GEOLOGICAL AND RESERVOIR ENGINEERING CONSIDERATIONS FOR THE NORTH RANKIN GAS RECYCLING PROJECT

Abstract

The North Rankin Field, situated in the Dampier Sub-basin offshore north-western Australia, is being developed as part of the North West Shelf Development Project. Substantial excess production capacity from the field exists until liquified natural gas exports to Japan plateau in late 1993. A recycling project was proposed to utilise this spare capacity. Gas produced in excess of current sales volumes is stripped of its condensate, and the lean gas is reinjected into the reservoir thereby increasing condensate production and the total recovery.

The North Rankin A Gas Recycling Project was commissioned in June 1987 and its performance to date has been better than original expectations. The average recycling rate during the first few months of operation was approximately 13 x 106sm3/d (450 MMscfd) with associated condensate production of 1600 m3/d (10,000 STB/d). Studies suggest that the total recycled volume for the project, up until late 1993, may be in the order of 19 x 109sm3 (0.7 Tcf) with additional condensate recovery of approximately 2 million cubic metres (13 MMSTB).

The design and implementation of subsurface aspects of the recycling project required close co-operation between the geological and reservoir engineering disciplines. Detailed studies were undertaken to determine reservoir extent and communication within the field in order to identify likely paths for lateral and vertical gas movement. Estimates of swept reservoir volumes were used to determine injected lean gas distributions. These gas distributions, in conjunction with operational aspects, formed the basis for the production and injection policy developed to maximise sweep efficiency and optimise condensate recovery from the recycling project.

Multiple zone through tubing perforation was needed in most injection wells. The initial perforation was performed with an optimal underbalance pressure. Subsequent perforation was neutrally balanced. The degree of underbalance while perforating has a significant effect on perforation efficiency and simple models were used to predict this.

Accurate estimates of well injectivity must be made to ensure that the perforation design allows well target rates to be met. Numerical reservoir and wellbore models were constructed to evaluate well injection potentials and gas distributions.

To verify the design procedures and well performance, well testing and production logging was carried out before and after the start up of gas recycling. Production logging surveys in the injection wells and measuring condensate gas ratios in the producers are essential in monitoring the performance of gas recycling.