Stabilized and Increased Well Production Using Automatic Choke Control

Abstract

Abstract Process fluctuations are often equivalent to lost production as the necessary margins to process constraints need to take into account the fluctuations. Better operation is achieved by reduced process fluctuations so that the average production may be closer to the constraints. This is achieved by automatic process control which also reduces operator load. Wells which produce oil and free gas (coning) from the reservoir are sensitive to changes in operating conditions, and the gas and oil rates drift over time. This is natural because variations in coned free gas generate variations in wellhead pressure. This will in turn affect the production rate and the well drawdown. In manual operation, a common strategy is to operate with an excess of free gas or use artificial gas lift and manipulate the wellhead choke manually. An automatic wellhead choke control solution is developed to reduce fluctuations and increase production rates. The two main elements in the automatic wellhead choke control are: A robust measure of the controlled variable (i.e. the gas flow rate) A robust control strategy Since the gas flow rate is not directly measured, a gas flow rate estimator was developed, with the premise that no hardware modification should be necessary: The gas fraction in the well stream is determined from the pressure drop from the bottomhole to the wellhead The total multiphase well flow is calculated from either a venturi meter (if available) or the well head choke (based on choke pressure drop, travel, and characteristics) The control strategy is to ensure fast corrections to keep the gas flow rate close to the specified set point, thus enabling a close-to-collapse operation of the established gas cone. The gas flow rate set point is specified to ensure that the well head pressure is sufficiently high, i.e. higher than the pressure downstream the choke. The solution is implemented on several wells at various Statoil operated fields with gas coning into the wells. The solution gives stable well flow rates, and thus enables increased production. The operator workload related to well supervision has also been reduced. The method could also be applied to other that should remain constant, e.g. water cut, water flow, or GOR, as long as the quantity can be estimated (or measured), and will respond to well head choke manipulations.