Intelligent Automation for Intermittent-Gas-Lift Oil Wells

Abstract

Intermittent gas lift is widely used in the PETROBRAS onshore petroleum fields. The intermittent gas injection in the annulus between the tubing and casing causes a gas lift valve to open. This gas lift valve is a pressure driven valve installed in the lower part of the tubing and controls the amount of gas injected at the bottom of a liquid column held inside the tubing. The injected gas becomes a big gas bubble that carries the liquid column up to the surface. The intermittent cycles are obtained by a motor-valve installed at the surface opening and closing at fixed time intervals (Time cycle - Tc) and remaining open during a fixed amount of time (Time injection - Ti). This work presents the concepts use in a firmware for Programmable logic Control (PLC) especially designed to optimize both Tc and Ti. Based on the tubing and casing pressure continuously acquired by a pair of electronic pressure transmitters (PT) and on artificial intelligence (AI) concepts such as neural nets and fuzzy logic, this firmware performs the pattern recognition of the casing and tubing pressure behavior, executes an algorithm implemented by the user and gives a diagnostic about the performance of the system. With the diagnostic and changing the values of Ti and Tc, the electronic device improves the system performance, reducing the gas consumption andincreasing the liquid production. The electronic device is installed at the well site and send to the central office, by request, the data acquired from the well. At the central office, a PC computer with a modem performs a data pooling, well by well, and stores the acquired data in MS-Access type data base for further statistical and historical analysis. Introduction The Intermittent Gas Lift operation consists of the intermittent gas injection, from the casing into the tubing, in way to drag to the surface the liquid column accumulated in the lower part of the tubing. The gas injection at the bottom of the well is accomplished through a valve (gas-lift valve) installed in the lower part of the tubing, seated or retrieved by wire line units. At the surface the gas injection in the casing is accomplished using a Motor Valve, as shown in Fig.1. The complete cycle of the intermittent gas-lift is composed basically of three periods1, given below:Accumulation: Period during which doesn't happen injection of gas in the casing at the surface or in the tubing at the bottom (motor valve and gas-lift valves closed). Fluid from the formation accumulates in the tubing until a desirable length of slug is obtained.Elevation: Period during which the liquid slug is thrown to the surface by the injection and expansion of the gas at the bottom of the tubing. In this period, gas in the casing is injected in the tubing through a gas-lift valve.Reduction of Pressure: Period in which the tubing pressure is reduced to a minimum, through the removal of the gas used for elevation of the slug. The system optimization is obtained setting up the Fall-Back and the Injection Gas-Liquid Ratio (IGLR). Fall-back is the liquid portion that is not produced during the cycle - it is the fluid volume that remains in the tubing after the slug reaches the surface. IGLR is the relationship between the injected gas volume and the fluid volume produced in each cycle. Intermittent Gas-Lift optimization consists of to maximize the oil production, reducing the fall-back, with a minimum consumption of gas, reducing the IGLR. Time cycle and gas-lift valve design are the key to reduce fall back and improve the system efficiency. The time cycle is divided in two parts:Injection Time (IT): it is the period of time that the Motor Valve at the surface remains opened, allowing injection of gas into the casing, andTime cycle (Tc):is the period of time the Motor Valve remains closed plus the Injection time.