Despite concerns about proppant embedment in extremely soft chalk, Cook demonstrates that wide propped fractures have substantial benefits in the oil bearing chalk of the Eldfisk field of the Norwegian North Sea.
The Eldfisk oil field is a high porosity, low permeability reservoir of soft chalk in the Norwegian North Sea. Commercial oil production from the Eldfisk field is dependent on well stimulations involving hydraulic fractures. To date, pseudo limited entry (PLE) acid fracturing has been the standard completion technique that initiates relatively high (5000 bopd) individual well oil production rates. Over time however, many of the Eldfisk wells have experienced rapidly declining oil production. The main causes of the decreased well productivity are attributed to hydraulic fracture closure, solids production and casing collapse. Initial calculated skin values of -4.5 on average increase annually at a rate of +0.6. Through year 1999 over 100 casing deformations have been logged in the reservoir.
The root problem behind the productivity deteriorations is the Eldfisk field soft chalk nature where the Brinell Hardness Number (BHN) is commonly less than 5 for the most productive zones. In late 1997, an alternative completion technique involving four hydraulic propped fractures was successfully executed in a horizontal well. Evaluation of rate-time flow efficiency for the propped fractured well versus acid fractured wells with the use of an analytical model is reported in this paper. In addition, the analytical model is used to run predictions of increased multiple propped fractures. This paper relates formation BHN to the hydraulic fracture design criteria and concludes that productivity for hydraulic propped fractured wells in soft chalk formations is preserved over time to a greater extent versus the current standard PLE acid fracture technique. This paper concludes that an additional horizontal well with eight hydraulic propped fractured zones is warranted to improve understanding of the cost effectiveness of this alternative oil recovery completion technique for the Eldfisk chalk field.
Author(s): C.C. Cook, Norsk Hydro ASA; K. Brekke, PTI-Scandpower AS
Paper Number: SPE 73757