manufactured by Remote Operating Systems (ROS) of San Antonio, sends a digital signal that can be transmitted through a telephone modem to any desired office. Union Pacific Resources (UPRC), the operator, conducted the study with Scurlock Permian Corp. (SPC) during 1993. In 1994, the oil and gas division of the Railroad Commission of Texas accepted the digital level controller as an alternative measurement device to tank gauging and lease automated custody transfer in Texas.
SENSOR DESCRIPTION
The sensor
(Fig. 1)(81432 bytes) is installed in oil storage tanks and consists of a 1.5-in. sealed tube surrounded by a circular float. Measurement electronics are in the tube, and the only moving part is the float containing a magnet that activates switches as the float moves. The manufacturer says that the accuracy can be designed to be 0.1 in.
UPRC STUDY
UPRC initiated automation of its Austin Chalk oil and gas production around its La Grange, Tex., operations office in 1990. During a period of 2 years, it automated over 125 wells and over 600 oil and water tanks. According to UPRC, automation reduced travel and manpower costs and increased production efficiency. To further save costs and increase efficiency, UPRC said it tested the digital level sensor to determine if it could reduce the 20% of the field pumper's time spent on the custody transfer process of collecting, checking, processing, and reporting both production and oil sales. A successful test would also reduce the administrative and accounting support required for handling trip tickets. The digital level sensor would serve as a custody transfer device for top and bottom gauges and for determining the average temperature of oil sold. UPRC described the field test as involving 9 of its 130 wells and included 36 oil tanks equipped with permanently installed level sensors as part of the production automation system. Remote Terminal Units (RTUs) electronically read the sensor levels and temperatures and reported by radio to PC-based master terminal units. The test involved over 30 different tank truck drivers who transferred 417 loads of oil. They followed normal gauging procedures along with recording digital level sensor top and bottom levels and associated oil temperatures on each trip ticket. UPRC's RTUs also electronically recorded the complete transaction. In addition, specially installed electronic temperature sensors in the flow line compared the tank sensor and driver measurements. This flow line temperature involved 82 loads over several months with more than 230 discrete temperature calibration comparisons in nine well tanks over 6 months. UPRC's report said that during the 9 month test on 417 tank truck loads, about 45,036 in. of oil transferred, the difference between the hand recorded and sensor measurement was 29.25 in. or -0.7 in./load. Temperatures were found to be within 0.2 F. for the compared loads. The study team concluded that this accuracy was acceptable for custody transfer.
Copyright 1995 Oil & Gas Journal. All Rights Reserved.