STUDY OF EXCESS WATER PRODUCTION FROM A SUDANESE OIL FIELD (STRATEGY FOR SOLUTION)

Abstract

Main H- Field now is producing oil rate under high water cut 80% and the reasons to the current high water cut phenomenon could be divided into two areas – un-controllable and controllable parameters. The uncontrollable parameters are related to the reservoir and fluid properties such as higher viscosity (4.3 to 45cp), 28-35 API, high permeability of the formation (100 7000 md), very strong aquifer support (Infinite) and absence of solution gas, Traces-7.7 SCF/STB. While the controllable parameters are well production rate which seem to be higher than the well critical rate, completion, commingling strategy between two or more reservoirs with varying fluid properties and development well location (well density).

The objective of this thesis is a straightforward strategy for diagnosing excess water production problems in H-Field. For effective treatment, the nature of the problem must first be correctly identified. The strategy advocates that the easiest problems should be attacked first and diagnosis of water production problems should begin with information already at hand.

WOR & WOR’ Diagnostic plots which was carried out to most of main H-Field wells found that this high water cut mostly related to coning/cusping problem due to unfavorable mobility ratio, high off take rate (maximum draw down caused by high capacity of ESP), most wells have thin, poor or even no shale break between lowest perforation and OWC, which is favorable for water coning and most wells were perforated too close to the OWC.

Simulation study by using single well model (He-23 as back ground) the result showed that, the critical flow rate ( oil free rate qo,crit ) for coning is so low (150 bbl/d) that mean the well cannot produce dry oil economically. Most likely decision would be to produce at a high rate rather than the critical rate.

Too high production rate may result high oil production rate in first period but with in quick water coning and fast water fingering, which lower down the efficiency of water displacement, hence reducing the well recoverable reserve. Production performance plots analysis showed that using ESP with high off take rate instead of PCP with control off take rate in most of main H-Field wells show the same oil rate with only increase in water rate.

Except for He-01 well, decline curve analysis for Group-1 wells in main H-Field showed most of wells Cum oil Production or EUR seems to be lower under ESP (high off take rate) compared to under PCP (control off take rate). This means the reserve or Cum oil Production is sensitive to max liquid rate especially in wells without thick localized shale barrier between OWC and the bottom of perforation interval.

He-01 showed that high off take rate (ESP) is better than using low off take rate (PCP) for well Cum oil production or recoverable reserve. This due to well location at the crest and there is a thick localized and laterally extensive shale barrier between OWC and the well bottom perforation interval.

The high off take rate must be controllable depending on many factor such as well location must be at crest to avoid water coning/cusping and perforation strategy should be okay and the bottom of perforation interval from OWC must be more than 10 m, Shale break between bottom of perforation interval and OWC should be thick (over 4 m) and the quality of the shale is it permeable or impermeable must be confirmed by logging interpretation, X- Section of the layers of the certain well and near by wells at least 3 wells should be carried out so as to show the continuity and laterally extensive of the shale in different direction.

In reservoir produced by age water drive mechanism the mobility ratio (viscosity) is main factor for early water break through or water cusping, so for that in all wells produced commingle layer the horizontal permeability for each layers should be the same value (close) to avoid water cusping or creating channel between the aquifer and perforations Interval. Productivity index of the well must be high with maximum draw down cased by the pump.

The sensitivity study for different liquid rate which was carried out by simulation using single well model the result showed that the optimum liquid rate is (5000 bbl/d) , the free water period is longer, the cum. Oil volume is bigger ; the EUR is increasing with the this rate in 20 years taken the 95% water cut as a economical limitation and the shale break radius is 270 m with vertical permeability 10 md.

Also the EUR is almost same when the rate is larger or equals to 5000 blpd. That means if we used high off- take rate more than 5000 the Cum of Oil will be same only increase Cum of water which is not necessary to produced while the operation cost of disposal water, surface facility and Pump rental will increase when the rate is larger and water encroachment will affect the reservoir parameter such as Porosity, Permeability and water saturation and might be created a channeled between the aquifer and well perforation interval because since the water touch the rock surface it will change the saturation and rock tendency to water and if we shut the well for long term and re-open again this well will produce high water cut.