Application of technically sound well and reservoir management (WRM) principles together with cost reduction measures utilising the latest technologies are the key to maximising economic recovery from mature reservoirs. The fields are in one cluster in Petroleum Development Oman. More than 250 major well entries are executed each year. The main measures that are applied to improve production and reduce costs are: integrated reservoir surveillance plans, better sequencing of the workover units, muli-disciplinary input to workover proposals, ESP monitoring and the introduction of new technology such as one-rig-up coiled tubing for cleanout, saturation-logging, perforation & stimulation. Together these processes have reduced well intervention costs by 20% and increased net oil gains by 10%. This paper aims to demonstrate that the application of intensive WRM techniques and appropriate technologies can reduce the unit cost for well interventions.
Introduction
The cluster consists of two fields in North Oman and both fields produce from the Lower Cretaceous Shuaiba Formation, which is heavily faulted and consists of intrashelf basin floor carbonate muds and limestone reef. Porosities ranges from 12 to 35% while matrix permeability ranges between 1 md to 20md. Permeabilities in fault, fracture & leached zones are believed to be orders of magnitude higher than the average matrix permeability.Oil is relatively light (40° API). The two fields produce from around 350 active wells, over 90% of which are horizontal wells. One of the two fields has a strong aquifer, whereas the other field has a major waterflood, with some 130 active water injection wells. Both fields produce at a high watercut of around 90%. All the producers are completed with either a gaslift or electrical submersible pump artificial lift system.
In these fields, minimising the decline by restoring and optimising production is a major effort. Every year typically 250 major well entries are executed by a Workover Unit or a Coiled Tubing Unit.
The major interventions are focussed on
Restoring integrity: The wells are typically some 20–30 years old and exhibit integrity problems.Replace failed ESPsOptimise the inflow profile, by adding perforations or water shut-offs, in order to improve reservoir sweep.Stimulations to remove damage to inflow and outflow of wells.Well conversions from producer to injector or from gaslift to ESP.
The high number of major well entries has an associated high opex cost. In order to reduce unit opex costs, focus has been given to improve the efficiency of well interventions.
This paper describes the improvements that were done to reduce the costs of the major well interventions, while at the same time improving the production from these interventions.
Improvements
These improvements come from the following elements:Selection of jobs to be executedESP managementManagement of the workover unitsApply ‘Service-the-Limit’ techniquesUse of new technology, specificallyOne rig-up CT systemUse of diverting agents for stimulations
