Productivity Preservation via Hydraulic Propped Fractures in the Eldfisk North Sea Chalk Field

Abstract

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. Introduction Since the early 1970's, petroleum engineers working on North Sea chalk fields have been challenged to design well stimulation treatments focused on hydraulic fracturing of the low permeability, fragile chalk formations all the while with an eye on costs. Fracturing is necessary to make the low permeability chalk formations produce economically. Conventional acid fracturing has been tried on all of the chalk fields. Both field experience and research (Ref. 1) have shown that acid fracturing render attractive near term oil production, however, for some of the chalk fields, the medium term production period is poor. The fall off in production is attributed to low hardness and homogeneous nature of the chalk whereby the etched surfaces of the hydraulic acid fractures close with increasing stress. To date, there has been oil production from over ten chalk fields in the North Sea. It would be convenient if one standard well completion would serve all the fields, however each individual North Sea chalk field has its own unique static and dynamic rock parameters that dictate optimum completion design. Geographically, starting with the southern North Sea Danish chalk fields, after initially stimulating with acid fracture treatments, there have been over 100 propped fracture treatments placed in horizontal wells in the Dan field (Ref. 2). Offset to the Dan field is the Gorm field, which has also utilized hydraulic propped fractures in some its water injection wells (Ref. 3). Further north, still in Danish waters, the South Arne chalk field development strategy has focused on the drilling of horizontal wells with completions involving multiple propped fractures (Ref. 4). The advent of propped fractures in Norwegian chalk reservoirs was first introduced at the Valhall field (Ref. 5). The evolution from exclusive use of acid fracturing to propped fractures for all of the above mentioned fields has centered on one mutual goal, namely productivity preservation.

Proving It - A Review of 80 Published Field Studies Demonstrating the Importance of Increased Fracture Conductivity

“…99 Most wells in this region are currently stimulated with high-rate, multistage acid fracturing treatments. Although providing acceptable initial rates, most acid fractured wells suffer rapid decline, often accompanied by solids production and casing deformations.…”