In the 2018 intervention campaign, CNOOC aimed to improve production by retrofitting a horizontal well offshore of China. Water mobility in the reservoir was at least 20 times higher than oil mobility and a strong aquifer was located below the well. The well was drilled along a heterogeneous formation with varying properties resulting in an uneven reservoir influx toward the wellbore. Although the well was already completed with passive ICDs, oil production from the well started suffering severely from an early water breakthrough in a couple of weeks after starting production.
It was recognized that the challenges could be mitigated by deploying Autonomous Inflow Control Devices (AICDs) which can control the reservoir fluid influx toward the wellbore and therefore optimise the well performance. An AICD is an active flow control device that delivers a variable flow restriction in response to the properties of a fluid and the rate of flow passing through.
An integrated workflow comprising history matching and performance evaluation of the existing completion and sensitivity analyses was adopted to determine the best retrofit completion for the well. A well with a horizontal length of 300m was drilled in a thin formation with the oil column averaging 15ft. The optimum retrofit completion was to install a 2 3/8″ inner string consisting of AICD subs, swellable packers inside the existing ICD completion. The well was segmented in three compartments and a tailored AICD completion based on log data from the well was designed to properly restrict the production of water. The string was then connected to a redesigned ESP pump lifting the fluids to the surface. Through teamwork between the companies, the well was successfully re-completed with RCP AICD completions.
Over a 9-month period of production, the well performance has been optimised with AICD devices. The AICDs significantly reduced the water cut (WC) of the well from 97% to 87% helping produce 200% more oil compared to production prior to re-completion. This application not only saved the cost of treating extra water but also added value by producing more oil. It also facilitated the connection of another well to the production system due to the enhanced capacity of the system which was then producing a lower volume of liquid.
This well is an example that demonstrates the possibility of retrofitting AICDs in existing screens successfully. AICD completions ensured a balanced contribution from all reservoir sections and limited water production significantly. The lessons learnt from pre and post-installation studies will be discussed throughout. The AICD completions enabled the operator to implement an optimum reservoir drainage strategy that uses downhole control that can be manipulated autonomously based on well dynamic conditions to produce more oil.