Successful Mud Acid Stimulations Maintain Productivity in Gravelpacked Wells at Heidrun

Abstract

Abstract The Heidrun Field is located on the Haltenbanken area in the Norwegian Sea was discovered in 1985, and has been producing since October 1995. The hydrocarbons are present in 3 reservoirs of Jurassic age, the high quality Fangst group, and the more heterogeneous Tilje and Are formations. The initial reservoir pressure is 250 bar and the reservoir temperature is 88 °C. The drainage strategy utilizes injection of produced gas and seawater for pressure support. The reservoir sands are in general poorly consolidated and contain fairly high quantities of clay with kaolinite content well above 25 % in Fangst and Tilje. Most of the producing wells have sand control devices (gravel pack or screen) installed. The cased-hole gravelpacks in the high-permeable Fangst formation have shown extensive productivity decline after water breakthrough. Initial oil rates in these producers exceed 6000 Sm3/d, but the rates drop rapidly even at very low water cuts. A team of engineers and scientist from the operator and service companies analyzed the problem and started to develop solutions. Migration of kaolinite fines was identified as the major cause for productivity decline. A mud acid/clay acid system to remove damage and maintain productivity was designed. Several successful treatments with this fluid system have now been carried out. Detailed monitoring and analyses of the fluids that are backproduced after the treatments have proven to be crucial in continuous improvement of the stimulation system. An extra oil production of approximately 500 000 Sm3 is ascribed to successful stimulation treatments in the first year following the treatment. It is now recognized that successful acid stimulation will be necessary to maintain production rates from the Heidrun field in years to come. Details about the acid stimulation design, pumping schedule and ion composition analysis of the spent acid return will be given in the presentation.