Challenging Chemical Gas Shut off in a Fractured Carbonate Reservoir: Case Studies

Al-Sharji, Hamed Hamoud (Petroleum Development Oman) | Ali, Ehtesham (Petroleum Development Oman) | Kosztin, Bela (Petroleum Development Oman) | Edwards, Clement (ADCO) | Neyaei, Fardin Ali (Schlumberger) | Shaheen, Tarek (Schlumberger Well Services)

OnePetro 


This paper discusses the gas shut-off treatments carried out in a fractured carbonate field in north Oman and also describes the good practices and lessons learnt from a number of jobs. In addition to the technical analysis, the paper also addresses the economic value of the campaign.

Oil production from this field with complex geology and reservoir mechanism was negatively affected by gas breakthroughs in several wells. The constraints on gas handling capacity resulted in shutting-in a number of high GOR wells. These wells were required to be treated to shut-off source of the gas breakthrough in order to restore oil production. Challenges faced in shutting off these gas zones included: 1) Poor cement bond behind the liner shoe. 2) Massive fractures resulting in loss circulation. 3) Uncertainty with fractures volume estimation. 4) Fracture shut-off in open-hole sections. 5) Treatment execution under sub-hydrostatic conditions.

To overcome these challenges a robust chemical shut off methodology had to be innovated. This methodology consisted of the following main pillars: a) Utilize various reservoir diagnostics tools to identify fractures and sources of high GOR. b) Use of flowing cross-linked polymer gel combined with a ringing type of cross-linked polymer gel as a capping fluid. c) Utilize an on-fly mixing system that enables volume and concentration adjustment as plugging progression dictates. d) Utilize matrix diagnostics plot along with modified hall plot in real-time to continually estimate flowing gel volume. e) Deploy a fit-for-purpose gel placement assembly for treatment under Sub-hydrostatic conditions.